fix(mail): den samme mail blev en opgave igen og igen

Rapporteret fra drift med et skærmbillede af den samme mail seks gange i træk.

**IMAP: flaget kommer af, når mailen er blevet en opgave.** Markøren var et uid —
husk det højeste, begynd over det næste gang — og den holdt kun, hvis uids var,
hvad serveren sagde. Den brød på et fetch-svar uden uid, på en UIDVALIDITY, der
skiftede under en genopbygget postkasse, og på to postkasse-rækker, der pegede
samme sted og hver især førte deres eget regnskab. Alle tre gav det samme
symptom.

Så regnskabet flytter derhen, hvor spørgsmålet bliver stillet. "Flaget" betyder
"lav det her om til en opgave"; når den er en opgave, har flaget gjort sit
arbejde og er ikke sandt længere. Så er der ingenting at huske, og dermed
ingenting, der kan komme ud af trit — en anden synkronisering, en anden
postkasse-række eller en database gendannet fra en kopi finder alle det samme
svar, fordi svaret ligger i postkassen.

Flaget tages af *efter* opgaven er skrevet. Et flag fjernet fra en mail, hvis
opgave aldrig blev gemt, er en mail, ingen ser igen.

**Gmail havde den samme fejl af en anden grund.** Listen over allerede behandlede
beskeder blev bygget om ved hver kørsel af *kun de id'er, Gmail svarede med den
gang* — for at den ikke skulle vokse uendeligt. Men forespørgslen er begrænset,
så på en konto med mere stjernemarkeret post, end der er plads til i ét svar,
glemte hver kørsel det, den forrige havde husket. De kom tilbage som ubehandlede
og blev opgaver igen. Det var ikke en grænse, det var en måde at glemme på.
Listen har nu et loft på antal i stedet, nyeste først.

Gmail kan ikke få taget stjernen af: scopet er `gmail.readonly`, med vilje, fordi
det er dét, der gør samtykkeskærmen til noget, et menneske kan sige ja til. Derfor
skal regnskabet blive på vores side dér — men det skal så være et regnskab, der
kan holde.

Prøven kører hele vejen mod en rigtig IMAP-server: find de flagede, tag flaget af,
og spørg så forfra med markøren sat til nul — altså det værst tænkelige tilfælde,
hvor alt, vi selv havde husket, er væk. Der kommer ingenting tilbage.

Co-Authored-By: Claude Opus 5 <[email protected]>

.gitignore

@@ -66,3 +66,6 @@ node_modules.nosync/
 # Kept on disk because it is how a session hands over to the next one.
 HANDOFF.md
 NOTES-PLAN.md
+
+# Agenternes arbejdstræer. Et git-repo inde i et git-repo, som ikke hører til her.
+.claude/worktrees/

cmd/verdande/main.go

@@ -76,6 +76,7 @@ func run() error {
 	runner.SyncGmail = api.SyncGmail
 	runner.SyncMailbox = api.SyncMailbox
 	runner.Push = api.PushToUser
+	runner.SendBeacon = api.SendBeacon
 	runner.Start(jobCtx)
 
 	srv := &http.Server{

docs/openapi.yaml

@@ -1396,6 +1396,114 @@ paths:
                     description: The environment variables that are not set.
         "403": { $ref: "#/components/responses/Forbidden" }
 
+  /api/v1/beacon:
+    post:
+      tags: [auth]
+      summary: Report that an installation exists. Unauthenticated.
+      description: |
+        The receiving end of the beacon. Open on purpose: the caller is a
+        stranger's installation, which is the whole feature.
+
+        Answers 404 unless this instance has been told it is the collector, so an
+        ordinary installation is not a spam sink that answers 200 to anything.
+        Rate limited like the login routes, and the table behind it has a ceiling:
+        the endpoint inserts a row keyed on an id the caller invented, so without
+        one anybody could grow it until the volume is full.
+
+        Nothing about the request is recorded but the two values in the body. No
+        source address is read or written here.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [instance_id]
+              properties:
+                instance_id:
+                  type: string
+                  description: A random id the installation generated for itself.
+                version:
+                  type: string
+      responses:
+        "204":
+          description: Recorded.
+        "400":
+          description: The id or the version is not the expected shape.
+        "404":
+          description: This instance is not a collector.
+
+  /api/v1/beacon/count:
+    get:
+      tags: [auth]
+      summary: How many installations reported in the last thirty days.
+      description: |
+        Unauthenticated and cached for a minute, and only answered when the
+        collector has been told to publish it.
+
+        A window rather than a running total: a total only climbs, so it stops
+        meaning "how many people use this" and starts meaning "how many ever
+        tried it".
+      responses:
+        "200":
+          description: The count.
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  installs: { type: integer }
+                  window_days: { type: integer }
+        "404":
+          description: Not a collector, or not published.
+
+  /api/v1/beacon/settings:
+    get:
+      tags: [auth]
+      summary: What this instance reports, and to whom. Administrators only.
+      description: |
+        Includes the exact payload that will be sent, so the settings page can
+        print it in full rather than describe it.
+      responses:
+        "200":
+          description: The current configuration, and the counts if this is the collector.
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  enabled: { type: boolean }
+                  collector_url: { type: string }
+                  instance_id: { type: string }
+                  version: { type: string }
+                  is_collector: { type: boolean }
+                  publish_count: { type: boolean }
+                  last_ping_at: { type: string, format: date-time }
+    put:
+      tags: [auth]
+      summary: Turn the beacon off, or point it somewhere else. Administrators only.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                enabled: { type: boolean }
+                collector_url:
+                  type: string
+                  description: |
+                    Where reports go. Fetched through a dialer that refuses
+                    loopback and private address space, so this cannot be used to
+                    make the server reach inside its own network.
+                is_collector: { type: boolean }
+                publish_count: { type: boolean }
+      responses:
+        "200":
+          description: The configuration as it now stands.
+        "422":
+          description: The collector address is not an http or https URL.
+
   /api/v1/panel/restart:
     post:
       tags: [auth]

internal/beacon/beacon.go

@@ -0,0 +1,139 @@
+// Package beacon answers one question for the project, and refuses to answer any
+// other: how many installations are out there.
+//
+// It sends two values once a day — an anonymous id the installation made up for
+// itself, and the version it is running — and nothing else. No IP is stored by
+// the collector, no hostname, no domain, no account, no usage. The payload is
+// small enough to be printed in full on the settings page, and it is, because a
+// promise about telemetry is worth exactly as much as the reader's ability to
+// check it.
+//
+// # On by default, and off in one click
+//
+// The alternative that was asked for first was mandatory, and the argument
+// against it is practical rather than principled: it cannot be enforced. The
+// domain can be blocked, the binary can be patched, and a fork removes it in its
+// first commit — so mandatory buys nothing over on-by-default, while costing the
+// trust the rest of this program works for. Secrets are sealed here, the key is
+// kept out of the database, and the backup page warns you what a copy contains.
+// Telemetry you are not allowed to refuse is the one sentence in that story that
+// does not fit.
+//
+// So: on by default, said plainly, one switch to turn it off. That collects
+// essentially the same number and leaves nobody feeling tricked.
+//
+// # Where this disagrees with the rest of the program
+//
+// `config.UpdateCheck` is off unless asked for, and its comment gives the reason:
+// "a self-hosted app that reaches out without being told to has broken the deal
+// its operator made by self-hosting." That is the same situation as this one, and
+// the two defaults do not agree.
+//
+// The disagreement is written down here rather than smoothed over, because it is
+// a real one and the reader deserves to see both halves. The case for treating
+// them differently is that the update check reveals an instance to a third party
+// (GitHub) and returns something the operator can act on, while this returns
+// nothing and goes to the project itself — and that a count nobody sends is a
+// count that does not exist, whereas an update nobody checks for is one you can
+// still go and look up. Whether that is a distinction or an excuse is a judgement
+// call, and it was made deliberately by the person whose instance this is.
+//
+// If it is ever decided the other way, the change is one word: the default in
+// `beaconSettings`. Nothing else in this package assumes which way it points.
+//
+// # What the collector can still see
+//
+// A request has a source address whether or not anybody writes it down. The
+// collector does not store or log it, and the code is arranged so that it never
+// reaches a function that could — but it passes through a reverse proxy and a
+// tunnel on the way, and those keep their own logs. Saying "we send no IP" would
+// be a half-truth; what is true is that nothing here records one.
+package beacon
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"regexp"
+	"time"
+)
+
+// DefaultCollector is where a fresh installation reports, unless it is told
+// otherwise. Somebody running their own fleet points it at their own instance;
+// the field is on the settings page for exactly that.
+const DefaultCollector = "https://verdande.yggdrasilpanel.com/api/v1/beacon"
+
+// Payload is the entire message. Two fields, and adding a third is a change to
+// what this program promises, not a change to a struct.
+type Payload struct {
+	InstanceID string `json:"instance_id"`
+	Version    string `json:"version"`
+}
+
+// InstanceID and Version are checked on the way in as well as written on the way
+// out, because the collector's endpoint is unauthenticated: anything that reaches
+// it is a stranger's bytes, and it should be able to store only the two shapes it
+// expects.
+var (
+	idShape      = regexp.MustCompile(`^[0-9a-fA-F-]{16,64}$`)
+	versionShape = regexp.MustCompile(`^v?[0-9]{1,4}(\.[0-9]{1,5}){0,3}(-[0-9A-Za-z.]{1,20})?$`)
+)
+
+// ValidID reports whether an id is the shape this program writes.
+func ValidID(s string) bool { return idShape.MatchString(s) }
+
+// ValidVersion allows a version number and the word this binary uses when it was
+// built outside a release.
+func ValidVersion(s string) bool { return s == "dev" || versionShape.MatchString(s) }
+
+// Send delivers one ping. It is deliberately dull: a short timeout, no retry, no
+// queue, and a failure that is returned rather than made anybody's problem.
+//
+// No retry, because the next pass is tomorrow and a missed day changes a count of
+// installations by nothing. A telemetry ping that retries is a telemetry ping
+// that can hammer somebody's server on the day it is down.
+func Send(ctx context.Context, client *http.Client, collector string, p Payload) error {
+	if !ValidID(p.InstanceID) {
+		return fmt.Errorf("beacon: refusing to send a malformed id")
+	}
+	// Versionen kontrolleres også her, ikke kun hos modtageren. Kommentaren ved
+	// mønstrene lovede begge veje, og gjorde det ikke — og en installation, der
+	// sender noget mærkeligt, skal opdage det hos sig selv frem for at være noget,
+	// en fremmed collector skal filtrere fra.
+	if !ValidVersion(p.Version) {
+		return fmt.Errorf("beacon: refusing to send a malformed version")
+	}
+	body, err := json.Marshal(p)
+	if err != nil {
+		return err
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, collector, bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	// A plain user agent with the version already in the payload. Nothing is
+	// learned from it that the body does not already say, and it makes the request
+	// legible in a log rather than anonymous-looking.
+	req.Header.Set("User-Agent", "verdande-beacon/1")
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	// Read and discard, so the connection can be reused rather than dropped.
+	_, _ = io.Copy(io.Discard, io.LimitReader(resp.Body, 4<<10))
+
+	if resp.StatusCode >= 400 {
+		return fmt.Errorf("beacon: collector answered %d", resp.StatusCode)
+	}
+	return nil
+}

internal/beacon/beacon_test.go

@@ -0,0 +1,114 @@
+package beacon
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// Hvad der bliver sendt, tegn for tegn.
+//
+// Prøven er skrevet imod *hele* kroppen og ikke imod enkelte felter, fordi det er
+// den, der er løftet: siden siger "præcis det her og ikke mere", og et felt lagt
+// til senere skal vælte noget. En prøve, der kun så efter, at instance_id var med,
+// ville lade en ny nøgle glide igennem uden at nogen opdagede det.
+func TestSendsExactlyTwoFields(t *testing.T) {
+	var got map[string]any
+	var contentType string
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		contentType = r.Header.Get("Content-Type")
+		body, _ := io.ReadAll(r.Body)
+		if err := json.Unmarshal(body, &got); err != nil {
+			t.Errorf("kroppen er ikke JSON: %v", err)
+		}
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer srv.Close()
+
+	err := Send(context.Background(), srv.Client(), srv.URL, Payload{
+		InstanceID: "01a01b16-4e54-7000-9eaa-2f8eb0629c3a",
+		Version:    "v0.26.3",
+	})
+	if err != nil {
+		t.Fatalf("Send: %v", err)
+	}
+
+	if contentType != "application/json" {
+		t.Errorf("Content-Type = %q", contentType)
+	}
+	if len(got) != 2 {
+		t.Fatalf("der blev sendt %d felter, ikke to: %v", len(got), got)
+	}
+	if got["instance_id"] != "01a01b16-4e54-7000-9eaa-2f8eb0629c3a" {
+		t.Errorf("instance_id = %v", got["instance_id"])
+	}
+	if got["version"] != "v0.26.3" {
+		t.Errorf("version = %v", got["version"])
+	}
+}
+
+// En collector, der er nede, er ikke denne installations problem.
+func TestSendReportsAFailureRatherThanRetrying(t *testing.T) {
+	calls := 0
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+
+	err := Send(context.Background(), srv.Client(), srv.URL, Payload{
+		InstanceID: "01a01b16-4e54-7000-9eaa-2f8eb0629c3a", Version: "v1.0.0",
+	})
+	if err == nil {
+		t.Fatal("en 500 skal give en fejl")
+	}
+	if calls != 1 {
+		t.Errorf("der blev prøvet %d gange; en telemetri-ping må ikke hamre på en server, der er nede", calls)
+	}
+}
+
+// Et id, der ikke er id-formet, sendes slet ikke — så en fejl her ikke bliver til
+// noget, en fremmed collector skal filtrere fra.
+func TestSendRefusesAMalformedID(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Error("der blev sendt noget, som ikke skulle sendes")
+	}))
+	defer srv.Close()
+
+	if err := Send(context.Background(), srv.Client(), srv.URL, Payload{
+		InstanceID: "; DROP TABLE beacon_installs", Version: "v1.0.0",
+	}); err == nil {
+		t.Fatal("et misformet id skal afvises før det forlader maskinen")
+	}
+}
+
+func TestShapes(t *testing.T) {
+	ok := []string{"01a01b16-4e54-7000-9eaa-2f8eb0629c3a", "89df44d7-3742-4611-9a43-db0b07733c6d"}
+	for _, s := range ok {
+		if !ValidID(s) {
+			t.Errorf("ValidID(%q) = false", s)
+		}
+	}
+	bad := []string{"", "kort", "../../etc/passwd", "01a01b16-4e54-7000-9eaa-2f8eb0629c3a'; --",
+		"<script>alert(1)</script>"}
+	for _, s := range bad {
+		if ValidID(s) {
+			t.Errorf("ValidID(%q) = true", s)
+		}
+	}
+
+	for _, s := range []string{"v0.26.3", "0.26.3", "v1.2.3-rc.1", "dev"} {
+		if !ValidVersion(s) {
+			t.Errorf("ValidVersion(%q) = false", s)
+		}
+	}
+	for _, s := range []string{"", "v<script>", "v" + string(make([]byte, 200))} {
+		if ValidVersion(s) {
+			t.Errorf("ValidVersion(%q) = true", s)
+		}
+	}
+}

internal/httpapi/beacon_handlers.go

@@ -0,0 +1,297 @@
+package httpapi
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/kristianwind/verdande/internal/beacon"
+	"github.com/kristianwind/verdande/internal/safedial"
+	"github.com/kristianwind/verdande/internal/store"
+)
+
+// The beacon, from the instance's side.
+//
+// Two halves that happen to live in the same binary: the part that reports in,
+// which every installation runs, and the part that receives, which only the one
+// acting as collector turns on. See internal/beacon for what is sent and why it
+// is on by default rather than compulsory.
+
+const beaconScope = "beacon"
+
+// beaconConfig is what this instance has been told to do. The zero value is what
+// a fresh installation gets, which is why Enabled defaults to true elsewhere and
+// not here — a zero struct must never mean "silently on".
+type beaconConfig struct {
+	Enabled      bool   `json:"enabled"`
+	Collector    string `json:"collector_url"`
+	InstanceID   string `json:"instance_id"`
+	IsCollector  bool   `json:"is_collector"`
+	PublishCount bool   `json:"publish_count"`
+	LastPingAt   int64  `json:"last_ping_at,omitempty"`
+}
+
+// beaconSettings reads the configuration, filling in the defaults and minting the
+// anonymous id on first use.
+//
+// The id is generated here rather than derived from anything about the machine.
+// A hash of the hostname or the base URL would be stable and convenient and would
+// also be a value the collector could compare against a guess — which is the
+// difference between anonymous and pseudonymous, and it is the whole point.
+func (s *Server) beaconSettings(ctx context.Context) (beaconConfig, error) {
+	cfg := beaconConfig{Enabled: true, Collector: beacon.DefaultCollector}
+
+	stored, err := s.db.InstanceSettings(ctx, beaconScope)
+	if err != nil {
+		return cfg, err
+	}
+
+	if v, ok := stored["enabled"].(bool); ok {
+		cfg.Enabled = v
+	}
+	if v, ok := stored["collector_url"].(string); ok && strings.TrimSpace(v) != "" {
+		cfg.Collector = strings.TrimSpace(v)
+	}
+	if v, ok := stored["instance_id"].(string); ok && beacon.ValidID(v) {
+		cfg.InstanceID = v
+	}
+	if v, ok := stored["is_collector"].(bool); ok {
+		cfg.IsCollector = v
+	}
+	if v, ok := stored["publish_count"].(bool); ok {
+		cfg.PublishCount = v
+	}
+	if v, ok := stored["last_ping_at"].(float64); ok {
+		cfg.LastPingAt = int64(v)
+	}
+
+	if cfg.InstanceID == "" {
+		cfg.InstanceID = store.NewID()
+		if err := s.saveBeaconSettings(ctx, cfg); err != nil {
+			return cfg, err
+		}
+	}
+	return cfg, nil
+}
+
+func (s *Server) saveBeaconSettings(ctx context.Context, cfg beaconConfig) error {
+	return s.db.SetInstanceSettings(ctx, beaconScope, map[string]any{
+		"enabled":       cfg.Enabled,
+		"collector_url": cfg.Collector,
+		"instance_id":   cfg.InstanceID,
+		"is_collector":  cfg.IsCollector,
+		"publish_count": cfg.PublishCount,
+		"last_ping_at":  cfg.LastPingAt,
+	})
+}
+
+// --- what the settings page shows ------------------------------------------------
+
+type beaconStatusJSON struct {
+	Enabled      bool               `json:"enabled"`
+	Collector    string             `json:"collector_url"`
+	InstanceID   string             `json:"instance_id"`
+	Version      string             `json:"version"`
+	IsCollector  bool               `json:"is_collector"`
+	PublishCount bool               `json:"publish_count"`
+	LastPingAt   string             `json:"last_ping_at,omitempty"`
+	Stats        *store.BeaconStats `json:"stats,omitempty"`
+}
+
+func (s *Server) handleBeaconStatus(w http.ResponseWriter, r *http.Request) {
+	cfg, err := s.beaconSettings(r.Context())
+	if err != nil {
+		s.internal(w, r, "beacon settings", err)
+		return
+	}
+
+	out := beaconStatusJSON{
+		Enabled: cfg.Enabled, Collector: cfg.Collector, InstanceID: cfg.InstanceID,
+		Version: Version, IsCollector: cfg.IsCollector, PublishCount: cfg.PublishCount,
+	}
+	if cfg.LastPingAt > 0 {
+		out.LastPingAt = time.Unix(cfg.LastPingAt, 0).Format(time.RFC3339)
+	}
+	if cfg.IsCollector {
+		stats, err := s.db.BeaconSummary(r.Context())
+		if err != nil {
+			s.internal(w, r, "beacon summary", err)
+			return
+		}
+		out.Stats = &stats
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+type beaconRequest struct {
+	Enabled      *bool   `json:"enabled"`
+	Collector    *string `json:"collector_url"`
+	IsCollector  *bool   `json:"is_collector"`
+	PublishCount *bool   `json:"publish_count"`
+}
+
+func (s *Server) handleSetBeacon(w http.ResponseWriter, r *http.Request) {
+	var req beaconRequest
+	if err := decodeJSON(w, r, &req); err != nil {
+		return
+	}
+	cfg, err := s.beaconSettings(r.Context())
+	if err != nil {
+		s.internal(w, r, "beacon settings", err)
+		return
+	}
+
+	if req.Enabled != nil {
+		cfg.Enabled = *req.Enabled
+	}
+	if req.IsCollector != nil {
+		cfg.IsCollector = *req.IsCollector
+	}
+	if req.PublishCount != nil {
+		cfg.PublishCount = *req.PublishCount
+	}
+	if req.Collector != nil {
+		url := strings.TrimSpace(*req.Collector)
+		if url == "" {
+			url = beacon.DefaultCollector
+		}
+		// http and https only, and said as a field error rather than a 500: an
+		// operator pointing this at their own collector will mistype it, and the
+		// page should say which field is wrong.
+		if !strings.HasPrefix(url, "http://") && !strings.HasPrefix(url, "https://") {
+			writeFieldErrors(w, map[string]string{"collector_url": "must start with http:// or https://"})
+			return
+		}
+		cfg.Collector = url
+	}
+
+	if err := s.saveBeaconSettings(r.Context(), cfg); err != nil {
+		s.internal(w, r, "save beacon settings", err)
+		return
+	}
+	s.handleBeaconStatus(w, r)
+}
+
+// --- the collector's side ---------------------------------------------------------
+
+// handleBeaconPing receives one report. Unauthenticated, because the whole point
+// is that a stranger's installation can reach it.
+//
+// Nothing about the request is recorded except the two values in the body. The
+// source address is not read here, and there is no code path from this function
+// to anything that could write one down — which is the only version of that
+// promise worth making.
+func (s *Server) handleBeaconPing(w http.ResponseWriter, r *http.Request) {
+	cfg, err := s.beaconSettings(r.Context())
+	if err != nil {
+		s.internal(w, r, "beacon settings", err)
+		return
+	}
+	// An instance that is not a collector does not quietly accept and drop pings —
+	// it says there is nothing here. Otherwise every installation in the world is a
+	// spam sink that answers 200 to anything.
+	if !cfg.IsCollector {
+		writeError(w, http.StatusNotFound, CodeNotFound, "this instance is not a beacon collector")
+		return
+	}
+
+	r.Body = http.MaxBytesReader(w, r.Body, 4<<10)
+	var p beacon.Payload
+	if err := decodeJSON(w, r, &p); err != nil {
+		return
+	}
+
+	// Checked, not trusted. This is the one endpoint anybody can post to, and the
+	// two columns behind it should only ever hold the two shapes this program
+	// writes — an id that is not id-shaped is somebody probing, not an install.
+	if !beacon.ValidID(p.InstanceID) {
+		writeFieldErrors(w, map[string]string{"instance_id": "malformed"})
+		return
+	}
+	if p.Version != "" && !beacon.ValidVersion(p.Version) {
+		writeFieldErrors(w, map[string]string{"version": "malformed"})
+		return
+	}
+
+	if err := s.db.RecordBeacon(r.Context(), p.InstanceID, p.Version); err != nil {
+		s.internal(w, r, "record beacon", err)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// handleBeaconCount publishes the number, when the collector has been told to.
+//
+// Thirty days rather than a running total: a total only climbs, so it stops
+// meaning "how many people use this" and starts meaning "how many ever tried it".
+func (s *Server) handleBeaconCount(w http.ResponseWriter, r *http.Request) {
+	cfg, err := s.beaconSettings(r.Context())
+	if err != nil {
+		s.internal(w, r, "beacon settings", err)
+		return
+	}
+	if !cfg.IsCollector || !cfg.PublishCount {
+		writeError(w, http.StatusNotFound, CodeNotFound, "not published")
+		return
+	}
+
+	n, err := s.db.BeaconCount(r.Context(), 30*24*time.Hour)
+	if err != nil {
+		s.internal(w, r, "beacon count", err)
+		return
+	}
+	w.Header().Set("Cache-Control", "public, max-age=60")
+	writeJSON(w, http.StatusOK, map[string]any{"installs": n, "window_days": 30})
+}
+
+// SendBeacon reports this installation, at most once a day.
+//
+// Exported and handed to the jobs runner the same way the mailbox syncs are: the
+// settings, the anonymous id and the version all live on this side, and the
+// runner should not have to know about any of them.
+//
+// The day is counted from the last successful send, not from a wall-clock hour.
+// A fleet of installations that all pinged at 03:00 would arrive in one spike;
+// counted this way they spread themselves out by when each was started.
+func (s *Server) SendBeacon(ctx context.Context) error {
+	cfg, err := s.beaconSettings(ctx)
+	if err != nil {
+		return err
+	}
+	if !cfg.Enabled {
+		return nil
+	}
+	// The collector does not report to itself over the network. It would work, and
+	// it would also mean the one instance whose count matters most depends on its
+	// own tunnel being up to be counted.
+	if cfg.IsCollector {
+		if err := s.db.RecordBeacon(ctx, cfg.InstanceID, Version); err != nil {
+			return err
+		}
+		cfg.LastPingAt = time.Now().Unix()
+		return s.saveBeaconSettings(ctx, cfg)
+	}
+
+	if time.Since(time.Unix(cfg.LastPingAt, 0)) < 24*time.Hour {
+		return nil
+	}
+
+	// safedial, ikke http.DefaultClient. Adressen kan skrives i fladen, og en
+	// administrator, der peger den på 127.0.0.1 eller på 169.254.169.254, ville
+	// ellers have en maskine, der henter indefra på kommando — og svaret på om den
+	// svarede, er i sig selv det, sådan et forsøg leder efter.
+	if err := beacon.Send(ctx, safedial.Client(15*time.Second), cfg.Collector, beacon.Payload{
+		InstanceID: cfg.InstanceID,
+		Version:    Version,
+	}); err != nil {
+		// Logged at info, not error. The collector being unreachable is not this
+		// installation's problem and must never look like one — it is somebody
+		// else's server, and the only consequence is a number being one lower.
+		s.log.Info("beacon not delivered", "err", err)
+		return nil
+	}
+
+	cfg.LastPingAt = time.Now().Unix()
+	return s.saveBeaconSettings(ctx, cfg)
+}

internal/httpapi/gmail_handlers.go

@@ -251,6 +251,17 @@ func (s *Server) handleGmailSyncNow(w http.ResponseWriter, r *http.Request) {
 //
 // Returns how many were made. Also used by the background job, which is why it
 // takes a context and a user rather than a request.
+// maxGmailSeen bounds the list of message ids already turned into tasks.
+//
+// Gmail is read-only here — the scope is `gmail.readonly`, deliberately, because
+// asking for less is what makes the consent screen something a person can agree
+// to — so verdande cannot unstar a message the way the IMAP side unflags one. The
+// record has to live on this side, and a record on this side has to be bounded.
+//
+// A count rather than the previous rule of "whatever Gmail returned this run",
+// which was not a bound but a way of forgetting.
+const maxGmailSeen = 2000
+
 func (s *Server) SyncGmail(ctx context.Context, user *store.User) (int, error) {
 	box, err := s.db.MailboxOfKind(ctx, user.ID, "gmail")
 	if err != nil {
@@ -362,14 +373,35 @@ func (s *Server) SyncGmail(ctx context.Context, user *store.User) (int, error) {
 	}
 
 	if created > 0 {
-		// Only the ids Gmail still returns are kept, so the list cannot grow
-		// without bound as messages fall out of the 30-day window.
-		keep := make([]string, 0, len(ids))
+		// The ids this run saw, on top of the ones already known.
+		//
+		// It used to keep only the ids Gmail returned *this run*, to stop the list
+		// growing without bound. That threw away every id that was not on the page
+		// it happened to fetch — and the query is capped, so on an account with more
+		// starred mail than one page, each run forgot the messages the last one had
+		// remembered. They came back unseen, and became tasks again, and again. That
+		// is the duplicate.
+		//
+		// Bounded by a count instead. Newest first, and a ceiling far above any
+		// plausible page, so an id leaves the list because it is old rather than
+		// because it was not in the last answer.
+		keep := make([]string, 0, len(ids)+len(box.Seen))
+		added := map[string]bool{}
 		for _, id := range ids {
-			if seen[id] {
+			if seen[id] && !added[id] {
 				keep = append(keep, id)
+				added[id] = true
 			}
 		}
+		for _, id := range box.Seen {
+			if !added[id] {
+				keep = append(keep, id)
+				added[id] = true
+			}
+		}
+		if len(keep) > maxGmailSeen {
+			keep = keep[:maxGmailSeen]
+		}
 		box.Seen = keep
 		box.LastSyncAt = time.Now()
 		if err := s.db.SaveMailbox(ctx, box); err != nil {

internal/httpapi/mailbox_handlers.go

@@ -175,6 +175,10 @@ func (s *Server) SyncMailbox(ctx context.Context, user *store.User, m *store.Mai
 
 	created := 0
 	highest := m.LastUID
+	// The uids of the mails that actually became tasks. Only those get unflagged,
+	// and only after the task is committed: a flag removed from a mail whose task
+	// was never written is a mail nobody will ever see again.
+	done := make([]uint32, 0, len(messages))
 	for _, msg := range messages {
 		if ctx.Err() != nil {
 			// What was made so far is already committed and counted, and the marker
@@ -204,12 +208,27 @@ func (s *Server) SyncMailbox(ctx context.Context, user *store.User, m *store.Mai
 			continue
 		}
 		created++
+		done = append(done, msg.UID)
 		if msg.UID > highest {
 			highest = msg.UID
 		}
 		s.hub.Publish(projectID, "task.created", toTaskJSON(*task))
 	}
 
+	// The flag comes off what became a task. This is what actually stops a mail
+	// arriving twice; the marker below is now a second line rather than the only
+	// one. See imap.Unflag for why the record belongs in the mailbox and not here.
+	//
+	// Outside the request's context, like the marker: a cancelled request that
+	// already created tasks must still take the flags off them, or the next run
+	// makes them all again.
+	if err := client.Unflag(done); err != nil {
+		// Not fatal, and not the caller's problem. A folder somebody opened
+		// read-only, or a server that refuses the STORE, still gets the uid marker
+		// underneath — worse, but not broken.
+		s.log.Warn("could not unflag imported mail", "err", err, "mailbox", m.ID, "count", len(done))
+	}
+
 	// A run that got all the way through moves the marker to the whole of what the
 	// search considered, not merely to the last message it happened to read. That
 	// is what stops the same mail becoming a task again on the next run when the

internal/httpapi/server.go

@@ -143,6 +143,19 @@ func New(cfg *config.Config, db *store.DB, log *slog.Logger, web fs.FS) *Server
 			writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
 		})
 
+		// The beacon's receiving end, and the number it produces. Both without a
+		// session, because the caller is a stranger's installation — that is the
+		// whole feature. Rate limited like the login routes: it is the one endpoint
+		// anybody on the internet can write to, and a row per request is a row per
+		// request whether or not the request is honest.
+		//
+		// Refused with a 404 unless this instance has been told it is the collector,
+		// so an ordinary installation is not a spam sink that answers 200 to
+		// anything.
+		r.Method(http.MethodPost, "/beacon",
+			s.rateLimit(s.loginLimiter, http.HandlerFunc(s.handleBeaconPing)))
+		r.Get("/beacon/count", s.handleBeaconCount)
+
 		// Reachable without being signed in — these are how you become signed in.
 		r.Route("/auth", func(r chi.Router) {
 			r.Get("/setup", s.handleSetupState)
@@ -390,6 +403,11 @@ func New(cfg *config.Config, db *store.DB, log *slog.Logger, web fs.FS) *Server
 				// `:latest` on the way, which is what makes a restart an update.
 				r.Get("/panel", s.handlePanelStatus)
 				r.Post("/panel/restart", s.handleRestartFromPanel)
+				// Who this instance reports to, and whether it does. An
+				// administrator's decision, not a person's: there is one
+				// installation and it either phones home or it does not.
+				r.Get("/beacon/settings", s.handleBeaconStatus)
+				r.Put("/beacon/settings", s.handleSetBeacon)
 				// A backup file is a complete copy of the database, so this whole
 				// group is sessions-only as well as administrators-only: a leaked
 				// token must not be able to download everybody's data.

internal/imap/imap.go

@@ -310,3 +310,61 @@ func cut(text string) string {
 	}
 	return text
 }
+
+// Unflag takes the flag off the messages that have become tasks.
+//
+// This is what stops the same mail arriving twice, and it replaces a marker that
+// could not be made to hold. The marker was a uid: remember the highest one seen,
+// start above it next time. That is correct only if uids are what the server says
+// they are, and it broke in every way such a scheme can — a fetch reply without a
+// uid, a UIDVALIDITY that changed under a rebuilt mailbox, two mailbox rows
+// pointing at the same folder and each keeping its own marker. Every one of those
+// produced the same symptom: the same mail became a task again, and again.
+//
+// Unflagging moves the record out of our database and into the mailbox, where the
+// question is asked. "Flagged" means "turn this into a task"; once it is a task,
+// the flag has done its job and is not true any more. Nothing has to be
+// remembered, so there is nothing to get out of step — a second sync, a second
+// mailbox row, or a database restored from a backup all find the same answer,
+// because the answer is in the mailbox.
+//
+// It is destructive in the small way that matters: it changes something in
+// somebody's mail. That is the trade, and it is the right way round — a flag is a
+// note to self that this needs doing, and it has been done.
+//
+// The folder is re-selected writable. Since() opens it read-only on purpose, and
+// a search should not need write access just because a later step might.
+func (c *Client) Unflag(uids []uint32) error {
+	if len(uids) == 0 {
+		return nil
+	}
+	if _, err := c.c.Select(c.folder, nil).Wait(); err != nil {
+		return fmt.Errorf("open %s for writing: %w", c.folder, err)
+	}
+
+	set := imap.UIDSetNum()
+	for _, u := range uids {
+		if u == 0 {
+			// No uid means nothing to address. The message keeps its flag and will
+			// be seen again — which is the failure this whole function exists to
+			// prevent, so it is worth saying rather than skipping quietly.
+			continue
+		}
+		set.AddNum(imap.UID(u))
+	}
+	if len(set) == 0 {
+		return nil
+	}
+
+	// Silent: the server has nothing to tell us that we would act on, and the
+	// untagged replies would only have to be collected and thrown away.
+	cmd := c.c.Store(set, &imap.StoreFlags{
+		Op:     imap.StoreFlagsDel,
+		Silent: true,
+		Flags:  []imap.Flag{imap.FlagFlagged},
+	}, nil)
+	if err := cmd.Close(); err != nil {
+		return fmt.Errorf("unflag in %s: %w", c.folder, err)
+	}
+	return nil
+}

internal/imap/roundtrip_test.go

@@ -316,3 +316,81 @@ func TestNothingAtOrBelowTheMarkerComesBack(t *testing.T) {
 		}
 	}
 }
+
+// Den samme mail må ikke kunne blive en opgave to gange.
+//
+// Prøven kører hele vejen imod en rigtig server: den finder de flagede mails,
+// tager flaget af dem, og spørger så forfra som en ny synkronisering ville — med
+// markøren sat til nul, altså det værst tænkelige tilfælde, hvor alt, hvad vi
+// selv havde husket, er væk. Der skal ikke komme noget tilbage.
+//
+// Det er det, markøren ikke kunne love. Den holdt kun, hvis uids var, hvad
+// serveren sagde, og fejlede på et fetch-svar uden uid, på en UIDVALIDITY, der
+// skiftede, og på to postkasse-rækker, der pegede samme sted og hver især førte
+// deres eget regnskab. Alle tre gav det samme: mailen blev en opgave igen.
+func TestAnUnflaggedMailDoesNotComeBack(t *testing.T) {
+	addr, pool := serveMailbox(t, []mail{
+		{subject: "Faktura 4711", from: "[email protected]", body: "Vedhæftet.", flagged: true},
+		{subject: "Nyhedsbrev", from: "[email protected]", body: "Læs mere", flagged: false},
+		{subject: "Kontrakt", from: "[email protected]", body: "Til underskrift", flagged: true},
+	})
+
+	client, err := Dial(Account{Host: addr, Username: "kw", Password: "hemmelig", RootCAs: pool})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	msgs, _, err := client.Since(0, 25)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(msgs) != 2 {
+		t.Fatalf("fandt %d flagede mails, ventede 2", len(msgs))
+	}
+
+	uids := make([]uint32, 0, len(msgs))
+	for _, m := range msgs {
+		uids = append(uids, m.UID)
+	}
+	if err := client.Unflag(uids); err != nil {
+		t.Fatalf("Unflag: %v", err)
+	}
+
+	// Forfra, og fra nul: selv en instans, der intet husker, må ikke finde dem igen.
+	again, _, err := client.Since(0, 25)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(again) != 0 {
+		t.Fatalf("%d mails kom igen efter at flaget var taget af — den første af dem er %q",
+			len(again), again[0].Subject)
+	}
+}
+
+// Et tomt kald må ikke åbne mappen skrivbar for ingenting.
+func TestUnflagWithNothingToDoIsQuiet(t *testing.T) {
+	addr, pool := serveMailbox(t, []mail{
+		{subject: "Faktura", from: "[email protected]", body: "x", flagged: true},
+	})
+	client, err := Dial(Account{Host: addr, Username: "kw", Password: "hemmelig", RootCAs: pool})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	if err := client.Unflag(nil); err != nil {
+		t.Errorf("Unflag(nil) = %v", err)
+	}
+	// Kun nuller: der er intet at adressere, og mailen skal beholde sit flag.
+	if err := client.Unflag([]uint32{0, 0}); err != nil {
+		t.Errorf("Unflag(nuller) = %v", err)
+	}
+	msgs, _, err := client.Since(0, 25)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(msgs) != 1 {
+		t.Errorf("mailen mistede sit flag, uden at nogen havde adresseret den")
+	}
+}

internal/jobs/jobs.go

@@ -45,6 +45,11 @@ type Runner struct {
 	// layer, which owns the VAPID keys and the subscription list.
 	Push func(userID, title, body, projectID string)
 
+	// SendBeacon reports this installation to the collector, if it is switched on
+	// and a day has passed. Supplied by the HTTP layer for the same reason as the
+	// syncs: that is where the instance settings and the version live.
+	SendBeacon func(ctx context.Context) error
+
 	wg sync.WaitGroup
 }
 
@@ -69,6 +74,10 @@ func (r *Runner) Start(ctx context.Context) {
 	// account for a to-do app. Ten minutes is fast enough for something somebody
 	// starred and slow enough not to look like abuse.
 	r.every(ctx, "mailboxes", 10*time.Minute, r.syncMailboxes)
+	// Checked hourly, sent at most daily — the same shape as the backup, and for
+	// the same reason: a container asleep at the scheduled minute would otherwise
+	// skip the day. SendBeacon decides whether a day has passed.
+	r.every(ctx, "beacon", time.Hour, r.sendBeacon)
 }
 
 func (r *Runner) Wait() { r.wg.Wait() }
@@ -103,6 +112,17 @@ func (r *Runner) every(ctx context.Context, name string, interval time.Duration,
 	}()
 }
 
+// sendBeacon reports that this installation exists, at most once a day.
+//
+// Every decision about what is sent, and why this is on by default rather than
+// compulsory, lives in internal/beacon. This is only the clock.
+func (r *Runner) sendBeacon(ctx context.Context) error {
+	if r.SendBeacon == nil {
+		return nil
+	}
+	return r.SendBeacon(ctx)
+}
+
 // --- reminders ------------------------------------------------------------------
 
 // sendDueReminders delivers everything that has come due since the last pass.

internal/safedial/safedial.go

@@ -0,0 +1,106 @@
+// Package safedial refuses to open a connection to this machine or to the
+// network it sits on.
+//
+// The problem it solves is the same one three times: somewhere a user supplies a
+// URL or a host, and the server fetches it. An AI provider's base address, a push
+// endpoint, an IMAP host, a beacon collector. Each of them is a feature, and each
+// of them is also a way to ask the server to reach something the caller cannot —
+// the panel next door, a metadata endpoint, a database on the same bridge — and
+// to report back whether it answered. On a homelab that is the whole point of the
+// box being where it is.
+//
+// # Why the check is on the address and not on the URL
+//
+// Parsing the URL and refusing "localhost" or "127.0.0.1" is the obvious version
+// and it does not work: a name resolves to whatever its owner says, so
+// `evil.example.com` can answer 127.0.0.1 — and can answer differently the second
+// time, after a parse-time check has passed. That is DNS rebinding, and the only
+// place it can be caught is after resolution and before the connection, which is
+// exactly where net.Dialer's Control hook runs.
+//
+// # What is refused
+//
+// Loopback, link-local (including 169.254.169.254, which is where cloud metadata
+// lives), the private ranges, unique-local IPv6, and the unspecified address.
+// Everything else is allowed: this is not an allowlist of the internet, it is a
+// wall around the inside.
+package safedial
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"syscall"
+	"time"
+)
+
+// ErrPrivate is returned when a connection was refused for where it pointed. It
+// deliberately does not say which address it resolved to: the answer is itself
+// the thing an attacker is fishing for.
+type ErrPrivate struct{ Host string }
+
+func (e *ErrPrivate) Error() string {
+	return fmt.Sprintf("refusing to connect to %s: it resolves to an address on this machine or its private network", e.Host)
+}
+
+// Blocked reports whether an address must not be dialled.
+func Blocked(ip net.IP) bool {
+	return ip == nil ||
+		ip.IsLoopback() ||
+		ip.IsUnspecified() ||
+		ip.IsLinkLocalUnicast() ||
+		ip.IsLinkLocalMulticast() ||
+		ip.IsInterfaceLocalMulticast() ||
+		ip.IsMulticast() ||
+		ip.IsPrivate()
+}
+
+// Control is a net.Dialer Control function that refuses a private destination.
+//
+// It runs after the name has been resolved and before the socket is connected,
+// which is the only moment where the address that will actually be used is known.
+func Control(network, address string, _ syscall.RawConn) error {
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		return err
+	}
+	ip := net.ParseIP(host)
+	if Blocked(ip) {
+		return &ErrPrivate{Host: host}
+	}
+	return nil
+}
+
+// Dialer is a net.Dialer that will not reach inside.
+func Dialer(timeout time.Duration) *net.Dialer {
+	return &net.Dialer{Timeout: timeout, Control: Control}
+}
+
+// Client is an http.Client for fetching something a user named.
+//
+// Redirects are followed but capped, and every hop goes through the same dialer —
+// a redirect to 127.0.0.1 is the oldest way around a check that only looked at the
+// URL somebody typed.
+func Client(timeout time.Duration) *http.Client {
+	return &http.Client{
+		Timeout: timeout,
+		Transport: &http.Transport{
+			DialContext:           Dialer(timeout).DialContext,
+			TLSHandshakeTimeout:   timeout,
+			ResponseHeaderTimeout: timeout,
+		},
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			if len(via) >= 5 {
+				return fmt.Errorf("too many redirects")
+			}
+			return nil
+		},
+	}
+}
+
+// AllowPrivate is the same client without the wall, for a destination the
+// operator configured rather than a user — the panel, and nothing else. Named so
+// that using it is a decision somebody wrote down.
+func AllowPrivate(timeout time.Duration) *http.Client {
+	return &http.Client{Timeout: timeout}
+}

internal/safedial/safedial_test.go

@@ -0,0 +1,55 @@
+package safedial
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestBlockedCoversTheInside(t *testing.T) {
+	blocked := []string{
+		"127.0.0.1", "::1", "0.0.0.0",
+		// Skyens metadata-endepunkt. Den ene adresse, enhver SSRF sigter efter.
+		"169.254.169.254",
+		"10.0.0.5", "172.16.0.1", "192.168.1.10", "fd00::1", "fe80::1",
+	}
+	for _, s := range blocked {
+		if !Blocked(net.ParseIP(s)) {
+			t.Errorf("Blocked(%s) = false", s)
+		}
+	}
+	for _, s := range []string{"93.184.216.34", "8.8.8.8", "2606:2800:220:1:248:1893:25c8:1946"} {
+		if Blocked(net.ParseIP(s)) {
+			t.Errorf("Blocked(%s) = true, men det er en almindelig adresse", s)
+		}
+	}
+}
+
+// En httptest-server lytter på loopback, hvilket gør den til den perfekte prøve:
+// den findes, den svarer, og den må ikke kunne nås.
+func TestClientRefusesLoopback(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Error("forbindelsen blev oprettet; den skulle være afvist")
+	}))
+	defer srv.Close()
+
+	_, err := Client(3 * time.Second).Get(srv.URL)
+	if err == nil {
+		t.Fatal("en forespørgsel til loopback skal afvises")
+	}
+}
+
+func TestAllowPrivateStillReaches(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer srv.Close()
+
+	resp, err := AllowPrivate(3 * time.Second).Get(srv.URL)
+	if err != nil {
+		t.Fatalf("den skal kunne nå det, operatøren selv har peget på: %v", err)
+	}
+	resp.Body.Close()
+}

internal/store/beacon.go

@@ -0,0 +1,138 @@
+package store
+
+import (
+	"context"
+	"time"
+)
+
+// The collector's side of the beacon: what has reported in, and how many.
+//
+// Every method here is about counting installations, never about identifying one.
+// There is no lookup by anything but the id an installation made up for itself,
+// and there is nothing to join it against.
+
+// BeaconStats is the whole of what the collector can say.
+type BeaconStats struct {
+	Total      int            `json:"total"`
+	Active7d   int            `json:"active_7d"`
+	Active30d  int            `json:"active_30d"`
+	ByVersion  map[string]int `json:"by_version"`
+	LastPingAt *time.Time     `json:"last_ping_at,omitempty"`
+}
+
+// maxBeaconInstalls bounds the table.
+//
+// The receiving endpoint is unauthenticated — it has to be, since the caller is a
+// stranger's installation — and it inserts a row keyed on an id the caller made
+// up. Anybody can therefore mint ids and grow this table until the volume the
+// database lives on is full. A ceiling turns that from a disk-fill into a number
+// that is merely wrong, which is the difference between an outage and an
+// annoyance. A hundred thousand is far above any plausible real count and far
+// below anything that matters on disk.
+const maxBeaconInstalls = 100_000
+
+// RecordBeacon writes down that an installation is alive, and on which version.
+//
+// `first_seen` is kept from the original row on purpose: it is the only thing
+// that distinguishes an installation that has been running for a year from one
+// that appeared this morning, and it costs nothing to keep.
+func (db *DB) RecordBeacon(ctx context.Context, instanceID, version string) error {
+	now := time.Now().Unix()
+
+	// An installation already known keeps reporting whatever the ceiling says: the
+	// cap must never stop counting the real ones, only stop new rows from being
+	// invented without limit.
+	var known int
+	if err := db.QueryRowContext(ctx,
+		`SELECT count(*) FROM beacon_installs WHERE instance_id = ?`, instanceID).Scan(&known); err != nil {
+		return err
+	}
+	if known == 0 {
+		var total int
+		if err := db.QueryRowContext(ctx,
+			`SELECT count(*) FROM beacon_installs`).Scan(&total); err != nil {
+			return err
+		}
+		if total >= maxBeaconInstalls {
+			// Not an error to the caller. Their installation did nothing wrong, and
+			// there is nothing for them to do about it.
+			return nil
+		}
+	}
+
+	_, err := db.ExecContext(ctx, `
+		INSERT INTO beacon_installs (instance_id, version, first_seen, last_seen)
+		VALUES (?, ?, ?, ?)
+		ON CONFLICT (instance_id) DO UPDATE SET
+		    version   = excluded.version,
+		    last_seen = excluded.last_seen`,
+		instanceID, version, now, now)
+	return err
+}
+
+// BeaconCount is how many installations have reported within the window.
+//
+// A window rather than a running total, and that is not a detail. A total only
+// ever climbs: every instance anybody ever started for ten minutes stays in it
+// forever, so the number stops meaning "how many people use this" and starts
+// meaning "how many people have ever tried it". The question being asked is the
+// first one.
+func (db *DB) BeaconCount(ctx context.Context, within time.Duration) (int, error) {
+	var n int
+	err := db.QueryRowContext(ctx,
+		`SELECT count(*) FROM beacon_installs WHERE last_seen >= ?`,
+		time.Now().Add(-within).Unix()).Scan(&n)
+	return n, err
+}
+
+// BeaconSummary is everything the settings page shows.
+func (db *DB) BeaconSummary(ctx context.Context) (BeaconStats, error) {
+	out := BeaconStats{ByVersion: map[string]int{}}
+
+	if err := db.QueryRowContext(ctx,
+		`SELECT count(*) FROM beacon_installs`).Scan(&out.Total); err != nil {
+		return out, err
+	}
+
+	var err error
+	if out.Active7d, err = db.BeaconCount(ctx, 7*24*time.Hour); err != nil {
+		return out, err
+	}
+	if out.Active30d, err = db.BeaconCount(ctx, 30*24*time.Hour); err != nil {
+		return out, err
+	}
+
+	// Versions of the ones still alive, not of everything ever seen: an instance
+	// that stopped reporting two years ago should not still be counted as running
+	// the version it was on when it stopped.
+	rows, err := db.QueryContext(ctx, `
+		SELECT version, count(*) FROM beacon_installs
+		 WHERE last_seen >= ? AND version <> ''
+		 GROUP BY version`, time.Now().Add(-30*24*time.Hour).Unix())
+	if err != nil {
+		return out, err
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var v string
+		var n int
+		if err := rows.Scan(&v, &n); err != nil {
+			return out, err
+		}
+		out.ByVersion[v] = n
+	}
+	if err := rows.Err(); err != nil {
+		return out, err
+	}
+
+	var last *int64
+	if err := db.QueryRowContext(ctx,
+		`SELECT max(last_seen) FROM beacon_installs`).Scan(&last); err != nil {
+		return out, err
+	}
+	if last != nil {
+		t := time.Unix(*last, 0)
+		out.LastPingAt = &t
+	}
+	return out, nil
+}

internal/store/migrations/0022_beacon.sql

@@ -0,0 +1,26 @@
+-- One row per installation that has ever reported in.
+--
+-- This table only exists on the instance that is acting as the collector. Every
+-- other instance has it too, empty, because the schema is the same everywhere —
+-- and because "am I the collector" is a setting, not a build.
+--
+-- What is deliberately NOT here is the point of the whole feature: there is no
+-- column for an IP address, a hostname, a domain, an account, a count of tasks,
+-- or anything else. The two things stored are the two things sent, and the table
+-- is the promise written down where it cannot drift from the prose — a column
+-- added here later is a promise broken, and it should be as awkward to add as
+-- possible.
+--
+-- `instance_id` is the primary key rather than a rowid alias, because the same
+-- installation reporting daily must update its row rather than grow a new one.
+-- What is wanted is "how many installations", not "how many pings".
+CREATE TABLE beacon_installs (
+    instance_id TEXT PRIMARY KEY,
+    version     TEXT NOT NULL DEFAULT '',
+    first_seen  INTEGER NOT NULL,
+    last_seen   INTEGER NOT NULL
+);
+
+-- The count is always "seen in the last N days", so last_seen is what every query
+-- sorts and filters on.
+CREATE INDEX beacon_by_last_seen ON beacon_installs (last_seen DESC);

web/e2e/smoke.spec.js

@@ -2396,3 +2396,41 @@ test('lister overlever en gemning — også i et citat, og også med numre', asy
 
 	expect(trouble).toEqual([]);
 });
+
+test('en note kan ikke smugle et script ind gennem et billedes alt-tekst', async ({ page }) => {
+	const trouble = watchForTrouble(page);
+	await page.goto('/noter');
+
+	// Undslap `alt` ikke sine anførselstegn, lukkede den første af dem attributten,
+	// og resten stod som markup: et img med en onerror på. En note deles gennem et
+	// projekt, så det ville køre på vores eget domæne, hos den der åbnede noten.
+	const id = await page.evaluate(async () => {
+		const r = await fetch('/api/v1/notes', {
+			method: 'POST',
+			credentials: 'include',
+			headers: { 'Content-Type': 'application/json', 'Sec-Fetch-Site': 'same-origin' },
+			body: JSON.stringify({
+				body: '# Ondsindet\n\n![" onerror="window.__pwned = true](/api/v1/attachments/aaaabbbbccccdddd)\n'
+			})
+		});
+		return (await r.json()).id;
+	});
+
+	await page.goto('/noter?note=' + id);
+	const ed = page.getByRole('textbox', { name: 'Notens tekst' });
+	await expect(ed.locator('img')).toHaveCount(1);
+	// Billedet peger ingen steder, så det fejler at indlæse — hvilket er præcis
+	// den vej, en onerror ville blive kaldt ad.
+	await page.waitForTimeout(800);
+
+	expect(await page.evaluate(() => window.__pwned)).toBeUndefined();
+	// Og hændelsen står som tekst i alt-attributten, hvor den hører hjemme.
+	const alt = await ed.locator('img').getAttribute('alt');
+	expect(alt).toContain('onerror');
+	expect(await ed.locator('img').evaluate((el) => el.hasAttribute('onerror'))).toBe(false);
+
+	// Billedets egen 404 hører til prøven: adressen peger med vilje på et bilag,
+	// der ikke findes, fordi det er den vej en onerror ville blive kaldt ad. Alt
+	// andet skal stadig være stille.
+	expect(trouble.filter((t) => !t.includes('/api/v1/attachments/'))).toEqual([]);
+});

web/src/lib/api.js

@@ -227,6 +227,9 @@ export const api = {
 	gmailClient: () => get('/gmail/client'),
 	setGmailClient: (data) => put('/gmail/client', data),
 
+	beacon: () => get('/beacon/settings'),
+	setBeacon: (body) => put('/beacon/settings', body),
+
 	listBackups: () => get('/backups'),
 	runBackup: () => post('/backups'),
 	// A link rather than a fetch: the browser's own download handles a file of

web/src/lib/locales/da.js

@@ -29,6 +29,21 @@ export const da = {
 	'notes.sortUpdated': 'Senest rørt',
 	'notes.sortCreated': 'Oprettet',
 	'notes.sortTitle': 'Navn',
+	'beacon.title': 'Tælling af installationer',
+	'beacon.hint': 'Én gang i døgnet melder denne installation, at den findes, så projektet kan se hvor mange der bruger det. Der sendes **kun** de to værdier herunder — et tilfældigt id og versionen — og intet andet: ingen IP bliver gemt, ingen navne, ingen adresser, intet om hvad du bruger det til. Slået til som udgangspunkt, og du kan slå den fra her.',
+	'beacon.send': 'Send en anonym daglig melding',
+	'beacon.exactly': 'Præcis det her — ikke mere:',
+	'beacon.lastPing': 'Sidst sendt: {when}',
+	'beacon.collector': 'Modtagerens adresse',
+	'beacon.collectorHint': 'Hvor meldingen sendes hen. Lad den stå, medmindre du kører din egen tæller.',
+	'beacon.beCollector': 'Denne instans er tælleren',
+	'beacon.receiving': 'Modtager anonyme meldinger — id og version, aldrig en IP.',
+	'beacon.total': 'I alt',
+	'beacon.active7': 'Aktive 7 dage',
+	'beacon.active30': 'Aktive 30 dage',
+	'beacon.byVersion': 'Efter version (30 dage):',
+	'beacon.publish': 'Offentliggør tallet på /api/v1/beacon/count',
+	'beacon.publishHint': 'Uden login, mellemlagret et minut. Viser installationer set de seneste 30 dage — ikke et løbende total, som kun ville stige.',
 	'notes.strike': 'Gennemstreget',
 	'notes.showSource': 'Vis som kode',
 	'notes.source': 'Notens kildetekst',

web/src/lib/locales/en.js

@@ -25,6 +25,21 @@ export const en = {
 	'notes.sortUpdated': 'Last touched',
 	'notes.sortCreated': 'Created',
 	'notes.sortTitle': 'Name',
+	'beacon.title': 'Counting installations',
+	'beacon.hint': 'Once a day this installation reports that it exists, so the project can see how many people run it. It sends **only** the two values below — a random id and the version — and nothing else: no IP is stored, no names, no addresses, nothing about what you use it for. On by default, and you can turn it off here.',
+	'beacon.send': 'Send an anonymous daily report',
+	'beacon.exactly': 'Exactly this — nothing more:',
+	'beacon.lastPing': 'Last sent: {when}',
+	'beacon.collector': 'Collector address',
+	'beacon.collectorHint': 'Where the report goes. Leave it unless you run your own collector.',
+	'beacon.beCollector': 'This instance is the collector',
+	'beacon.receiving': 'Receiving anonymous reports — id and version, never an IP.',
+	'beacon.total': 'Total',
+	'beacon.active7': 'Active 7 days',
+	'beacon.active30': 'Active 30 days',
+	'beacon.byVersion': 'By version (30 days):',
+	'beacon.publish': 'Publish the count at /api/v1/beacon/count',
+	'beacon.publishHint': 'No login, cached for a minute. Reports installations seen in the last 30 days — not a running total, which would only ever climb.',
 	'notes.strike': 'Strikethrough',
 	'notes.showSource': 'Show as code',
 	'notes.source': 'Note source',

web/src/lib/richtext.js

@@ -37,28 +37,61 @@ const BLOCKS = [
 const escapeHtml = (s) =>
 	s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
 
+/**
+ * En værdi, der skal stå inde i en attribut.
+ *
+ * escapeHtml er skrevet til tekst mellem to tags, hvor et anførselstegn ikke
+ * betyder noget, og lader dem derfor stå. Inde i en attribut lukker de den — og
+ * så står resten af strengen som markup. Det er den ene forskel, og den er hele
+ * forskellen mellem tekst og et onerror.
+ */
+const attr = (s) => escapeHtml(s).replace(/"/g, '&quot;').replace(/'/g, '&#39;');
+
+/**
+ * Billeder løftes ud af teksten, før noget andet rører den.
+ *
+ * De stod før først i kæden, og så kørte de fire resterende regler hen over en
+ * streng, der nu indeholdt rigtig HTML — ind i den `alt`, der lige var lavet.
+ * Det gik godt, så længe ingen af erstatningerne indeholdt et anførselstegn, og
+ * det er en betingelse, der stod skrevet ingen steder: den dag nogen tilføjer en
+ * regel for links, er den brudt, og det er præcis den vej, XSS-en kom ind ad.
+ *
+ * Værre endnu var undslupningen. `escapeHtml` kørte på hele teksten, og `attr`
+ * kørte den igen på `alt` — så et og-tegn blev til en dobbelt undslupning, der
+ * voksede ved hver eneste åbning og gemning. En note med "Kaffe & te" i et
+ * billede blev længere, hver gang nogen så på den.
+ *
+ * En pladsholder løser begge dele. Billedet tages ud af den rå tekst, resten
+ * undslippes og formateres som altid, og billedet lægges tilbage til sidst med en
+ * `alt`, der er undsluppet nøjagtig én gang. Rækkefølgen af de øvrige regler kan
+ * så ikke længere betyde noget.
+ *
+ * Nul-tegnet som mærke, fordi det er det ene tegn, en note ikke kan indeholde:
+ * det overlever hverken en contenteditable eller en Markdown-fil.
+ */
+const IMAGE = /!\[([^\]]*)\]\((\/api\/v1\/attachments\/[0-9a-f-]+)\)/g;
+const MARK = "\u0000";
+
 /** Inline marks, innermost first so a bold word inside italics survives both. */
 function inlineToHtml(text) {
-	return (
-		escapeHtml(text)
-			// Billedet først, og før alt andet.
-			//
-			// Der var ingen regel for det overhovedet, så en note fra Apple Noter
-			// viste `![](/api/v1/attachments/…)` som ord. Billedet var hentet, gemt og
-			// hæftet på noten — det eneste, der manglede, var at nogen tegnede det.
-			//
-			// Kun vores egen adresse. Et `![](http://…)` fra en indsat tekst ville
-			// ellers hente fra en fremmed vært, når noten åbnes, og fortælle den, at
-			// den er blevet læst — og en note er det sidste sted, man vil have en
-			// sporingspixel. Alt andet står som den tekst, det er.
-			.replace(/!\[([^\]]*)\]\((\/api\/v1\/attachments\/[0-9a-f-]+)\)/g,
-				(_, alt, src) => `<img src="${src}" alt="${alt}">`)
-			.replace(/&lt;u&gt;(.+?)&lt;\/u&gt;/g, '<u>$1</u>')
-			.replace(/`([^`]+)`/g, '<code>$1</code>')
-			.replace(/\*\*(.+?)\*\*/g, '<strong>$1</strong>')
-			.replace(/~~(.+?)~~/g, '<s>$1</s>')
-			.replace(/(^|[^*])\*([^*]+)\*/g, '$1<em>$2</em>')
-	);
+	// Kun vores egen adresse bliver til et billede. Et `![](http://…)` fra en
+	// indsat tekst ville ellers hente fra en fremmed vært, når noten åbnes, og
+	// fortælle den, at den er blevet læst — og en note er det sidste sted, man vil
+	// have en sporingspixel. Alt andet står som den tekst, det er.
+	const images = [];
+	const lifted = (text ?? '').replace(IMAGE, (_, alt, src) => {
+		images.push(`<img src="${src}" alt="${attr(alt)}">`);
+		return `${MARK}${images.length - 1}${MARK}`;
+	});
+
+	const marked = escapeHtml(lifted)
+		.replace(/&lt;u&gt;(.+?)&lt;\/u&gt;/g, '<u>$1</u>')
+		.replace(/`([^`]+)`/g, '<code>$1</code>')
+		.replace(/\*\*(.+?)\*\*/g, '<strong>$1</strong>')
+		.replace(/~~(.+?)~~/g, '<s>$1</s>')
+		.replace(/(^|[^*])\*([^*]+)\*/g, '$1<em>$2</em>');
+
+	return marked.replace(new RegExp(MARK + '(\\d+)' + MARK, 'g'), (_, i) => images[Number(i)] ?? '');
 }
 
 /**
@@ -121,7 +154,12 @@ export function markdownToHtml(markdown) {
 		if (fenced) {
 			closeList();
 			fence = fenced[1] ?? '';
-			out.push(`<pre data-lang="${escapeHtml(fence)}"><code>`);
+			// attr, ikke escapeHtml: værdien står inde i en attribut. Den er sikker i
+			// dag alene fordi hegn-regexen er \w*, som ikke rummer anførselstegn — og
+			// den dag nogen udvider den for at tage imod "c++" eller "f#", er det en
+			// XSS. Undslupningen skal passe til stedet, ikke til det, der
+			// tilfældigvis kan stå der nu.
+			out.push(`<pre data-lang="${attr(fence)}"><code>`);
 			continue;
 		}
 

web/src/routes/indstillinger/data/+page.svelte

@@ -5,7 +5,29 @@
 	import { app } from '$lib/stores.svelte.js';
 	import { goto } from '$app/navigation';
 	import { ago } from '$lib/when.js';
-	import { t } from '$lib/i18n.svelte.js';
+	import { t, tag } from '$lib/i18n.svelte.js';
+
+	// --- beaconet -----------------------------------------------------------------
+
+	// Hvad denne installation melder, og til hvem. Se internal/beacon for hvorfor
+	// den er slået til som udgangspunkt frem for at være obligatorisk.
+	let beacon = $state(null);
+
+	$effect(() => {
+		if (!app.user?.is_admin) return;
+		api.beacon().then((r) => (beacon = r)).catch(() => {});
+	});
+
+	async function saveBeacon(change) {
+		try {
+			beacon = await api.setBeacon(change);
+		} catch (e) {
+			app.toast(humanMessage(e));
+			// Læst forfra, så en afvist ændring ikke bliver stående som et flueben,
+			// der ikke svarer til, hvad serveren gør.
+			api.beacon().then((r) => (beacon = r)).catch(() => {});
+		}
+	}
 
 	// --- backups ---------------------------------------------------------------------
 
@@ -486,6 +508,95 @@
 	{/if}
 </section>
 
+<!-- Beaconet.
+     Under Data, fordi det er her alt om "hvad forlader denne instans" står — en
+     sikkerhedskopi, en eksport, en import. Telemetri hører til i samme rude som
+     dem, ikke gemt væk under en fane, ingen åbner. -->
+<section class="panel">
+	<header>
+		<h2>{t('beacon.title')}</h2>
+		<p class="hint">{t('beacon.hint')}</p>
+	</header>
+
+	{#if beacon}
+		<label class="check">
+			<input
+				type="checkbox"
+				checked={beacon.enabled}
+				onchange={(e) => saveBeacon({ enabled: e.currentTarget.checked })}
+			/>
+			<span>{t('beacon.send')}</span>
+		</label>
+
+		<!-- Hele beskeden, ord for ord. Et løfte om telemetri er præcis så meget
+		     værd som læserens mulighed for at efterprøve det, så den står her frem
+		     for som en sætning om, hvad der cirka bliver sendt. -->
+		<pre class="payload">{t('beacon.exactly')}
+{JSON.stringify({ instance_id: beacon.instance_id, version: beacon.version }, null, 2)}</pre>
+
+		{#if beacon.last_ping_at}
+			<p class="hint">{t('beacon.lastPing', { when: new Date(beacon.last_ping_at).toLocaleString(tag()) })}</p>
+		{/if}
+
+		<div class="field">
+			<label for="collector">{t('beacon.collector')}</label>
+			<input
+				id="collector"
+				value={beacon.collector_url}
+				onchange={(e) => saveBeacon({ collector_url: e.currentTarget.value })}
+			/>
+			<p class="hint">{t('beacon.collectorHint')}</p>
+		</div>
+
+		<label class="check">
+			<input
+				type="checkbox"
+				checked={beacon.is_collector}
+				onchange={(e) => saveBeacon({ is_collector: e.currentTarget.checked })}
+			/>
+			<span>{t('beacon.beCollector')}</span>
+		</label>
+
+		{#if beacon.is_collector && beacon.stats}
+			<p class="hint">{t('beacon.receiving')}</p>
+			<div class="counts">
+				<div class="count">
+					<strong>{beacon.stats.total}</strong>
+					<span>{t('beacon.total')}</span>
+				</div>
+				<div class="count">
+					<strong>{beacon.stats.active_7d}</strong>
+					<span>{t('beacon.active7')}</span>
+				</div>
+				<div class="count">
+					<strong>{beacon.stats.active_30d}</strong>
+					<span>{t('beacon.active30')}</span>
+				</div>
+			</div>
+
+			{#if Object.keys(beacon.stats.by_version ?? {}).length}
+				<p class="hint">
+					{t('beacon.byVersion')}
+					{Object.entries(beacon.stats.by_version)
+						.sort((a, b) => b[1] - a[1])
+						.map(([v, n]) => `${v} ×${n}`)
+						.join(', ')}
+				</p>
+			{/if}
+
+			<label class="check">
+				<input
+					type="checkbox"
+					checked={beacon.publish_count}
+					onchange={(e) => saveBeacon({ publish_count: e.currentTarget.checked })}
+				/>
+				<span>{t('beacon.publish')}</span>
+			</label>
+			<p class="hint">{t('beacon.publishHint')}</p>
+		{/if}
+	{/if}
+</section>
+
 <style>
 	/* The backup list, which is a list of rows with a right-hand action — the same
 	   shape the settings pages use elsewhere, said once here because this is the
@@ -596,4 +707,48 @@
 	.link:hover {
 		text-decoration: underline;
 	}
+
+	/* Beskeden som den sendes. Monospace, fordi den skal kunne sammenlignes tegn
+	   for tegn med det, der står i dokumentationen. */
+	.payload {
+		font-family: var(--font-mono);
+		font-size: var(--text-xs);
+		background: var(--surface-sunken);
+		border: 1px solid var(--line);
+		border-radius: var(--radius-sm);
+		padding: var(--s2) var(--s3);
+		overflow-x: auto;
+		margin: var(--s2) 0;
+		white-space: pre-wrap;
+		overflow-wrap: anywhere;
+	}
+
+	.counts {
+		display: flex;
+		gap: var(--s2);
+		margin: var(--s2) 0;
+		flex-wrap: wrap;
+	}
+
+	.count {
+		flex: 1 1 7rem;
+		display: flex;
+		flex-direction: column;
+		gap: 2px;
+		padding: var(--s3);
+		border: 1px solid var(--line);
+		border-radius: var(--radius);
+	}
+
+	.count strong {
+		font-size: var(--text-xl);
+		line-height: 1;
+	}
+
+	.count span {
+		font-size: var(--text-xs);
+		color: var(--ink-faint);
+		text-transform: uppercase;
+		letter-spacing: 0.06em;
+	}
 </style>