Wait on a busy SQLite database and turn on WAL mode (closes #198)

Requests and the eviction pass write on separate connections, and with
no busy timeout a write that met another one failed at once with
"database is locked" and was lost. internal/database now adds
_pragma=busy_timeout(5000) to every db_url, the default or one the
operator sets, so such a write waits up to five seconds. The default
db_url's _journal_mode=WAL is not a parameter the driver reads, so it
is now _pragma=journal_mode(WAL). README.md and config.example.yml say
what pixa adds to db_url.

Model: opus-5-5

README.md

@@ -490,6 +490,12 @@ Key settings in more detail:
   waits for an upstream connection and for a processing slot (up to 10 seconds
   each), so keep it longer than `upstream_fetch_timeout` plus 20 seconds
 - `signing_key` — HMAC secret for URL signatures
+- `db_url` — the SQLite database to open; omitted, it is
+  `file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)`, which keeps the
+  database in WAL mode. pixa adds `_pragma=busy_timeout(5000)` to any `db_url`,
+  so a write that finds another in progress waits up to five seconds for it
+  instead of failing. WAL mode comes only from the URL: keep
+  `_pragma=journal_mode(WAL)` in one you set
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
   disk cache entirely; omitted defaults to 75% of the sum of the free space on
   the filesystem containing `<state_dir>/cache/` and the bytes of source and

TODO.md

@@ -31,6 +31,12 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 SQLite writes no longer fail with "database is locked" (closes
+  #198): pixa adds `_pragma=busy_timeout(5000)` to every `db_url`, so a write
+  that finds another in progress on another connection waits up to five seconds
+  for it, and the default `db_url` turns on WAL mode with
+  `_pragma=journal_mode(WAL)`. The old default's `_journal_mode=WAL` is not a
+  parameter the driver reads, so the database was never in WAL mode.
 - 2026-10-04 `TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup`
   only passes through a periodic pass (closes #189): it slept for three
   eviction intervals before writing its file, and a startup pass still running

config.example.yml

@@ -34,9 +34,10 @@ maintenance_mode: false
 state_dir: ./data
 
 # SQLite database URL (default:
-# file:<state_dir>/state.sqlite3?_journal_mode=WAL). An empty value aborts
-# startup; leave the key out to use the default.
-# db_url: "file:./data/state.sqlite3?_journal_mode=WAL"
+# file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)). pixa adds
+# _pragma=busy_timeout(5000) to it. An empty value aborts startup; leave the
+# key out to use the default.
+# db_url: "file:./data/state.sqlite3?_pragma=journal_mode(WAL)"
 
 # Image proxy settings
 # HMAC signing key for URL signatures (required, at least 32 characters)

internal/config/config.go

@@ -320,7 +320,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 				settingName(keyDBURL), errValueEmpty)
 		}
 
-		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_journal_mode=WAL", c.StateDir)
+		// The driver sets the journal mode only through a _pragma parameter.
+		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_pragma=journal_mode(WAL)", c.StateDir)
 	}
 
 	if loader.err != nil {

internal/config/config_validation_internal_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"database/sql"
 	"log/slog"
 	"os"
 	"path/filepath"
@@ -9,6 +10,8 @@ import (
 	"time"
 
 	"git.eeqj.de/sneak/smartconfig"
+
+	_ "modernc.org/sqlite" // SQLite driver registration
 )
 
 // validTestSigningKey is a 32-character signing key that satisfies the
@@ -94,12 +97,43 @@ func TestOmittedValuesUseDefaults(t *testing.T) {
 		t.Errorf("AllowlistHosts = %v, want empty", c.AllowlistHosts)
 	}
 
-	wantDBURL := "file:" + DefaultStateDir + "/state.sqlite3?_journal_mode=WAL"
+	wantDBURL := "file:" + DefaultStateDir +
+		"/state.sqlite3?_pragma=journal_mode(WAL)"
 	if c.DBURL != wantDBURL {
 		t.Errorf("DBURL = %q, want derived default %q", c.DBURL, wantDBURL)
 	}
 }
 
+// TestDefaultDBURLOpensTheDatabaseInWALMode opens the db_url derived from
+// state_dir with the SQLite driver pixad uses and checks that the database
+// is in WAL mode: the driver ignores any parameter it does not know.
+func TestDefaultDBURLOpensTheDatabaseInWALMode(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+"state_dir: "+t.TempDir()+"\n")
+	if err != nil {
+		t.Fatalf("config with only state_dir set should be valid, got: %v", err)
+	}
+
+	db, err := sql.Open("sqlite", c.DBURL)
+	if err != nil {
+		t.Fatalf("failed to open %q: %v", c.DBURL, err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	var journalMode string
+
+	err = db.QueryRowContext(t.Context(), "PRAGMA journal_mode").Scan(&journalMode)
+	if err != nil {
+		t.Fatalf("failed to read the journal mode of %q: %v", c.DBURL, err)
+	}
+
+	if journalMode != "wal" {
+		t.Errorf("journal mode of %q = %q, want wal", c.DBURL, journalMode)
+	}
+}
+
 func TestExplicitValidValuesAreUsed(t *testing.T) {
 	t.Parallel()
 

internal/database/concurrent_writes_internal_test.go

@@ -0,0 +1,153 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"log/slog"
+	"path/filepath"
+	"sync"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// TestConcurrentWritesAllSucceed opens a database the way pixad does and
+// writes to it from several goroutines at once, so the writes run on
+// separate connections, as one request's writes and the background eviction
+// pass do. Every write must succeed, none failing with "database is locked",
+// whether or not db_url already has parameters, and the parameters it has
+// must still apply.
+func TestConcurrentWritesAllSucceed(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name            string
+		query           string
+		wantJournalMode string
+	}{
+		{
+			name:            "db_url without parameters",
+			query:           "",
+			wantJournalMode: "delete",
+		},
+		{
+			name:            "db_url with the WAL parameter",
+			query:           "?_pragma=journal_mode(WAL)",
+			wantJournalMode: "wal",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			dbURL := "file:" + filepath.Join(t.TempDir(), "state.sqlite3") + tt.query
+
+			d := &Database{
+				log:    slog.New(slog.DiscardHandler),
+				config: &config.Config{DBURL: dbURL},
+			}
+
+			err := d.connect(t.Context())
+			if err != nil {
+				t.Fatalf("failed to connect to %q: %v", dbURL, err)
+			}
+
+			t.Cleanup(func() { _ = d.db.Close() })
+
+			writeConcurrently(t, d.db)
+
+			var journalMode string
+
+			err = d.db.QueryRowContext(t.Context(), "PRAGMA journal_mode").
+				Scan(&journalMode)
+			if err != nil {
+				t.Fatalf("failed to read the journal mode: %v", err)
+			}
+
+			if journalMode != tt.wantJournalMode {
+				t.Errorf("journal mode = %q, want %q", journalMode, tt.wantJournalMode)
+			}
+		})
+	}
+}
+
+// writeConcurrently runs writeLikeOneRequest from several goroutines at once
+// and checks that every write was made.
+func writeConcurrently(t *testing.T, db *sql.DB) {
+	t.Helper()
+
+	const (
+		writers       = 4
+		requestsEach  = 20
+		totalRequests = writers * requestsEach
+	)
+
+	ctx := t.Context()
+
+	var wg sync.WaitGroup
+
+	for writer := range writers {
+		wg.Go(func() {
+			for request := range requestsEach {
+				key := fmt.Sprintf("%d-%d", writer, request)
+
+				err := writeLikeOneRequest(ctx, db, key)
+				if err != nil {
+					t.Errorf("writer %d: %v", writer, err)
+
+					return
+				}
+			}
+		})
+	}
+
+	wg.Wait()
+
+	var hits, sources int
+
+	err := db.QueryRowContext(ctx, `
+		SELECT hit_count, (SELECT COUNT(*) FROM source_content)
+		FROM cache_stats WHERE id = 1
+	`).Scan(&hits, &sources)
+	if err != nil {
+		t.Fatalf("failed to count the writes: %v", err)
+	}
+
+	if hits != totalRequests || sources != totalRequests {
+		t.Errorf("hit_count = %d and %d source_content rows, want %d of each",
+			hits, sources, totalRequests)
+	}
+}
+
+// writeLikeOneRequest makes the writes one request and the eviction pass
+// make: it counts a cache hit, stores a source, records a transformed image
+// and deletes that record again.
+func writeLikeOneRequest(ctx context.Context, db *sql.DB, key string) error {
+	_, err := db.ExecContext(ctx,
+		`UPDATE cache_stats SET hit_count = hit_count + 1 WHERE id = 1`)
+	if err != nil {
+		return fmt.Errorf("counting a cache hit: %w", err)
+	}
+
+	_, err = db.ExecContext(ctx, `INSERT INTO source_content
+		(content_hash, content_type, size_bytes) VALUES (?, 'image/png', 1)`, key)
+	if err != nil {
+		return fmt.Errorf("storing a source: %w", err)
+	}
+
+	_, err = db.ExecContext(ctx, `INSERT INTO variant_content
+		(cache_key, size_bytes, content_type) VALUES (?, 1, 'image/png')`, key)
+	if err != nil {
+		return fmt.Errorf("recording a transformed image: %w", err)
+	}
+
+	_, err = db.ExecContext(ctx,
+		`DELETE FROM variant_content WHERE cache_key = ?`, key)
+	if err != nil {
+		return fmt.Errorf("evicting a transformed image: %w", err)
+	}
+
+	return nil
+}

internal/database/database.go

@@ -243,7 +243,17 @@ func (s *Database) DB() *sql.DB {
 }
 
 func (s *Database) connect(ctx context.Context) error {
-	dbURL := s.config.DBURL
+	// Requests and the eviction pass write on separate connections. With
+	// a busy timeout, a write that finds another one in progress waits up
+	// to five seconds for it instead of failing at once with "database is
+	// locked". The driver runs each _pragma parameter on every connection
+	// it opens.
+	separator := "?"
+	if strings.Contains(s.config.DBURL, "?") {
+		separator = "&"
+	}
+
+	dbURL := s.config.DBURL + separator + "_pragma=busy_timeout(5000)"
 
 	s.log.Info("connecting to database", "url", dbURL)