fix(state): durable config writes wait for another writer

.changeset/durable-config-write-lock-first.md

@@ -0,0 +1,5 @@
+---
+"ftw": patch
+---
+
+A driver's rotated sign-in token, a settings save and a vehicle wake reservation no longer fail at once with "database is locked (SQLITE_BUSY)" while another part of FTW writes to the database. They now wait up to five seconds for it, as other writes already did. A MyUplink token lost this way could stop the driver signing in after the next restart.

go/internal/state/configuration.go

@@ -145,6 +145,15 @@ func (s *Store) durableConfigWriteContext(parent context.Context, write func(*sq
 		return err
 	}
 	defer tx.Rollback()
+	// Take the write lock before write reads anything. SQLite does not wait
+	// out busy_timeout for a transaction that read first and then writes: it
+	// returns SQLITE_BUSY at once while another connection is writing, since
+	// waiting there could deadlock. SaveDriverSecret and SaveConfiguration
+	// both read first, so a rotated OAuth token was refused by any
+	// concurrent write. The statement changes no row.
+	if _, err := tx.ExecContext(ctx, `UPDATE config SET value = value WHERE 0`); err != nil {
+		return err
+	}
 	err = write(tx)
 	if err == nil {
 		err = tx.Commit()

go/internal/state/configuration_test.go

@@ -1,10 +1,12 @@
 package state
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"path/filepath"
 	"testing"
+	"time"
 )
 
 func TestConfigurationCommitIsDurableAndAtomic(t *testing.T) {
@@ -103,3 +105,43 @@ func TestBackupExportsTheCapturedConfiguration(t *testing.T) {
 		t.Fatalf("current revision: %+v %v", current, err)
 	}
 }
+
+// SaveDriverSecret and SaveConfiguration read before they write. SQLite then
+// skips busy_timeout and refuses the write at once while another connection
+// writes. A rotated OAuth token lost that way can lock a driver out at its
+// next restart, so both must wait for the other writer instead.
+func TestDurableConfigWriteWaitsForAnotherWriter(t *testing.T) {
+	s := freshStore(t)
+	rev, err := s.SaveConfiguration([]byte(`{"config":{"drivers":[{"name":"myuplink"}]}}`), 0, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	holdWriteLock := func(hold time.Duration) {
+		t.Helper()
+		ctx := context.Background()
+		conn, err := s.db.Conn(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if _, err := conn.ExecContext(ctx, `BEGIN IMMEDIATE`); err != nil {
+			t.Fatal(err)
+		}
+		time.AfterFunc(hold, func() {
+			_, _ = conn.ExecContext(ctx, `COMMIT`)
+			conn.Close()
+		})
+	}
+
+	holdWriteLock(300 * time.Millisecond)
+	if err := s.SaveDriverSecret("myuplink", "refresh_token", "RT-rotated"); err != nil {
+		t.Fatalf("rotated token refused while another connection wrote: %v", err)
+	}
+	if got, ok := s.LoadConfig(DriverSecretKey("myuplink", "refresh_token")); !ok || got != "RT-rotated" {
+		t.Fatalf("stored token = %q, %v", got, ok)
+	}
+
+	holdWriteLock(300 * time.Millisecond)
+	if _, err := s.SaveConfiguration([]byte(`{"config":{"drivers":[{"name":"myuplink"}]}}`), rev, nil); err != nil {
+		t.Fatalf("settings save refused while another connection wrote: %v", err)
+	}
+}