Catch and avoid DB errors on witness

internal/witness/cmd/feeder/main.go

@@ -198,6 +198,7 @@ func (l *ctLog) feedOnce(ctx context.Context, w *wh.Witness) error {
 	}
 	if errors.Is(err, wh.ErrSTHTooOld) {
 		glog.Infof("STH mismatch at log size %d for %s", wSize, l.name)
+		glog.Infof("%s", wsthRaw)
 	}
 	// Parse the STH it returns.
 	var wsthJSON ct.GetSTHResponse

internal/witness/cmd/witness/impl/witness.go

@@ -98,6 +98,10 @@ func Main(ctx context.Context, opts ServerOpts) error {
 	if err != nil {
 		return fmt.Errorf("failed to connect to DB: %w", err)
 	}
+
+	// Avoid multiple writes colliding and resulting in a "database locked" error.
+	db.SetMaxOpenConns(1)
+
 	// Load log configuration into the map.
 	logMap, err := buildLogMap(opts.Config)
 	if err != nil {

internal/witness/cmd/witness/internal/witness/witness.go

@@ -174,6 +174,8 @@ func (w *Witness) Update(ctx context.Context, logID string, nextRaw []byte, proo
 	if err != nil {
 		return nil, fmt.Errorf("couldn't create db tx: %v", err)
 	}
+	defer tx.Rollback()
+
 	// Get the latest STH (if one exists).
 	prevRaw, err := w.getLatestSTH(tx.QueryRow, logID)
 	if err != nil {
@@ -264,8 +266,8 @@ func (w *Witness) getLatestSTH(queryRow func(query string, args ...interface{})
 
 // setSTH writes the STH to the database for a given log.
 func (w *Witness) setSTH(tx *sql.Tx, logID string, sth []byte) error {
-	tx.Exec(`INSERT INTO sths (logID, sth) VALUES (?, ?)
-		 ON CONFLICT(logID) DO UPDATE SET sth=excluded.sth`,
-		logID, sth)
+	if _, err := tx.Exec(`INSERT OR REPLACE INTO sths (logID, sth) VALUES (?, ?)`, logID, sth); err != nil {
+		return fmt.Errorf("failed to update STH; %v", err)
+	}
 	return tx.Commit()
 }