Log the start-up checkpoint's figures under their own names (closes #117)

Initialize scanned the row of PRAGMA wal_checkpoint(TRUNCATE), which is
the busy flag, the frames in the WAL and the frames copied, into
wal_pages, checkpointed and moved; and a completed TRUNCATE reads 0 for
both counts. The line now logs busy, wal_frames and checkpointed_frames,
plus wal_bytes, the WAL file's size read before the checkpoint, and is a
warning when busy is 1. Tests open a database a second time while the
first stays open, with and without a read open on the first, and check
the line's figures.

Model: opus-5-5

internal/database/database.go

@@ -245,6 +245,12 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 	return database, nil
 }
 
+// The messages Initialize logs the start-up WAL checkpoint with.
+const (
+	startupCheckpointMessage           = "WAL checkpoint on startup"
+	startupCheckpointIncompleteMessage = "WAL checkpoint on startup incomplete"
+)
+
 // Initialize creates the database schema if it doesn't exist.
 func (d *Database) Initialize() error {
 	// Set SQLite pragmas for performance. Per-connection settings (cache_size,
@@ -266,19 +272,44 @@ func (d *Database) Initialize() error {
 		}
 	}
 
-	// Run WAL checkpoint on startup to consolidate any existing WAL
-	var walPages, checkpointedPages, movedPages int
+	// The start-up checkpoint copies the WAL a previous run left into the
+	// database file and empties the WAL file; its frame counts then read 0,
+	// so the WAL file's size is read first. There is no WAL file when the
+	// database is new or the last run closed the database, which deletes it.
+	var walBytes int64
 
-	err := d.writer.QueryRowContext(context.Background(),
+	wal, err := os.Stat(d.path + walFileSuffix)
+	if err == nil {
+		walBytes = wal.Size()
+	} else if !errors.Is(err, os.ErrNotExist) {
+		d.logger.Warn("Failed to read the WAL file's size on startup",
+			"error", err)
+	}
+
+	var busy, walFrames, checkpointedFrames int
+
+	err = d.writer.QueryRowContext(context.Background(),
 		"PRAGMA wal_checkpoint(TRUNCATE)",
-	).Scan(&walPages, &checkpointedPages, &movedPages)
-	if err != nil {
+	).Scan(&busy, &walFrames, &checkpointedFrames)
+
+	switch {
+	case err != nil:
 		d.logger.Warn("Failed to checkpoint WAL on startup", "error", err)
-	} else {
-		d.logger.Info("WAL checkpoint on startup",
-			"wal_pages", walPages,
-			"checkpointed", checkpointedPages,
-			"moved", movedPages,
+	case busy != 0:
+		// Another process using the database kept the checkpoint from
+		// copying or emptying all of the WAL.
+		d.logger.Warn(startupCheckpointIncompleteMessage,
+			"wal_bytes", walBytes,
+			"busy", busy,
+			"wal_frames", walFrames,
+			"checkpointed_frames", checkpointedFrames,
+		)
+	default:
+		d.logger.Info(startupCheckpointMessage,
+			"wal_bytes", walBytes,
+			"busy", busy,
+			"wal_frames", walFrames,
+			"checkpointed_frames", checkpointedFrames,
 		)
 	}