Checkpoint without waiting on readers and log a busy checkpoint (closes #102)

The 5-second checkpoint ran in TRUNCATE mode, which waits up to the
5-second busy timeout for every open read while holding the database
lock every batch write needs. It now runs in PASSIVE mode, which never
waits. A WAL file over 64 MiB instead gets one TRUNCATE checkpoint whose
wait for reads is capped at 500 ms and logged; it empties the file once
no read is open, and while reads follow one another with no pause it
can give up every time. A checkpoint that leaves part of the WAL
uncopied returns ErrCheckpointIncomplete, which the maintainer logs as
incomplete instead of completed. The writing connection's
journal_size_limit is 64 MiB.

Model: opus-5-5

internal/routewatch/dbmaintainer.go

@@ -2,6 +2,7 @@ package routewatch
 
 import (
 	"context"
+	"errors"
 	"log/slog"
 	"sync"
 	"time"
@@ -34,6 +35,12 @@ const (
 	analyzeTimeout = 5 * time.Minute
 )
 
+// checkpointIncompleteMessage is logged, at debug level like a completed
+// checkpoint, when a checkpoint leaves part of the WAL for a later one: while
+// reads overlap, most do. The database warns when the WAL file stays over its
+// size limit.
+const checkpointIncompleteMessage = "WAL checkpoint incomplete"
+
 // DBMaintainer handles background database maintenance tasks.
 type DBMaintainer struct {
 	db     database.Store
@@ -123,9 +130,13 @@ func (m *DBMaintainer) runCheckpoint(ctx context.Context) {
 	duration, err := runWithTimeout(ctx, checkpointTimeout, m.db.Checkpoint)
 	m.recordResult(&m.lastCheckpoint, &m.lastCheckpointError, &m.checkpointCount, err)
 
-	if err != nil {
+	switch {
+	case errors.Is(err, database.ErrCheckpointIncomplete):
+		m.logger.Debug(checkpointIncompleteMessage,
+			"error", err, "duration", duration)
+	case err != nil:
 		m.logger.Error("WAL checkpoint failed", "error", err, "duration", duration)
-	} else {
+	default:
 		m.logger.Debug("WAL checkpoint completed", "duration", duration)
 	}
 }