Fix snapshot-only restore

This commit fixes a bug introduced by parallel restore (03831e2)
where snapshot-only restores were not being handled correctly and
Litestream would hang indefinitely. Now the restore will check
explicitly for snapshot-only restores and exit the restore process
early to avoid WAL handling completely.

db.go

@@ -1454,7 +1454,7 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 	if err != nil {
 		return fmt.Errorf("cannot find max wal index for restore: %w", err)
 	}
-	logger.Printf("%s: starting restore: generation %s, index %08x-%08x", logPrefix, opt.Generation, minWALIndex, maxWALIndex)
+	snapshotOnly := maxWALIndex == -1
 
 	// Initialize starting position.
 	pos := Pos{Generation: opt.Generation, Index: minWALIndex}
@@ -1466,6 +1466,15 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 		return fmt.Errorf("cannot restore snapshot: %w", err)
 	}
 
+	// If no WAL files available, move snapshot to final path & exit early.
+	if snapshotOnly {
+		logger.Printf("%s: snapshot only, finalizing database", logPrefix)
+		return os.Rename(tmpPath, opt.OutputPath)
+	}
+
+	// Begin processing WAL files.
+	logger.Printf("%s: restoring wal files: generation=%s index=[%08x,%08x]", logPrefix, opt.Generation, minWALIndex, maxWALIndex)
+
 	// Fill input channel with all WAL indexes to be loaded in order.
 	ch := make(chan int, maxWALIndex-minWALIndex+1)
 	for index := minWALIndex; index <= maxWALIndex; index++ {
@@ -1476,7 +1485,7 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 	// Track load state for each WAL.
 	var mu sync.Mutex
 	cond := sync.NewCond(&mu)
-	ready := make([]bool, maxWALIndex-minWALIndex+1)
+	walStates := make([]walRestoreState, maxWALIndex-minWALIndex+1)
 
 	parallelism := opt.Parallelism
 	if parallelism < 1 {
@@ -1499,20 +1508,23 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 					}
 
 					startTime := time.Now()
-					if err = downloadWAL(ctx, r, opt.Generation, index, tmpPath); os.IsNotExist(err) && index == minWALIndex && index == maxWALIndex {
-						logger.Printf("%s: no wal available, snapshot only", logPrefix)
-						continue // snapshot file only, ignore error
-					} else if err != nil {
-						cond.Broadcast()
-						return fmt.Errorf("cannot download wal %s/%08x: %w", opt.Generation, index, err)
+
+					err := downloadWAL(ctx, r, opt.Generation, index, tmpPath)
+					if err != nil {
+						err = fmt.Errorf("cannot download wal %s/%08x: %w", opt.Generation, index, err)
 					}
 
 					// Mark index as ready-to-apply and notify applying code.
 					mu.Lock()
-					ready[index-minWALIndex] = true
+					walStates[index-minWALIndex] = walRestoreState{ready: true, err: err}
 					mu.Unlock()
 					cond.Broadcast()
 
+					// Returning the error here will cancel the other goroutines.
+					if err != nil {
+						return err
+					}
+
 					logger.Printf("%s: downloaded wal %s/%08x elapsed=%s",
 						logPrefix, opt.Generation, index,
 						time.Since(startTime).String(),
@@ -1526,12 +1538,15 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 	for index := minWALIndex; index <= maxWALIndex; index++ {
 		// Wait until next WAL file is ready to apply.
 		mu.Lock()
-		for !ready[index-minWALIndex] {
+		for !walStates[index-minWALIndex].ready {
 			if err := ctx.Err(); err != nil {
 				return err
 			}
 			cond.Wait()
 		}
+		if err := walStates[index-minWALIndex].err; err != nil {
+			return err
+		}
 		mu.Unlock()
 
 		// Apply WAL to database file.
@@ -1545,6 +1560,12 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 		)
 	}
 
+	// Ensure all goroutines finish. All errors should have been handled during
+	// the processing of WAL files but this ensures that all processing is done.
+	if err := g.Wait(); err != nil {
+		return err
+	}
+
 	// Copy file to final location.
 	logger.Printf("%s: renaming database from temporary location", logPrefix)
 	if err := os.Rename(tmpPath, opt.OutputPath); err != nil {
@@ -1554,6 +1575,11 @@ func RestoreReplica(ctx context.Context, r Replica, opt RestoreOptions) (err err
 	return nil
 }
 
+type walRestoreState struct {
+	ready bool
+	err   error
+}
+
 // CalcRestoreTarget returns a replica & generation to restore from based on opt criteria.
 func (db *DB) CalcRestoreTarget(ctx context.Context, opt RestoreOptions) (Replica, string, error) {
 	var target struct {