Add replicate flags to pump Litestream manually

For some special setups it is sometimes useful to run Litestream
manually instead of letting it replicate in the background.

This commit implements the following flags for replicate:

  * -once for doing synchronous replication and then exit
  * -force-snapshot to force a snapshot during -once
  * -enforce-retention to enforce retention rules during -once

Because running once does not respect the snapshot interval the
caller is expected to use -force-snapshot and -enforce-retention
regularly to ensure the replication targets stay clean.

For this to work correctly with a live database it needs to be opened
with auto checkpointing disabled and SQLITE_FCNTL_PERSIST_WAL.

Other uses include only using -force-snapshot to create regular backups
of the database instead of live replication.

Fixes #486

cmd/litestream/main.go

@@ -94,19 +94,19 @@ func (m *Main) Run(ctx context.Context, args []string) (err error) {
 
 		// Wait for signal to stop program.
 		select {
-		case err = <-c.execCh:
+		case err = <-c.runCh:
 			fmt.Println("subprocess exited, litestream shutting down")
 		case sig := <-signalCh:
 			fmt.Println("signal received, litestream shutting down")
 
-			if c.cmd != nil {
+			if c.runSignal != nil {
 				fmt.Println("sending signal to exec process")
-				if err := c.cmd.Process.Signal(sig); err != nil {
+				if err := c.runSignal(sig); err != nil {
 					return fmt.Errorf("cannot signal exec process: %w", err)
 				}
 
 				fmt.Println("waiting for exec process to close")
-				if err := <-c.execCh; err != nil && !strings.HasPrefix(err.Error(), "signal:") {
+				if err := <-c.runCh; err != nil && !strings.HasPrefix(err.Error(), "signal:") {
 					return fmt.Errorf("cannot wait for exec process: %w", err)
 				}
 			}

cmd/litestream/replicate.go

@@ -23,8 +23,12 @@ import (
 
 // ReplicateCommand represents a command that continuously replicates SQLite databases.
 type ReplicateCommand struct {
-	cmd    *exec.Cmd  // subcommand
-	execCh chan error // subcommand error channel
+	runSignal func(os.Signal) error // run cancel signaler
+	runCh     chan error            // run error channel
+
+	once             bool // replicate once and exit
+	forceSnapshot    bool // force snapshot to all replicas
+	enforceRetention bool // enforce retention of old snapshots
 
 	Config Config
 
@@ -34,14 +38,17 @@ type ReplicateCommand struct {
 
 func NewReplicateCommand() *ReplicateCommand {
 	return &ReplicateCommand{
-		execCh: make(chan error),
+		runCh: make(chan error),
 	}
 }
 
 // ParseFlags parses the CLI flags and loads the configuration file.
 func (c *ReplicateCommand) ParseFlags(ctx context.Context, args []string) (err error) {
 	fs := flag.NewFlagSet("litestream-replicate", flag.ContinueOnError)
 	execFlag := fs.String("exec", "", "execute subcommand")
+	onceFlag := fs.Bool("once", false, "replicate once and exit")
+	forceSnapshotFlag := fs.Bool("force-snapshot", false, "force snapshot when replicating once")
+	enforceRetentionFlag := fs.Bool("enforce-retention", false, "enforce retention of old snapshots")
 	tracePath := fs.String("trace", "", "trace path")
 	configPath, noExpandEnv := registerConfigFlag(fs)
 	fs.Usage = c.Usage
@@ -90,6 +97,22 @@ func (c *ReplicateCommand) ParseFlags(ctx context.Context, args []string) (err e
 		litestream.Tracef = log.New(f, "", log.LstdFlags|log.Lmicroseconds|log.LUTC|log.Lshortfile).Printf
 	}
 
+	// Once is mutually exclusive with exec
+	c.once = *onceFlag
+	if c.once && c.Config.Exec != "" {
+		return fmt.Errorf("cannot specify -once flag with exec")
+	}
+
+	c.forceSnapshot = *forceSnapshotFlag
+	if !c.once && c.forceSnapshot {
+		return fmt.Errorf("cannot specify -force-snapshot flag without -once")
+	}
+
+	c.enforceRetention = *enforceRetentionFlag
+	if !c.once && c.enforceRetention {
+		return fmt.Errorf("cannot specify -enforce-retention flag without -once")
+	}
+
 	return nil
 }
 
@@ -109,6 +132,14 @@ func (c *ReplicateCommand) Run() (err error) {
 			return err
 		}
 
+		// Disable monitors if we're running once.
+		if c.once {
+			db.MonitorInterval = 0
+			for _, r := range db.Replicas {
+				r.MonitorEnabled = false
+			}
+		}
+
 		// Open database & attach to program.
 		if err := db.Open(); err != nil {
 			return err
@@ -162,14 +193,65 @@ func (c *ReplicateCommand) Run() (err error) {
 			return fmt.Errorf("cannot parse exec command: %w", err)
 		}
 
-		c.cmd = exec.Command(execArgs[0], execArgs[1:]...)
-		c.cmd.Env = os.Environ()
-		c.cmd.Stdout = os.Stdout
-		c.cmd.Stderr = os.Stderr
-		if err := c.cmd.Start(); err != nil {
+		cmd := exec.Command(execArgs[0], execArgs[1:]...)
+		cmd.Env = os.Environ()
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		if err := cmd.Start(); err != nil {
 			return fmt.Errorf("cannot start exec command: %w", err)
 		}
-		go func() { c.execCh <- c.cmd.Wait() }()
+		c.runSignal = cmd.Process.Signal
+		go func() { c.runCh <- cmd.Wait() }()
+	} else if c.once {
+		// Run replication once for each replica with cancel.
+		ctx, cancel := context.WithCancel(context.Background())
+		c.runSignal = func(s os.Signal) error {
+			cancel()
+			return nil
+		}
+
+		go func() {
+			var err error
+
+			defer func() {
+				cancel()
+				c.runCh <- err
+			}()
+
+			for _, db := range c.DBs {
+				if c.forceSnapshot {
+					// Force next index with RESTART checkpoint.
+					db.MaxCheckpointPageN = 1
+				}
+
+				if err = db.Sync(ctx); err != nil {
+					return
+				}
+
+				// Prevent checkpointing on Close()
+				db.MinCheckpointPageN = 0
+				db.MaxCheckpointPageN = 0
+				db.TruncatePageN = 0
+				db.CheckpointInterval = 0
+
+				for _, r := range db.Replicas {
+					if c.forceSnapshot {
+						_, err = r.Snapshot(ctx)
+					} else {
+						err = r.Sync(ctx)
+					}
+					if err != nil {
+						return
+					}
+
+					if c.enforceRetention {
+						if err = r.EnforceRetention(ctx); err != nil {
+							return
+						}
+					}
+				}
+			}
+		}()
 	}
 
 	return nil
@@ -212,6 +294,15 @@ Arguments:
 	    Executes a subcommand. Litestream will exit when the child
 	    process exits. Useful for simple process management.
 
+	-once
+	    Execute replication once and exit.
+
+	-force-snapshot
+	    When replicating once, force taking a snapshot to all replicas.
+
+	-enforce-retention
+	    When replicating once, enforce rentention of old snapshots.
+
 	-no-expand-env
 	    Disables environment variable expansion in configuration file.
 

db.go

@@ -761,7 +761,7 @@ func (db *DB) Sync(ctx context.Context) (err error) {
 	checkpointMode := CheckpointModePassive
 	if db.MaxCheckpointPageN > 0 && newWALSize >= calcWALSize(db.pageSize, db.MaxCheckpointPageN) {
 		checkpoint, checkpointMode = true, CheckpointModeRestart
-	} else if newWALSize >= calcWALSize(db.pageSize, db.MinCheckpointPageN) {
+	} else if db.MinCheckpointPageN > 0 && newWALSize >= calcWALSize(db.pageSize, db.MinCheckpointPageN) {
 		checkpoint = true
 	} else if db.CheckpointInterval > 0 && !info.dbModTime.IsZero() && time.Since(info.dbModTime) > db.CheckpointInterval && newWALSize > calcWALSize(db.pageSize, 1) {
 		checkpoint = true