optionally protect against multiple starts with flock (#1873)

* optionally protect against multiple starts with flock

Fixes #1823

* use traditional .lock extension

* move config key to top level

irc/config.go

@@ -616,6 +616,8 @@ type Config struct {
 
 	languageManager *languages.Manager
 
+	LockFile string `yaml:"lock-file"`
+
 	Datastore struct {
 		Path        string
 		AutoUpgrade bool

irc/flock/flock.go

@@ -0,0 +1,24 @@
+//go:build !plan9
+
+package flock
+
+import (
+	"errors"
+
+	"github.com/gofrs/flock"
+)
+
+var (
+	CouldntAcquire = errors.New("Couldn't acquire flock (is another Ergo running?)")
+)
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	f := flock.New(path)
+	success, err := f.TryLock()
+	if err != nil {
+		return nil, err
+	} else if !success {
+		return nil, CouldntAcquire
+	}
+	return f, nil
+}

irc/flock/flock_iface.go

@@ -0,0 +1,14 @@
+package flock
+
+// documentation for github.com/gofrs/flock incorrectly claims that
+// Flock implements sync.Locker; it does not because the Unlock method
+// has a return type (err).
+type Flocker interface {
+	Unlock() error
+}
+
+type noopFlocker struct{}
+
+func (n *noopFlocker) Unlock() error {
+	return nil
+}

irc/flock/flock_plan9.go

@@ -0,0 +1,7 @@
+//go:build plan9
+
+package flock
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	return &noopFlocker{}, nil
+}

irc/server.go

@@ -25,6 +25,7 @@ import (
 	"github.com/ergochat/ergo/irc/caps"
 	"github.com/ergochat/ergo/irc/connection_limits"
 	"github.com/ergochat/ergo/irc/flatip"
+	"github.com/ergochat/ergo/irc/flock"
 	"github.com/ergochat/ergo/irc/history"
 	"github.com/ergochat/ergo/irc/logger"
 	"github.com/ergochat/ergo/irc/modes"
@@ -88,6 +89,7 @@ type Server struct {
 	whoWas            WhoWasList
 	stats             Stats
 	semaphores        ServerSemaphores
+	flock             flock.Flocker
 	defcon            uint32
 }
 
@@ -585,6 +587,19 @@ func (server *Server) applyConfig(config *Config) (err error) {
 
 	server.logger.Info("server", "Using config file", server.configFilename)
 
+	if initial {
+		if config.LockFile != "" {
+			server.flock, err = flock.TryAcquireFlock(config.LockFile)
+			if err != nil {
+				return fmt.Errorf("failed to acquire flock on %s: %w",
+					config.LockFile, err)
+			}
+		}
+		// the lock is never released until quit; we need to save a pointer
+		// to the (*flock.Flock) object so it doesn't get GC'ed, which would
+		// close the file and surrender the lock
+	}
+
 	// first, reload config sections for functionality implemented in subpackages:
 	wasLoggingRawIO := !initial && server.logger.IsLoggingRawIO()
 	err = server.logger.ApplyConfig(config.Logging)