fix: Pidfile handling and socket docs

cmd/finch-daemon/main.go

@@ -24,6 +24,7 @@ import (
 
 	"github.com/coreos/go-systemd/v22/activation"
 	"github.com/coreos/go-systemd/v22/daemon"
+	"github.com/gofrs/flock"
 	"github.com/moby/moby/pkg/pidfile"
 	"github.com/runfinch/finch-daemon/api/router"
 	"github.com/runfinch/finch-daemon/pkg/flog"
@@ -104,32 +105,39 @@ func getListener(options *DaemonOptions) (net.Listener, error) {
 	return listener, nil
 }
 
-func handlePidFileOptions(options *DaemonOptions) error {
-	if options.pidFile != "" {
-		if err := os.MkdirAll(filepath.Dir(options.pidFile), 0o640); err != nil {
-			return fmt.Errorf("failed to create pidfile directory %s", err)
-		}
-		if err := pidfile.Write(options.pidFile, os.Getpid()); err != nil {
-			return fmt.Errorf("failed to start daemon, ensure finch daemon is not running or delete %s %w", options.pidFile, err)
-		}
-	}
-	return nil
-}
-
 func run(options *DaemonOptions) error {
 	// This sets the log level of the dependencies that use logrus (e.g., containerd library).
 	if options.debug {
 		logrus.SetLevel(logrus.DebugLevel)
 	}
 
-	defer func() {
-		if err := os.Remove(options.pidFile); err != nil {
-			logrus.Errorf("failed to remove pidfile %s", options.pidFile)
+	if options.pidFile != "" {
+		if err := os.MkdirAll(filepath.Dir(options.pidFile), 0o600); err != nil {
+			return fmt.Errorf("failed to create pidfile directory %s", err)
 		}
-	}()
 
-	if err := handlePidFileOptions(options); err != nil {
-		return err
+		pidFileLock := flock.New(options.pidFile)
+
+		defer func() {
+			pidFileLock.Unlock()
+		}()
+
+		if isLocked, err := pidFileLock.TryLock(); err != nil || !isLocked {
+			return fmt.Errorf("failed to acquire lock on PID file (%s); ensure only one instance is using the PID file path", options.pidFile)
+		}
+
+		if err := pidfile.Write(options.pidFile, os.Getpid()); err != nil {
+			return fmt.Errorf("failed to start daemon, ensure finch daemon is not running or delete %s %w", options.pidFile, err)
+		}
+
+		pidFileLock.Unlock()
+
+		// Defer is at the end of trying to write to PID file so that it doesn't remove the pidfile created by another process when daemon fails to start
+		defer func() {
+			if err := os.Remove(options.pidFile); err != nil {
+				logrus.Errorf("failed to remove pidfile %s", options.pidFile)
+			}
+		}()
 	}
 
 	logger := flog.NewLogrus()

docs/sample-service-files/finch-socket-activation.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=finch daemon
+Documentation=https://runfinch.com
+After=network.target local-fs.target containerd.service finch.socket
+Wants=network.target containerd.service
+Requires=finch.socket
+
+[Service]
+ExecStart=/usr/local/bin/finch-daemon --debug --socket-addr fd://
+Type=notify
+Delegate=yes
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target

docs/sample-service-files/finch-socket-activation.socket

@@ -0,0 +1,11 @@
+[Unit]
+Description=Socket for finch daemon
+
+[Socket]
+ListenStream=/run/finch.sock
+SocketMode=0660
+SocketUser=root
+SocketGroup=root
+
+[Install]
+WantedBy=sockets.target

go.mod

@@ -84,6 +84,7 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-task/slim-sprig/v3 v3.0.0 // indirect
 	github.com/godbus/dbus/v5 v5.1.0 // indirect
+	github.com/gofrs/flock v0.12.1 // indirect
 	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
 	github.com/golang/protobuf v1.5.4 // indirect

go.sum

@@ -130,6 +130,8 @@ github.com/go-task/slim-sprig/v3 v3.0.0/go.mod h1:W848ghGpv3Qj3dhTPRyJypKRiqCdHZ
 github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
 github.com/godbus/dbus/v5 v5.1.0 h1:4KLkAxT3aOY8Li4FRJe/KvhoNFFxo0m6fNuFUO8QJUk=
 github.com/godbus/dbus/v5 v5.1.0/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
+github.com/gofrs/flock v0.12.1 h1:MTLVXXHf8ekldpJk3AKicLij9MdwOWkZ+a/jHHZby9E=
+github.com/gofrs/flock v0.12.1/go.mod h1:9zxTsyu5xtJ9DK+1tFZyibEV7y3uwDxPPfbxeeHCoD0=
 github.com/gogo/protobuf v1.3.2 h1:Ov1cvc58UF3b5XjBnZv7+opcTcQFZebYjWzi34vdm4Q=
 github.com/gogo/protobuf v1.3.2/go.mod h1:P1XiOD3dCwIKUDQYPy72D8LYyHL2YPYrpS2s69NZV8Q=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b/go.mod h1:SBH7ygxi8pfUlaOkMMuAQtPIUF8ecWP5IEl/CR7VP2Q=