fix: never remove a non-socket file at the server socket path

.env.example

@@ -34,7 +34,7 @@ TINYAUTH_RESOURCES_PATH="./resources"
 TINYAUTH_SERVER_PORT=3000
 # The address on which the server listens.
 TINYAUTH_SERVER_ADDRESS="0.0.0.0"
-# The path to the Unix socket.
+# Path of the Unix socket to listen on instead of TCP. An existing socket at this path is replaced, any other file is left untouched.
 TINYAUTH_SERVER_SOCKETPATH=
 
 # auth config

internal/bootstrap/router_bootstrap.go

@@ -6,12 +6,12 @@ import (
 	"fmt"
 	"net"
 	"net/http"
-	"os"
 	"time"
 
 	"github.com/tinyauthapp/tinyauth/internal/controller"
 	"github.com/tinyauthapp/tinyauth/internal/middleware"
 	"github.com/tinyauthapp/tinyauth/internal/model"
+	"github.com/tinyauthapp/tinyauth/internal/utils"
 	"go.uber.org/dig"
 
 	"github.com/gin-gonic/gin"
@@ -166,15 +166,16 @@ func (app *BootstrapApp) serveHTTP(ctx context.Context) error {
 }
 
 func (app *BootstrapApp) serveUnix(ctx context.Context) error {
-	_, err := os.Stat(app.config.Server.SocketPath)
+	removed, inUse, err := utils.RemoveExistingSocket(app.config.Server.SocketPath)
 
-	if err == nil {
-		app.log.App.Info().Msgf("Removing existing socket file %s", app.config.Server.SocketPath)
-		err := os.Remove(app.config.Server.SocketPath)
+	if err != nil {
+		return fmt.Errorf("failed to remove existing socket file: %w", err)
+	}
 
-		if err != nil {
-			return fmt.Errorf("failed to remove existing socket file: %w", err)
-		}
+	if inUse {
+		app.log.App.Warn().Msgf("Replaced socket %s that was still in use by another process, the socket path is where Tinyauth listens and should not be shared with other services", app.config.Server.SocketPath)
+	} else if removed {
+		app.log.App.Info().Msgf("Removed existing socket file %s", app.config.Server.SocketPath)
 	}
 
 	app.log.App.Info().Msgf("Starting server on unix socket %s", app.config.Server.SocketPath)

internal/model/config.go

@@ -137,7 +137,7 @@ type ResourcesConfig struct {
 type ServerConfig struct {
 	Port       int    `description:"The port on which the server listens." yaml:"port,omitempty"`
 	Address    string `description:"The address on which the server listens." yaml:"address,omitempty"`
-	SocketPath string `description:"The path to the Unix socket." yaml:"socketPath,omitempty"`
+	SocketPath string `description:"Path of the Unix socket to listen on instead of TCP. An existing socket at this path is replaced, any other file is left untouched." yaml:"socketPath,omitempty"`
 }
 
 type AuthConfig struct {

internal/utils/fs_utils.go

@@ -1,6 +1,11 @@
 package utils
 
-import "os"
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+)
 
 func ReadFile(file string) (string, error) {
 	_, err := os.Stat(file)
@@ -15,3 +20,33 @@ func ReadFile(file string) (string, error) {
 
 	return string(data), nil
 }
+
+// RemoveExistingSocket removes the unix socket left at path by a previous (or still running) instance so the server
+// can listen on it again. It reports whether a socket was removed and whether it was still accepting connections.
+// Anything that is not a socket (e.g. a regular file or a directory) is never removed.
+func RemoveExistingSocket(path string) (removed bool, inUse bool, err error) {
+	// stat follows symlinks, a symlink to a socket is replaced like the socket itself
+	info, err := os.Stat(path)
+	if os.IsNotExist(err) {
+		return false, false, nil
+	}
+	if err != nil {
+		return false, false, err
+	}
+
+	if info.Mode().Type() != os.ModeSocket {
+		return false, false, fmt.Errorf("refusing to remove %s, it is not a unix socket", path)
+	}
+
+	conn, err := net.DialTimeout("unix", path, time.Second)
+	if err == nil {
+		conn.Close()
+		inUse = true
+	}
+
+	if err := os.Remove(path); err != nil {
+		return false, inUse, err
+	}
+
+	return true, inUse, nil
+}

internal/utils/fs_utils_test.go

@@ -1,7 +1,9 @@
 package utils
 
 import (
+	"net"
 	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -30,3 +32,74 @@ func TestReadFile(t *testing.T) {
 	assert.ErrorContains(t, err, "no such file or directory")
 	assert.Equal(t, "", content)
 }
+
+func TestRemoveExistingSocket(t *testing.T) {
+	// Short directory, unix socket paths are limited to ~108 bytes
+	dir, err := os.MkdirTemp("", "ta")
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	// Non-existing path
+	removed, inUse, err := RemoveExistingSocket(filepath.Join(dir, "missing.sock"))
+	assert.NoError(t, err)
+	assert.False(t, removed)
+	assert.False(t, inUse)
+
+	// Regular file is never removed
+	file := filepath.Join(dir, "regular")
+	require.NoError(t, os.WriteFile(file, []byte("data"), 0600))
+	removed, _, err = RemoveExistingSocket(file)
+	assert.ErrorContains(t, err, "not a unix socket")
+	assert.False(t, removed)
+	assert.FileExists(t, file)
+
+	// Symlink to a regular file is never removed
+	fileLink := filepath.Join(dir, "regular.link")
+	require.NoError(t, os.Symlink(file, fileLink))
+	removed, _, err = RemoveExistingSocket(fileLink)
+	assert.ErrorContains(t, err, "not a unix socket")
+	assert.False(t, removed)
+	assert.FileExists(t, fileLink)
+
+	// Directory is never removed
+	removed, _, err = RemoveExistingSocket(dir)
+	assert.ErrorContains(t, err, "not a unix socket")
+	assert.False(t, removed)
+
+	// Socket still in use (e.g. the previous instance during a rolling update) is replaced and reported
+	live := filepath.Join(dir, "live.sock")
+	listener, err := net.Listen("unix", live)
+	require.NoError(t, err)
+	listener.(*net.UnixListener).SetUnlinkOnClose(false)
+	defer listener.Close()
+	removed, inUse, err = RemoveExistingSocket(live)
+	assert.NoError(t, err)
+	assert.True(t, removed)
+	assert.True(t, inUse)
+	assert.NoFileExists(t, live)
+
+	// Stale socket left behind by a previous run is removed
+	stale := filepath.Join(dir, "stale.sock")
+	staleListener, err := net.Listen("unix", stale)
+	require.NoError(t, err)
+	staleListener.(*net.UnixListener).SetUnlinkOnClose(false)
+	require.NoError(t, staleListener.Close())
+	removed, inUse, err = RemoveExistingSocket(stale)
+	assert.NoError(t, err)
+	assert.True(t, removed)
+	assert.False(t, inUse)
+	assert.NoFileExists(t, stale)
+
+	// Symlink to a socket is replaced like the socket itself
+	target := filepath.Join(dir, "target.sock")
+	targetListener, err := net.Listen("unix", target)
+	require.NoError(t, err)
+	targetListener.(*net.UnixListener).SetUnlinkOnClose(false)
+	require.NoError(t, targetListener.Close())
+	link := filepath.Join(dir, "link.sock")
+	require.NoError(t, os.Symlink(target, link))
+	removed, _, err = RemoveExistingSocket(link)
+	assert.NoError(t, err)
+	assert.True(t, removed)
+	assert.NoFileExists(t, link)
+}