Fix #205.

internal/dotlk/dotlk.go

@@ -0,0 +1,29 @@
+package dotlk
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+)
+
+// LockShm creates a directory on disk to prevent SQLite
+// from using this path for a shared memory file.
+func LockShm(name string) error {
+	err := os.Mkdir(name, 0777)
+	if errors.Is(err, fs.ErrExist) {
+		s, err := os.Lstat(name)
+		if err == nil && s.IsDir() {
+			return nil
+		}
+	}
+	return err
+}
+
+// Unlock removes the lock or shared memory file.
+func Unlock(name string) error {
+	err := os.Remove(name)
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil
+	}
+	return err
+}

internal/dotlk/dotlk_other.go

@@ -0,0 +1,13 @@
+//go:build !unix
+
+package dotlk
+
+import "os"
+
+// TryLock returns nil if it acquired the lock,
+// fs.ErrExist if another process has the lock.
+func TryLock(name string) error {
+	f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+	f.Close()
+	return err
+}

internal/dotlk/dotlk_unix.go

@@ -0,0 +1,50 @@
+//go:build unix
+
+package dotlk
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"strconv"
+
+	"golang.org/x/sys/unix"
+)
+
+// TryLock returns nil if it acquired the lock,
+// fs.ErrExist if another process has the lock.
+func TryLock(name string) error {
+	for retry := true; retry; retry = false {
+		f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+		if err == nil {
+			f.WriteString(strconv.Itoa(os.Getpid()))
+			f.Close()
+			return nil
+		}
+		if !errors.Is(err, fs.ErrExist) {
+			return err
+		}
+		if !removeStale(name) {
+			break
+		}
+	}
+	return fs.ErrExist
+}
+
+func removeStale(name string) bool {
+	buf, err := os.ReadFile(name)
+	if err != nil {
+		return errors.Is(err, fs.ErrNotExist)
+	}
+
+	pid, err := strconv.Atoi(string(buf))
+	if err != nil {
+		return false
+	}
+	if unix.Kill(pid, 0) == nil {
+		return false
+	}
+
+	err = os.Remove(name)
+	return err == nil || errors.Is(err, fs.ErrNotExist)
+}

vfs/os_dotlk.go

@@ -7,6 +7,8 @@ import (
 	"io/fs"
 	"os"
 	"sync"
+
+	"github.com/ncruces/go-sqlite3/internal/dotlk"
 )
 
 var (
@@ -28,12 +30,10 @@ func osGetSharedLock(file *os.File) _ErrorCode {
 	name := file.Name()
 	locker := vfsDotLocks[name]
 	if locker == nil {
-		f, err := os.OpenFile(name+".lock", os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
-		f.Close()
-		if errors.Is(err, fs.ErrExist) {
-			return _BUSY // Another process has the lock.
-		}
-		if err != nil {
+		if err := dotlk.TryLock(name + ".lock"); err != nil {
+			if errors.Is(err, fs.ErrExist) {
+				return _BUSY // Another process has the lock.
+			}
 			return _IOERR_LOCK
 		}
 		locker = &vfsDotLocker{}
@@ -114,8 +114,7 @@ func osReleaseLock(file *os.File, state LockLevel) _ErrorCode {
 	}
 
 	if locker.shared == 1 {
-		err := os.Remove(name + ".lock")
-		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+		if err := dotlk.Unlock(name + ".lock"); err != nil {
 			return _IOERR_UNLOCK
 		}
 		delete(vfsDotLocks, name)

vfs/shm_dotlk.go

@@ -6,11 +6,11 @@ import (
 	"context"
 	"errors"
 	"io/fs"
-	"os"
 	"sync"
 
 	"github.com/tetratelabs/wazero/api"
 
+	"github.com/ncruces/go-sqlite3/internal/dotlk"
 	"github.com/ncruces/go-sqlite3/internal/util"
 )
 
@@ -58,8 +58,7 @@ func (s *vfsShm) Close() error {
 		return nil
 	}
 
-	err := os.Remove(s.path)
-	if err != nil && !errors.Is(err, fs.ErrNotExist) {
+	if err := dotlk.Unlock(s.path); err != nil {
 		return _IOERR_UNLOCK
 	}
 	delete(vfsShmList, s.path)
@@ -82,9 +81,8 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 		return _OK
 	}
 
-	// Create a directory on disk to ensure only this process
-	// uses this path to register a shared memory.
-	err := os.Mkdir(s.path, 0777)
+	// Dead man's switch.
+	err := dotlk.LockShm(s.path)
 	if errors.Is(err, fs.ErrExist) {
 		return _BUSY
 	}