Dot file locking. (#179)

vfs/README.md

@@ -19,20 +19,19 @@ POSIX advisory locks, which SQLite uses on Unix, are
 On Linux and macOS, this package uses
 [OFD locks](https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html)
 to synchronize access to database files.
-OFD locks are fully compatible with POSIX advisory locks.
 
 This package can also use
 [BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2),
 albeit with reduced concurrency (`BEGIN IMMEDIATE` behaves like `BEGIN EXCLUSIVE`).
-On BSD, macOS, and illumos, BSD locks are fully compatible with POSIX advisory locks;
-on Linux and z/OS, they are fully functional, but incompatible;
-elsewhere, they are very likely broken.
 BSD locks are the default on BSD and illumos,
 but you can opt into them with the `sqlite3_flock` build tag.
 
 On Windows, this package uses `LockFileEx` and `UnlockFileEx`,
 like SQLite.
 
+You can also opt into a cross platform locking implementation
+with the `sqlite3_dotlk` build tag.
+
 Otherwise, file locking is not supported, and you must use
 [`nolock=1`](https://sqlite.org/uri.html#urinolock)
 (or [`immutable=1`](https://sqlite.org/uri.html#uriimmutable))
@@ -86,8 +85,8 @@ The implementation is compatible with SQLite's
 ### Build Tags
 
 The VFS can be customized with a few build tags:
-- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS to enable locking,
-  and elsewhere to test BSD locks.
+- `sqlite3_flock` forces the use of BSD locks.
+- `sqlite3_dotlk` forces the use of dot-file locks.
 - `sqlite3_nosys` prevents importing [`x/sys`](https://pkg.go.dev/golang.org/x/sys);
   disables locking _and_ shared memory on all platforms.
 - `sqlite3_noshm` disables shared memory on all platforms.
@@ -96,17 +95,19 @@ The VFS can be customized with a few build tags:
 > The default configuration of this package is compatible with the standard
 > [Unix and Windows SQLite VFSes](https://sqlite.org/vfs.html#multiple_vfses);
 > `sqlite3_flock` builds are compatible with the
-> [`unix-flock` VFS](https://sqlite.org/compile.html#enable_locking_style).
+> [`unix-flock` VFS](https://sqlite.org/compile.html#enable_locking_style);
+> `sqlite3_dotlk` builds are compatible with the
+> [`unix-dotfile` VFS](https://sqlite.org/compile.html#enable_locking_style).
 > If incompatible file locking is used, accessing databases concurrently with
 > _other_ SQLite libraries will eventually corrupt data.
 
 ### Custom VFSes
 
-- [`github.com/ncruces/go-sqlite3/vfs/adiantum`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/adiantum)
-  wraps a VFS to offer encryption at rest.
 - [`github.com/ncruces/go-sqlite3/vfs/memdb`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/memdb)
   implements an in-memory VFS.
 - [`github.com/ncruces/go-sqlite3/vfs/readervfs`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/readervfs)
   implements a VFS for immutable databases.
+- [`github.com/ncruces/go-sqlite3/vfs/adiantum`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/adiantum)
+  wraps a VFS to offer encryption at rest.
 - [`github.com/ncruces/go-sqlite3/vfs/xts`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/xts)
   wraps a VFS to offer encryption at rest.

vfs/file.go

@@ -35,10 +35,10 @@ func testSymlinks(path string) error {
 
 func (vfsOS) Delete(path string, syncDir bool) error {
 	err := os.Remove(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return _IOERR_DELETE_NOENT
+	}
 	if err != nil {
-		if errors.Is(err, fs.ErrNotExist) {
-			return _IOERR_DELETE_NOENT
-		}
 		return err
 	}
 	if runtime.GOOS != "windows" && syncDir {
@@ -151,6 +151,7 @@ func (f *vfsFile) Close() error {
 	if f.shm != nil {
 		f.shm.Close()
 	}
+	f.Unlock(LOCK_NONE)
 	return f.File.Close()
 }
 
@@ -206,10 +207,10 @@ func (f *vfsFile) HasMoved() (bool, error) {
 		return false, err
 	}
 	pi, err := os.Stat(f.Name())
+	if errors.Is(err, fs.ErrNotExist) {
+		return true, nil
+	}
 	if err != nil {
-		if errors.Is(err, fs.ErrNotExist) {
-			return true, nil
-		}
 		return false, err
 	}
 	return !os.SameFile(fi, pi), nil

vfs/lock.go

@@ -1,4 +1,4 @@
-//go:build (linux || darwin || windows || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && !sqlite3_nosys
+//go:build ((linux || darwin || windows || freebsd || openbsd || netbsd || dragonfly || illumos) && !sqlite3_nosys) || sqlite3_flock || sqlite3_dotlk
 
 package vfs
 

vfs/lock_other.go

@@ -1,4 +1,4 @@
-//go:build !(linux || darwin || windows || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) || sqlite3_nosys
+//go:build !(((linux || darwin || windows || freebsd || openbsd || netbsd || dragonfly || illumos) && !sqlite3_nosys) || sqlite3_flock || sqlite3_dotlk)
 
 package vfs
 

vfs/os_bsd.go

@@ -1,4 +1,4 @@
-//go:build (freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && !sqlite3_nosys
+//go:build ((freebsd || openbsd || netbsd || dragonfly || illumos) && !(sqlite3_dotlk || sqlite3_nosys)) || sqlite3_flock
 
 package vfs
 

vfs/os_dotlk.go

@@ -0,0 +1,143 @@
+//go:build sqlite3_dotlk
+
+package vfs
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"sync"
+)
+
+var (
+	// +checklocks:vfsDotLocksMtx
+	vfsDotLocks    = map[string]*vfsDotLocker{}
+	vfsDotLocksMtx sync.Mutex
+)
+
+type vfsDotLocker struct {
+	shared   int      // +checklocks:vfsDotLocksMtx
+	pending  *os.File // +checklocks:vfsDotLocksMtx
+	reserved *os.File // +checklocks:vfsDotLocksMtx
+}
+
+func osGetSharedLock(file *os.File) _ErrorCode {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		err := os.Mkdir(name+".lock", 0777)
+		if errors.Is(err, fs.ErrExist) {
+			return _BUSY // Another process has the lock.
+		}
+		if err != nil {
+			return _IOERR_LOCK
+		}
+		locker = &vfsDotLocker{}
+		vfsDotLocks[name] = locker
+	}
+
+	if locker.pending != nil {
+		return _BUSY
+	}
+	locker.shared++
+	return _OK
+}
+
+func osGetReservedLock(file *os.File) _ErrorCode {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		return _IOERR_LOCK
+	}
+
+	if locker.reserved != nil && locker.reserved != file {
+		return _BUSY
+	}
+	locker.reserved = file
+	return _OK
+}
+
+func osGetExclusiveLock(file *os.File, _ *LockLevel) _ErrorCode {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		return _IOERR_LOCK
+	}
+
+	if locker.pending != nil && locker.pending != file {
+		return _BUSY
+	}
+	locker.pending = file
+	if locker.shared > 1 {
+		return _BUSY
+	}
+	return _OK
+}
+
+func osDowngradeLock(file *os.File, _ LockLevel) _ErrorCode {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		return _IOERR_UNLOCK
+	}
+
+	if locker.reserved == file {
+		locker.reserved = nil
+	}
+	if locker.pending == file {
+		locker.pending = nil
+	}
+	return _OK
+}
+
+func osReleaseLock(file *os.File, state LockLevel) _ErrorCode {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		return _IOERR_UNLOCK
+	}
+
+	if locker.shared == 1 {
+		err := os.Remove(name + ".lock")
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			return _IOERR_UNLOCK
+		}
+		delete(vfsDotLocks, name)
+	}
+
+	if locker.reserved == file {
+		locker.reserved = nil
+	}
+	if locker.pending == file {
+		locker.pending = nil
+	}
+	locker.shared--
+	return _OK
+}
+
+func osCheckReservedLock(file *os.File) (bool, _ErrorCode) {
+	vfsDotLocksMtx.Lock()
+	defer vfsDotLocksMtx.Unlock()
+
+	name := file.Name()
+	locker := vfsDotLocks[name]
+	if locker == nil {
+		return false, _OK
+	}
+	return locker.reserved != nil, _OK
+}

vfs/os_ofd.go

@@ -1,4 +1,4 @@
-//go:build (linux || darwin) && !(sqlite3_flock || sqlite3_nosys)
+//go:build (linux || darwin) && !(sqlite3_flock || sqlite3_dotlk || sqlite3_nosys)
 
 package vfs
 

vfs/os_windows.go

@@ -1,4 +1,4 @@
-//go:build !sqlite3_nosys
+//go:build !(sqlite3_dotlk || sqlite3_nosys)
 
 package vfs
 

vfs/shm.go

@@ -1,4 +1,4 @@
-//go:build (darwin || linux || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !(sqlite3_noshm || sqlite3_nosys)
+//go:build (darwin || linux || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !(sqlite3_dotlk || sqlite3_noshm || sqlite3_nosys)
 
 package vfs
 

vfs/shm_bsd.go

@@ -1,4 +1,4 @@
-//go:build (freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !(sqlite3_noshm || sqlite3_nosys)
+//go:build (freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) && (386 || arm || amd64 || arm64 || riscv64 || ppc64le) && !(sqlite3_dotlk || sqlite3_noshm || sqlite3_nosys)
 
 package vfs
 

vfs/shm_other.go

@@ -1,4 +1,4 @@
-//go:build !(darwin || linux || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) || !(386 || arm || amd64 || arm64 || riscv64 || ppc64le) || sqlite3_noshm || sqlite3_nosys
+//go:build !(darwin || linux || freebsd || openbsd || netbsd || dragonfly || illumos || sqlite3_flock) || !(386 || arm || amd64 || arm64 || riscv64 || ppc64le) || sqlite3_dotlk || sqlite3_noshm || sqlite3_nosys
 
 package vfs