Improve Wal locking on BSD (#204)

Author: ncruces

README.md

@@ -74,7 +74,7 @@ This project aims for [high test coverage](https://github.com/ncruces/go-sqlite3
 It also benefits greatly from [SQLite's](https://sqlite.org/testing.html) and
 [wazero's](https://tetrate.io/blog/introducing-wazero-from-tetrate/#:~:text=Rock%2Dsolid%20test%20approach) thorough testing.
 
-Every commit is [tested](https://github.com/ncruces/go-sqlite3/wiki/Test-matrix) on
+Every commit is [tested](https://github.com/ncruces/go-sqlite3/wiki/Support-matrix) on
 Linux (amd64/arm64/386/riscv64/ppc64le/s390x), macOS (amd64/arm64),
 Windows (amd64), FreeBSD (amd64), OpenBSD (amd64), NetBSD (amd64),
 DragonFly BSD (amd64), illumos (amd64), and Solaris (amd64).

vfs/README.md

@@ -48,11 +48,6 @@ On Unix, this package may use `mmap` to implement
 [shared-memory for the WAL-index](https://sqlite.org/wal.html#implementation_of_shared_memory_for_the_wal_index),
 like SQLite.
 
-With [BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2)
-a WAL database can only be accessed by a single proccess.
-Other processes that attempt to access a database locked with BSD locks,
-will fail with the [`SQLITE_PROTOCOL`](https://sqlite.org/rescode.html#protocol) error code.
-
 On Windows, this package may use `MapViewOfFile`, like SQLite.
 
 You can also opt into a cross-platform, in-process, memory sharing implementation

vfs/os_bsd.go

@@ -9,11 +9,11 @@ import (
 )
 
 func osGetSharedLock(file *os.File) _ErrorCode {
-	return osLock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
+	return osFlock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
 }
 
 func osGetReservedLock(file *os.File) _ErrorCode {
-	rc := osLock(file, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK)
+	rc := osFlock(file, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK)
 	if rc == _BUSY {
 		// The documentation states that a lock is upgraded by
 		// releasing the previous lock, then acquiring the new lock.
@@ -37,7 +37,7 @@ func osGetExclusiveLock(file *os.File, state *LockLevel) _ErrorCode {
 }
 
 func osDowngradeLock(file *os.File, _ LockLevel) _ErrorCode {
-	rc := osLock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
+	rc := osFlock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
 	if rc == _BUSY {
 		// The documentation states that a lock is downgraded by
 		// releasing the previous lock then acquiring the new lock.
@@ -66,7 +66,36 @@ func osCheckReservedLock(file *os.File) (bool, _ErrorCode) {
 	return lock == unix.F_WRLCK, rc
 }
 
-func osLock(file *os.File, how int, def _ErrorCode) _ErrorCode {
+func osFlock(file *os.File, how int, def _ErrorCode) _ErrorCode {
 	err := unix.Flock(int(file.Fd()), how)
 	return osLockErrorCode(err, def)
 }
+
+func osReadLock(file *os.File, start, len int64) _ErrorCode {
+	return osLock(file, unix.F_RDLCK, start, len, _IOERR_RDLOCK)
+}
+
+func osWriteLock(file *os.File, start, len int64) _ErrorCode {
+	return osLock(file, unix.F_WRLCK, start, len, _IOERR_LOCK)
+}
+
+func osLock(file *os.File, typ int16, start, len int64, def _ErrorCode) _ErrorCode {
+	err := unix.FcntlFlock(file.Fd(), unix.F_SETLK, &unix.Flock_t{
+		Type:  typ,
+		Start: start,
+		Len:   len,
+	})
+	return osLockErrorCode(err, def)
+}
+
+func osUnlock(file *os.File, start, len int64) _ErrorCode {
+	err := unix.FcntlFlock(file.Fd(), unix.F_SETLK, &unix.Flock_t{
+		Type:  unix.F_UNLCK,
+		Start: start,
+		Len:   len,
+	})
+	if err != nil {
+		return _IOERR_UNLOCK
+	}
+	return _OK
+}

vfs/shm_bsd.go

@@ -4,7 +4,9 @@ package vfs
 
 import (
 	"context"
+	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"sync"
 
@@ -71,23 +73,16 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 		return _OK
 	}
 
-	// Always open file read-write, as it will be shared.
-	f, err := os.OpenFile(s.path,
-		os.O_RDWR|os.O_CREATE|_O_NOFOLLOW, 0666)
-	if err != nil {
-		return _CANTOPEN
-	}
-	// Closes file if it's not nil.
-	defer func() { f.Close() }()
+	vfsShmListMtx.Lock()
+	defer vfsShmListMtx.Unlock()
 
-	fi, err := f.Stat()
-	if err != nil {
+	// Stat file without opening it.
+	// Closing it would release all POSIX locks on it.
+	fi, err := os.Stat(s.path)
+	if err != nil && !errors.Is(err, fs.ErrNotExist) {
 		return _IOERR_FSTAT
 	}
 
-	vfsShmListMtx.Lock()
-	defer vfsShmListMtx.Unlock()
-
 	// Find a shared file, increase the reference count.
 	for _, g := range vfsShmList {
 		if g != nil && os.SameFile(fi, g.info) {
@@ -97,13 +92,35 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 		}
 	}
 
-	// Lock and truncate the file.
-	// The lock is only released by closing the file.
-	if rc := osLock(f, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK); rc != _OK {
+	// Always open file read-write, as it will be shared.
+	f, err := os.OpenFile(s.path,
+		os.O_RDWR|os.O_CREATE|_O_NOFOLLOW, 0666)
+	if err != nil {
+		return _CANTOPEN
+	}
+	// Closes file if it's not nil.
+	defer func() { f.Close() }()
+
+	// Dead man's switch.
+	if lock, rc := osTestLock(f, _SHM_DMS, 1); rc != _OK {
+		return _IOERR_LOCK
+	} else if lock == unix.F_WRLCK {
+		return _BUSY
+	} else if lock == unix.F_UNLCK {
+		if rc := osWriteLock(f, _SHM_DMS, 1); rc != _OK {
+			return rc
+		}
+		if err := f.Truncate(0); err != nil {
+			return _IOERR_SHMOPEN
+		}
+	}
+	if rc := osReadLock(f, _SHM_DMS, 1); rc != _OK {
 		return rc
 	}
-	if err := f.Truncate(0); err != nil {
-		return _IOERR_SHMOPEN
+
+	fi, err = f.Stat()
+	if err != nil {
+		return _IOERR_FSTAT
 	}
 
 	// Add the new shared file.
@@ -157,7 +174,30 @@ func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, ext
 func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) _ErrorCode {
 	s.Lock()
 	defer s.Unlock()
-	return s.shmMemLock(offset, n, flags)
+
+	// Check if we could obtain/release the lock locally.
+	rc := s.shmMemLock(offset, n, flags)
+	if rc != _OK {
+		return rc
+	}
+
+	// Obtain/release the appropriate file lock.
+	switch {
+	case flags&_SHM_UNLOCK != 0:
+		return osUnlock(s.File, _SHM_BASE+int64(offset), int64(n))
+	case flags&_SHM_SHARED != 0:
+		rc = osReadLock(s.File, _SHM_BASE+int64(offset), int64(n))
+	case flags&_SHM_EXCLUSIVE != 0:
+		rc = osWriteLock(s.File, _SHM_BASE+int64(offset), int64(n))
+	default:
+		panic(util.AssertErr())
+	}
+
+	// Release the local lock.
+	if rc != _OK {
+		s.shmMemLock(offset, n, flags^(_SHM_UNLOCK|_SHM_LOCK))
+	}
+	return rc
 }
 
 func (s *vfsShm) shmUnmap(delete bool) {