Locking improvements.

Author: ncruces

go.mod

@@ -6,6 +6,7 @@ toolchain go1.23.0
 
 require (
 	github.com/dchest/siphash v1.2.3
+	github.com/google/uuid v1.6.0
 	github.com/ncruces/julianday v1.0.0
 	github.com/ncruces/sort v0.1.2
 	github.com/psanford/httpreadat v0.1.0
@@ -17,6 +18,4 @@ require (
 	lukechampine.com/adiantum v1.1.1
 )
 
-require github.com/google/uuid v1.6.0
-
 retract v0.4.0 // tagged from the wrong branch

vfs/lock.go

@@ -75,19 +75,7 @@ func (f *vfsFile) Lock(lock LockLevel) error {
 		if f.lock <= LOCK_NONE || f.lock >= LOCK_EXCLUSIVE {
 			panic(util.AssertErr())
 		}
-		reserved := f.lock == LOCK_RESERVED
-		// A PENDING lock is needed before acquiring an EXCLUSIVE lock.
-		if f.lock < LOCK_PENDING {
-			// If we're already RESERVED, we can block indefinitely,
-			// since only incoming readers may briefly hold the PENDING lock.
-			if rc := osGetPendingLock(f.File, reserved /* block */); rc != _OK {
-				return rc
-			}
-			f.lock = LOCK_PENDING
-		}
-		// We are now PENDING, so we're just waiting for readers to leave.
-		// If we were RESERVED, we can block for a bit before invoking the busy handler.
-		if rc := osGetExclusiveLock(f.File, reserved /* block */); rc != _OK {
+		if rc := osGetExclusiveLock(f.File, &f.lock); rc != _OK {
 			return rc
 		}
 		f.lock = LOCK_EXCLUSIVE

vfs/os_bsd.go

@@ -4,31 +4,15 @@ package vfs
 
 import (
 	"os"
-	"time"
 
 	"golang.org/x/sys/unix"
 )
 
-func osUnlock(file *os.File, start, len int64) _ErrorCode {
-	if start == 0 && len == 0 {
-		err := unix.Flock(int(file.Fd()), unix.LOCK_UN)
-		if err != nil {
-			return _IOERR_UNLOCK
-		}
-	}
-	return _OK
-}
-
-func osLock(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, _ /*timeout*/ time.Duration) _ErrorCode {
+func osGetSharedLock(file *os.File) _ErrorCode {
 	return osLock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
 }
 
-func osWriteLock(file *os.File, _ /*start*/, _ /*len*/ int64, _ /*timeout*/ time.Duration) _ErrorCode {
+func osGetReservedLock(file *os.File) _ErrorCode {
 	rc := osLock(file, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK)
 	if rc == _BUSY {
 		// The documentation states the lock is upgraded by releasing the previous lock,
@@ -38,3 +22,40 @@ func osWriteLock(file *os.File, _ /*start*/, _ /*len*/ int64, _ /*timeout*/ time
 	}
 	return rc
 }
+
+func osGetExclusiveLock(file *os.File, state *LockLevel) _ErrorCode {
+	if *state >= LOCK_RESERVED {
+		return _OK
+	}
+	return osGetReservedLock(file)
+}
+
+func osDowngradeLock(file *os.File, _ LockLevel) _ErrorCode {
+	rc := osLock(file, unix.LOCK_SH|unix.LOCK_NB, _IOERR_RDLOCK)
+	if rc == _BUSY {
+		// The documentation states the lock is upgraded by releasing the previous lock,
+		// then acquiring the new lock.
+		// This is a race, so return IOERR_RDLOCK to ensure the transaction is aborted.
+		return _IOERR_RDLOCK
+	}
+	return _OK
+}
+
+func osReleaseLock(file *os.File, _ LockLevel) _ErrorCode {
+	err := unix.Flock(int(file.Fd()), unix.LOCK_UN)
+	if err != nil {
+		return _IOERR_UNLOCK
+	}
+	return _OK
+}
+
+func osCheckReservedLock(file *os.File) (bool, _ErrorCode) {
+	// Test the RESERVED lock.
+	lock, rc := osTestLock(file, _RESERVED_BYTE, 1)
+	return lock == unix.F_WRLCK, rc
+}
+
+func osLock(file *os.File, how int, def _ErrorCode) _ErrorCode {
+	err := unix.Flock(int(file.Fd()), how)
+	return osLockErrorCode(err, def)
+}

vfs/os_ofd.go

@@ -0,0 +1,59 @@
+//go:build (linux || darwin) && !(sqlite3_flock || sqlite3_nosys)
+
+package vfs
+
+import (
+	"os"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func osGetSharedLock(file *os.File) _ErrorCode {
+	// Test the PENDING lock before acquiring a new SHARED lock.
+	if lock, _ := osTestLock(file, _PENDING_BYTE, 1); lock == unix.F_WRLCK {
+		return _BUSY
+	}
+	// Acquire the SHARED lock.
+	return osReadLock(file, _SHARED_FIRST, _SHARED_SIZE, 0)
+}
+
+func osGetReservedLock(file *os.File) _ErrorCode {
+	// Acquire the RESERVED lock.
+	return osWriteLock(file, _RESERVED_BYTE, 1, 0)
+}
+
+func osGetExclusiveLock(file *os.File, state *LockLevel) _ErrorCode {
+	if *state == LOCK_RESERVED {
+		// A PENDING lock is needed before acquiring an EXCLUSIVE lock.
+		if rc := osWriteLock(file, _PENDING_BYTE, 1, -1); rc != _OK {
+			return rc
+		}
+		*state = LOCK_PENDING
+	}
+	// Acquire the EXCLUSIVE lock.
+	return osWriteLock(file, _SHARED_FIRST, _SHARED_SIZE, time.Millisecond)
+}
+
+func osDowngradeLock(file *os.File, state LockLevel) _ErrorCode {
+	if state >= LOCK_EXCLUSIVE {
+		// Downgrade to a SHARED lock.
+		if rc := osReadLock(file, _SHARED_FIRST, _SHARED_SIZE, 0); rc != _OK {
+			// notest // this should never happen
+			return _IOERR_RDLOCK
+		}
+	}
+	// Release the PENDING and RESERVED locks.
+	return osUnlock(file, _PENDING_BYTE, 2)
+}
+
+func osReleaseLock(file *os.File, _ LockLevel) _ErrorCode {
+	// Release all locks.
+	return osUnlock(file, 0, 0)
+}
+
+func osCheckReservedLock(file *os.File) (bool, _ErrorCode) {
+	// Test the RESERVED lock.
+	lock, rc := osTestLock(file, _RESERVED_BYTE, 1)
+	return lock == unix.F_WRLCK, rc
+}

vfs/os_unix.go

@@ -31,3 +31,41 @@ func osSetMode(file *os.File, modeof string) error {
 	}
 	return nil
 }
+
+func osTestLock(file *os.File, start, len int64) (int16, _ErrorCode) {
+	lock := unix.Flock_t{
+		Type:  unix.F_WRLCK,
+		Start: start,
+		Len:   len,
+	}
+	if unix.FcntlFlock(file.Fd(), unix.F_GETLK, &lock) != nil {
+		return 0, _IOERR_CHECKRESERVEDLOCK
+	}
+	return lock.Type, _OK
+}
+
+func osLockErrorCode(err error, def _ErrorCode) _ErrorCode {
+	if err == nil {
+		return _OK
+	}
+	if errno, ok := err.(unix.Errno); ok {
+		switch errno {
+		case
+			unix.EACCES,
+			unix.EAGAIN,
+			unix.EBUSY,
+			unix.EINTR,
+			unix.ENOLCK,
+			unix.EDEADLK,
+			unix.ETIMEDOUT:
+			return _BUSY
+		case unix.EPERM:
+			return _PERM
+		}
+		// notest // usually EWOULDBLOCK == EAGAIN
+		if errno == unix.EWOULDBLOCK && unix.EWOULDBLOCK != unix.EAGAIN {
+			return _BUSY
+		}
+	}
+	return def
+}

vfs/os_unix_lock.go

@@ -28,27 +28,25 @@ func osGetReservedLock(file *os.File) _ErrorCode {
 	return osWriteLock(file, _RESERVED_BYTE, 1, 0)
 }
 
-func osGetPendingLock(file *os.File, block bool) _ErrorCode {
-	var timeout time.Duration
-	if block {
-		timeout = -1
-	}
-
-	// Acquire the PENDING lock.
-	return osWriteLock(file, _PENDING_BYTE, 1, timeout)
-}
-
-func osGetExclusiveLock(file *os.File, block bool) _ErrorCode {
-	var timeout time.Duration
-	if block {
-		timeout = time.Millisecond
+func osGetExclusiveLock(file *os.File, state *LockLevel) _ErrorCode {
+	// A PENDING lock is needed before releasing the SHARED lock.
+	if *state < LOCK_PENDING {
+		// If we were RESERVED, we can block indefinitely.
+		var timeout time.Duration
+		if *state == LOCK_RESERVED {
+			timeout = -1
+		}
+		if rc := osWriteLock(file, _PENDING_BYTE, 1, timeout); rc != _OK {
+			return rc
+		}
+		*state = LOCK_PENDING
 	}
 
 	// Release the SHARED lock.
 	osUnlock(file, _SHARED_FIRST, _SHARED_SIZE)
 
 	// Acquire the EXCLUSIVE lock.
-	rc := osWriteLock(file, _SHARED_FIRST, _SHARED_SIZE, timeout)
+	rc := osWriteLock(file, _SHARED_FIRST, _SHARED_SIZE, time.Millisecond)
 
 	if rc != _OK {
 		// Reacquire the SHARED lock.
@@ -64,9 +62,7 @@ func osDowngradeLock(file *os.File, state LockLevel) _ErrorCode {
 
 		// Reacquire the SHARED lock.
 		if rc := osReadLock(file, _SHARED_FIRST, _SHARED_SIZE, 0); rc != _OK {
-			// This should never happen.
-			// We should always be able to reacquire the read lock.
-			// notest
+			// notest // this should never happen
 			return _IOERR_RDLOCK
 		}
 	}

vfs/os_windows.go

@@ -70,7 +70,7 @@ func (s *vfsShm) shmOpen() _ErrorCode {
 	}
 
 	// Dead man's switch.
-	if lock, rc := osGetLock(s.File, _SHM_DMS, 1); rc != _OK {
+	if lock, rc := osTestLock(s.File, _SHM_DMS, 1); rc != _OK {
 		return _IOERR_LOCK
 	} else if lock == unix.F_WRLCK {
 		return _BUSY