1 files changed, 51 insertions, 0 deletions
diff --git a/internal/fusefrontend/file_lock.go b/internal/fusefrontend/file_lock.go
new file mode 100644
index 0000000..c2965ae
--- /dev/null
+++ b/internal/fusefrontend/file_lock.go
@@ -0,0 +1,51 @@
+package fusefrontend
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
+	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
+)
+
+// SharedStorageLock conveniently wraps F_OFD_SETLKW.
+// It is a no-op unless args.SharedStorage is set.
+//
+// See https://man7.org/linux/man-pages/man2/fcntl.2.html -> "Open file description locks (non-POSIX)"
+//
+// lkType is one of:
+// * unix.F_RDLCK (shared read lock)
+// * unix.F_WRLCK (exclusive write lock)
+// * unix.F_UNLCK (unlock)
+//
+// This function is a no-op if args.SharedStorage == false.
+func (f *File) LockSharedStorage(lkType int16, lkStart int64, lkLen int64) (err error) {
+	if !f.rootNode.args.SharedStorage {
+		return nil
+	}
+	lk := unix.Flock_t{
+		Type:   lkType,
+		Whence: unix.SEEK_SET,
+		Start:  lkStart,
+		Len:    lkLen,
+	}
+	err = unix.FcntlFlock(uintptr(f.intFd()), syscallcompat.F_OFD_SETLK, &lk)
+	switch err {
+	case unix.EACCES, unix.EAGAIN:
+		tlog.Debug.Printf("LockSharedStorage: waiting for lock")
+	case nil:
+		return
+	}
+	for {
+		err = unix.FcntlFlock(uintptr(f.intFd()), syscallcompat.F_OFD_SETLKW, &lk)
+		if err == unix.EINTR {
+			tlog.Debug.Printf("LockSharedStorage: looping on EINTR")
+			continue
+		}
+		return
+	}
+}
+
+// UnlockSharedStorage calls LockSharedStorage with unix.F_UNLCK.
+func (f *File) UnlockSharedStorage(lkStart int64, lkLen int64) error {
+	return f.LockSharedStorage(unix.F_UNLCK, lkStart, lkLen)
+}