Add per-inode write mutex

At the moment, FUSE writes to a single file are serialized by the kernel.
However, it is unclear if this is guaranteed behaviour or may change
in the future.

This patch adds our own per-inode write lock to rule out races regardless
of kernel behavoir.

pathfs_frontend/file.go

@@ -43,6 +43,7 @@ type file struct {
 func NewFile(fd *os.File, writeOnly bool, cfs *cryptfs.CryptFS) nodefs.File {
 	var st syscall.Stat_t
 	syscall.Fstat(int(fd.Fd()), &st)
+	wlock.register(st.Ino)
 
 	return &file{
 		fd:        fd,
@@ -59,20 +60,6 @@ func (f *file) InnerFile() nodefs.File {
 func (f *file) SetInode(n *nodefs.Inode) {
 }
 
-// Ensure that all modifications to the file contents are serialized and no
-// reads happen concurrently.
-//
-// This prevents several races:
-// * getFileId vs Truncate
-// * zeroPad vs Read
-// * RMW vs Write
-func (f *file) wlock() {
-}
-func (f *file) rlock() {
-}
-func (f *file) unlock() {
-}
-
 // readHeader - load the file header from disk
 //
 // Returns io.EOF if the file is empty
@@ -252,6 +239,7 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 			cryptfs.Debug.Printf("len(oldData)=%d len(blockData)=%d", len(oldData), len(blockData))
 		}
 
+		// Encrypt
 		blockOffset, blockLen := b.CiphertextRange()
 		blockData = f.cfs.EncryptBlock(blockData, b.BlockNo, f.header.Id)
 		cryptfs.Debug.Printf("ino%d: Writing %d bytes to block #%d",
@@ -271,6 +259,7 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 		f.fdLock.Lock()
 		_, err = f.fd.WriteAt(blockData, int64(blockOffset))
 		f.fdLock.Unlock()
+
 		if err != nil {
 			cryptfs.Warn.Printf("doWrite: Write failed: %s", err.Error())
 			status = fuse.ToStatus(err)
@@ -284,6 +273,8 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 // Write - FUSE call
 func (f *file) Write(data []byte, off int64) (uint32, fuse.Status) {
 	cryptfs.Debug.Printf("ino%d: FUSE Write: offset=%d length=%d", f.ino, off, len(data))
+	wlock.lock(f.ino)
+	defer wlock.unlock(f.ino)
 
 	fi, err := f.fd.Stat()
 	if err != nil {
@@ -306,6 +297,7 @@ func (f *file) Release() {
 	f.fdLock.Lock()
 	f.fd.Close()
 	f.fdLock.Unlock()
+	wlock.unregister(f.ino)
 }
 
 // Flush - FUSE call
@@ -333,7 +325,11 @@ func (f *file) Fsync(flags int) (code fuse.Status) {
 	return r
 }
 
+// Truncate - FUSE call
 func (f *file) Truncate(newSize uint64) fuse.Status {
+	wlock.lock(f.ino)
+	defer wlock.unlock(f.ino)
+
 	// Common case first: Truncate to zero
 	if newSize == 0 {
 		f.fdLock.Lock()
@@ -343,7 +339,7 @@ func (f *file) Truncate(newSize uint64) fuse.Status {
 			cryptfs.Warn.Printf("Ftruncate(fd, 0) returned error: %v", err)
 			return fuse.ToStatus(err)
 		}
-		// A truncate to zero kills the file header
+		// Truncate to zero kills the file header
 		f.header = nil
 		return fuse.OK
 	}

pathfs_frontend/write_lock.go

@@ -0,0 +1,66 @@
+package pathfs_frontend
+
+import (
+	"sync"
+)
+
+func init() {
+	wlock.m = make(map[uint64]*refCntMutex)
+}
+
+// wlock - serializes write accesses to each file (identified by inode number)
+// Writing partial blocks means we have to do read-modify-write cycles. We
+// really don't want concurrent writes there.
+// Concurrent full-block writes could actually be allowed, but are not to
+// keep the locking simple.
+var wlock wlockMap
+
+// wlockMap - usage:
+// 1) register
+// 2) lock ... unlock ...
+// 3) unregister
+type wlockMap struct {
+	mapMutex sync.RWMutex
+	m        map[uint64]*refCntMutex
+}
+
+func (w *wlockMap) register(ino uint64) {
+	w.mapMutex.Lock()
+	r := w.m[ino]
+	if r == nil {
+		r = &refCntMutex{}
+		w.m[ino] = r
+	}
+	r.refCnt++ // this must happen inside the mapMutex lock
+	w.mapMutex.Unlock()
+}
+
+func (w *wlockMap) unregister(ino uint64) {
+	w.mapMutex.Lock()
+	r := w.m[ino]
+	r.refCnt--
+	if r.refCnt == 0 {
+		delete(w.m, ino)
+	}
+	w.mapMutex.Unlock()
+}
+
+func (w *wlockMap) lock(ino uint64) {
+	w.mapMutex.RLock()
+	r := w.m[ino]
+	w.mapMutex.RUnlock()
+	r.Lock() // this can take a long time - execute outside the mapMutex lock
+}
+
+func (w *wlockMap) unlock(ino uint64) {
+	w.mapMutex.RLock()
+	r := w.m[ino]
+	w.mapMutex.RUnlock()
+	r.Unlock()
+}
+
+// refCntMutex - mutex with reference count
+type refCntMutex struct {
+	sync.Mutex
+	refCnt int
+}