fusefronted: disallow writes running concurrently with reads

As uncovered by xfstests generic/465, concurrent reads and writes
could lead to this,

  doRead 3015532: corrupt block #1039: stupidgcm: message authentication failed,

as the read could pick up a block that has not yet been completely written -
write() is not atomic!

Now writes take ContentLock exclusively, while reads take it shared,
meaning that multiple reads can run in parallel with each other, but
not with a write.

This also simplifies the file header locking.

3 files changed, 45 insertions, 65 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index ffc41c6..32f615b 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -145,39 +145,34 @@ func (f *File) createHeader() (fileID []byte, err error) {
 //
 // Called by Read() for normal reading,
 // by Write() and Truncate() via doWrite() for Read-Modify-Write.
-//
-// doWrite() uses nolock=true because it makes sure the ID is in the cache and
-// HeaderLock is locked before calling doRead.
-func (f *File) doRead(dst []byte, off uint64, length uint64, nolock bool) ([]byte, fuse.Status) {
-	if !nolock {
-		// Make sure we have the file ID.
-		f.fileTableEntry.HeaderLock.RLock()
-		if f.fileTableEntry.ID == nil {
-			f.fileTableEntry.HeaderLock.RUnlock()
-			// Yes, somebody else may take the lock before we can. This will get
-			// the header read twice, but causes no harm otherwise.
-			f.fileTableEntry.HeaderLock.Lock()
-			tmpID, err := f.readFileID()
+func (f *File) doRead(dst []byte, off uint64, length uint64) ([]byte, fuse.Status) {
+	// Get the file ID, either from the open file table, or from disk.
+	var fileID []byte
+	f.fileTableEntry.IDLock.Lock()
+	if f.fileTableEntry.ID != nil {
+		// Use the cached value in the file table
+		fileID = f.fileTableEntry.ID
+	} else {
+		// Not cached, we have to read it from disk.
+		var err error
+		fileID, err = f.readFileID()
+		if err != nil {
+			f.fileTableEntry.IDLock.Unlock()
 			if err == io.EOF {
-				f.fileTableEntry.HeaderLock.Unlock()
+				// Empty file
 				return nil, fuse.OK
 			}
 			if err != nil {
-				f.fileTableEntry.HeaderLock.Unlock()
 				tlog.Warn.Printf("doRead %d: corrupt header: %v", f.qIno.Ino, err)
 				return nil, fuse.EIO
 			}
-			f.fileTableEntry.ID = tmpID
-			// Downgrade the lock.
-			f.fileTableEntry.HeaderLock.Unlock()
-			// The file ID may change in here. This does no harm because we
-			// re-read it after the RLock().
-			f.fileTableEntry.HeaderLock.RLock()
 		}
+		// Save into the file table
+		f.fileTableEntry.ID = fileID
 	}
-	fileID := f.fileTableEntry.ID
+	f.fileTableEntry.IDLock.Unlock()
 	if fileID == nil {
-		log.Panicf("filedID=%v, nolock=%v", fileID, nolock)
+		log.Panicf("fileID=%v", fileID)
 	}
 	// Read the backing ciphertext in one go
 	blocks := f.contentEnc.ExplodePlainRange(off, length)
@@ -189,10 +184,6 @@ func (f *File) doRead(dst []byte, off uint64, length uint64, nolock bool) ([]byt
 	ciphertext := f.fs.contentEnc.CReqPool.Get()
 	ciphertext = ciphertext[:int(alignedLength)]
 	n, err := f.fd.ReadAt(ciphertext, int64(alignedOffset))
-	// We don't care if the file ID changes after we have read the data. Drop the lock.
-	if !nolock {
-		f.fileTableEntry.HeaderLock.RUnlock()
-	}
 	if err != nil && err != io.EOF {
 		tlog.Warn.Printf("read: ReadAt: %s", err.Error())
 		return nil, fuse.ToStatus(err)
@@ -251,11 +242,14 @@ func (f *File) Read(buf []byte, off int64) (resultData fuse.ReadResult, code fus
 	f.fdLock.RLock()
 	defer f.fdLock.RUnlock()
 
+	f.fileTableEntry.ContentLock.RLock()
+	defer f.fileTableEntry.ContentLock.RUnlock()
+
 	tlog.Debug.Printf("ino%d: FUSE Read: offset=%d length=%d", f.qIno.Ino, len(buf), off)
 	if f.fs.args.SerializeReads {
 		serialize_reads.Wait(off, len(buf))
 	}
-	out, status := f.doRead(buf[:0], uint64(off), uint64(len(buf)), false)
+	out, status := f.doRead(buf[:0], uint64(off), uint64(len(buf)))
 	if f.fs.args.SerializeReads {
 		serialize_reads.Done()
 	}
@@ -277,31 +271,25 @@ func (f *File) Read(buf []byte, off int64) (resultData fuse.ReadResult, code fus
 // Empty writes do nothing and are allowed.
 func (f *File) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 	fileWasEmpty := false
-	// If the file ID is not cached, read it from disk
-	if f.fileTableEntry.ID == nil {
-		// Block other readers while we mess with the file header. Other writers
-		// are blocked by ContentLock already.
-		f.fileTableEntry.HeaderLock.Lock()
-		tmpID, err := f.readFileID()
+	// Get the file ID, create a new one if it does not exist yet.
+	var fileID []byte
+	// The caller has exclusively locked ContentLock, which blocks all other
+	// readers and writers. No need to take IDLock.
+	if f.fileTableEntry.ID != nil {
+		fileID = f.fileTableEntry.ID
+	} else {
+		// If the file ID is not cached, read it from disk
+		var err error
+		fileID, err = f.readFileID()
 		// Write a new file header if the file is empty
 		if err == io.EOF {
-			tmpID, err = f.createHeader()
+			fileID, err = f.createHeader()
 			fileWasEmpty = true
 		}
 		if err != nil {
-			f.fileTableEntry.HeaderLock.Unlock()
 			return 0, fuse.ToStatus(err)
 		}
-		f.fileTableEntry.ID = tmpID
-		if fileWasEmpty {
-			// The file was empty and we wrote a new file header. Keep the lock
-			// as we might have to kill the file header again if the data write
-			// fails.
-			defer f.fileTableEntry.HeaderLock.Unlock()
-		} else {
-			// We won't touch the header again, drop the lock immediately.
-			f.fileTableEntry.HeaderLock.Unlock()
-		}
+		f.fileTableEntry.ID = fileID
 	}
 	// Handle payload data
 	dataBuf := bytes.NewBuffer(data)
@@ -312,7 +300,7 @@ func (f *File) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 		// Incomplete block -> Read-Modify-Write
 		if b.IsPartial() {
 			// Read
-			oldData, status := f.doRead(nil, b.BlockPlainOff(), f.contentEnc.PlainBS(), true)
+			oldData, status := f.doRead(nil, b.BlockPlainOff(), f.contentEnc.PlainBS())
 			if status != fuse.OK {
 				tlog.Warn.Printf("ino%d fh%d: RMW read failed: %s", f.qIno.Ino, f.intFd(), status.String())
 				return 0, status
@@ -382,9 +370,7 @@ func (f *File) Write(data []byte, off int64) (uint32, fuse.Status) {
 	f.fdLock.RLock()
 	defer f.fdLock.RUnlock()
 	if f.released {
-		// The file descriptor has been closed concurrently, which also means
-		// the wlock has been freed. Exit here so we don't crash trying to access
-		// it.
+		// The file descriptor has been closed concurrently
 		tlog.Warn.Printf("ino%d fh%d: Write on released file", f.qIno.Ino, f.intFd())
 		return 0, fuse.EBADF
 	}
diff --git a/internal/fusefrontend/file_allocate_truncate.go b/internal/fusefrontend/file_allocate_truncate.go
index 81ac8d3..34fe2c0 100644
--- a/internal/fusefrontend/file_allocate_truncate.go
+++ b/internal/fusefrontend/file_allocate_truncate.go
@@ -111,9 +111,7 @@ func (f *File) Truncate(newSize uint64) fuse.Status {
 			return fuse.ToStatus(err)
 		}
 		// Truncate to zero kills the file header
-		f.fileTableEntry.HeaderLock.Lock()
 		f.fileTableEntry.ID = nil
-		f.fileTableEntry.HeaderLock.Unlock()
 		return fuse.OK
 	}
 	// We need the old file size to determine if we are growing or shrinking
@@ -144,7 +142,7 @@ func (f *File) Truncate(newSize uint64) fuse.Status {
 	var data []byte
 	if lastBlockLen > 0 {
 		var status fuse.Status
-		data, status = f.doRead(nil, plainOff, lastBlockLen, false)
+		data, status = f.doRead(nil, plainOff, lastBlockLen)
 		if status != fuse.OK {
 			tlog.Warn.Printf("Truncate: shrink doRead returned error: %v", err)
 			return status
@@ -206,8 +204,6 @@ func (f *File) truncateGrowFile(oldPlainSz uint64, newPlainSz uint64) fuse.Statu
 	if newPlainSz%f.contentEnc.PlainBS() == 0 {
 		// The file was empty, so it did not have a header. Create one.
 		if oldPlainSz == 0 {
-			f.fileTableEntry.HeaderLock.Lock()
-			defer f.fileTableEntry.HeaderLock.Unlock()
 			id, err := f.createHeader()
 			if err != nil {
 				return fuse.ToStatus(err)
diff --git a/internal/openfiletable/open_file_table.go b/internal/openfiletable/open_file_table.go
index d70bcfa..e21c96d 100644
--- a/internal/openfiletable/open_file_table.go
+++ b/internal/openfiletable/open_file_table.go
@@ -57,18 +57,16 @@ type table struct {
 
 // Entry is an entry in the open file table
 type Entry struct {
-	// Reference count
+	// Reference count. Protected by the table lock.
 	refCount int
-	// ContentLock guards the file content from concurrent writes. Every writer
+	// ContentLock protects on-disk content from concurrent writes. Every writer
 	// must take this lock before modifying the file content.
 	ContentLock countingMutex
-	// HeaderLock guards the file ID (in this struct) and the file header (on
-	// disk). Take HeaderLock.RLock() to make sure the file ID does not change
-	// behind your back. If you modify the file ID, you must take
-	// HeaderLock.Lock().
-	HeaderLock sync.RWMutex
 	// ID is the file ID in the file header.
 	ID []byte
+	// IDLock must be taken before reading or writing the ID field in this struct,
+	// unless you have an exclusive lock on ContentLock.
+	IDLock sync.Mutex
 }
 
 // Register creates an open file table entry for "qi" (or incrementes the
@@ -101,15 +99,15 @@ func Unregister(qi QIno) {
 
 // countingMutex incrementes t.writeLockCount on each Lock() call.
 type countingMutex struct {
-	sync.Mutex
+	sync.RWMutex
 }
 
 func (c *countingMutex) Lock() {
-	c.Mutex.Lock()
+	c.RWMutex.Lock()
 	atomic.AddUint64(&t.writeOpCount, 1)
 }
 
-// WriteOpCount returns the write lock counter value. This value is encremented
+// WriteOpCount returns the write lock counter value. This value is incremented
 // each time writeLock.Lock() on a file table entry is called.
 func WriteOpCount() uint64 {
 	return atomic.LoadUint64(&t.writeOpCount)