fusefrontend: replace unreliable "fd < 0" check

... with the "released" boolean.

For some reason, the "f.fd.Fd() < 0" check did not work reliably,
leading to nil pointer panics on the following wlock.lock().

The problem was discovered during fsstress testing and is unlikely
to happen in normal operations.

With this change, we passed 1700+ fsstress iterations.

1 files changed, 15 insertions, 5 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index e374956..3fa5a48 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -6,6 +6,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"log"
 	"os"
 	"sync"
 	"syscall"
@@ -21,11 +22,15 @@ import (
 // File - based on loopbackFile in go-fuse/fuse/nodefs/files.go
 type file struct {
 	fd *os.File
-
+	// Has Release() already been called on this file? This also means that the
+	// wlock entry has been freed, so let's not crash trying to access it.
+	// Due to concurrency, Release can overtake other operations. These will
+	// return EBADF in that case.
+	released bool
 	// fdLock prevents the fd to be closed while we are in the middle of
 	// an operation.
 	// Every FUSE entrypoint should RLock(). The only user of Lock() is
-	// Release(), which closes the fd, hence has to acquire an exclusive lock.
+	// Release(), which closes the fd and sets "released" to true.
 	fdLock sync.RWMutex
 
 	// Was the file opened O_WRONLY?
@@ -280,8 +285,9 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 func (f *file) Write(data []byte, off int64) (uint32, fuse.Status) {
 	f.fdLock.RLock()
 	defer f.fdLock.RUnlock()
-	if f.fd.Fd() < 0 {
+	if f.released {
 		// The file descriptor has been closed concurrently.
+		toggledlog.Warn.Printf("ino%d fh%d: Write on released file", f.ino, f.intFd())
 		return 0, fuse.EBADF
 	}
 	wlock.lock(f.ino)
@@ -308,7 +314,11 @@ func (f *file) Write(data []byte, off int64) (uint32, fuse.Status) {
 // Release - FUSE call, close file
 func (f *file) Release() {
 	f.fdLock.Lock()
+	if f.released {
+		log.Panicf("ino%d fh%d: double release", f.ino, f.intFd())
+	}
 	f.fd.Close()
+	f.released = true
 	f.fdLock.Unlock()
 
 	wlock.unregister(f.ino)
@@ -342,9 +352,9 @@ func (f *file) Fsync(flags int) (code fuse.Status) {
 func (f *file) Truncate(newSize uint64) fuse.Status {
 	f.fdLock.RLock()
 	defer f.fdLock.RUnlock()
-	if f.fd.Fd() < 0 {
+	if f.released {
 		// The file descriptor has been closed concurrently.
-		toggledlog.Warn.Printf("ino%d fh%d: Truncate on forgotten file", f.ino, f.intFd())
+		toggledlog.Warn.Printf("ino%d fh%d: Truncate on released file", f.ino, f.intFd())
 		return fuse.EBADF
 	}
 	wlock.lock(f.ino)