Call lseek(0, SEEK_CUR) unconditionally in runsc fsgofer's Readdir(offset=0).

9P2000.L is silent as to how readdir RPCs interact with directory mutation. The
most performant option is for Treaddir with offset=0 to restart iteration,
avoiding needing to walk+open+clunk a new directory fid between invocations of
getdents64(2), and the VFS2 gofer client assumes this is the case. Make this
actually true for the runsc fsgofer.

Fixes #3344, #3345, #3355

PiperOrigin-RevId: 324090384

1 files changed, 6 insertions, 3 deletions

diff --git a/runsc/fsgofer/fsgofer.go b/runsc/fsgofer/fsgofer.go
index ebefeacf2..c6694c278 100644
--- a/runsc/fsgofer/fsgofer.go
+++ b/runsc/fsgofer/fsgofer.go
@@ -979,9 +979,12 @@ func (l *localFile) Readdir(offset uint64, count uint32) ([]p9.Dirent, error) {
 
 	skip := uint64(0)
 
-	// Check if the file is at the correct position already. If not, seek to the
-	// beginning and read the entire directory again.
-	if l.lastDirentOffset != offset {
+	// Check if the file is at the correct position already. If not, seek to
+	// the beginning and read the entire directory again. We always seek if
+	// offset is 0, since this is side-effectual (equivalent to rewinddir(3),
+	// which causes the directory stream to resynchronize with the directory's
+	// current contents).
+	if l.lastDirentOffset != offset || offset == 0 {
 		if _, err := syscall.Seek(l.file.FD(), 0, 0); err != nil {
 			return nil, extractErrno(err)
 		}