cmd/mount2: fix NFS directory listings by supporting non-zero Seekdir offsets

Seekdir handled only a rewind to offset 0 and returned ENOTSUP otherwise.
The stateless kernel NFS server opens a fresh directory handle and seeks to
the last returned cookie on every readdir continuation, so any listing
spanning more than one readdir batch failed over NFS. dirStream is a
snapshot taken at Readdir time and go-fuse assigns each entry a sequential
offset, so seeking to off positions the stream at index off; off == 0 still
resets to the start, preserving the rewind/re-read behaviour.

Before: ls of a directory that spans more than one readdir batch failed
over NFS with "Unknown error 524".
After: it lists correctly.

Adds TestDirStreamSeekdir covering rewind, mid-stream resume and the EOF
clamp. The full NFS path was validated against a real Linux
nfs-kernel-server export over NFSv3, NFSv4.0 and NFSv4.2.

Fixes #9547

(cherry picked from commit 22aba81074)

cmd/mount2/node.go

@@ -271,16 +271,22 @@ func (ds *dirStream) Next() (de fuse.DirEntry, errno syscall.Errno) {
 func (ds *dirStream) Close() {
 }
 
-// Seekdir implements fusefs.FileSeekdirer so go-fuse can rewind the directory
-// stream when the kernel calls lseek(fd, 0, SEEK_SET) before a second getdents.
-// Without this, go-fuse returns ENOTSUP and ls returns empty on every call after
-// the first. See: https://github.com/hanwen/go-fuse/issues/549
+// Seekdir implements fusefs.FileSeekdirer so go-fuse can reposition the
+// directory stream. The kernel calls this both to rewind (lseek(fd, 0,
+// SEEK_SET) before a second getdents) and, for a kernel-NFS-exported mount,
+// to resume from a previously returned directory cookie: nfsd is stateless,
+// so it opens a fresh handle and seeks to the last offset on every readdir
+// continuation. Handling only offset 0 made go-fuse return ENOTSUP for those
+// resumes, so any listing spanning more than one readdir batch failed over NFS.
+//
+// dirStream is a snapshot taken at Readdir time and go-fuse assigns each entry
+// a sequential offset in Next() order (the entry yielded at index i carries
+// offset i+1), so repositioning to off means the next entry is the one at
+// index off. Values past the end clamp to EOF via HasNext.
+// See: https://github.com/hanwen/go-fuse/issues/549
 func (ds *dirStream) Seekdir(_ context.Context, off uint64) syscall.Errno {
-	if off == 0 {
-		ds.i = 0
-		return 0
-	}
-	return syscall.ENOTSUP
+	ds.i = int(off)
+	return 0
 }
 
 var _ fusefs.DirStream = (*dirStream)(nil)

cmd/mount2/node_test.go

@@ -0,0 +1,45 @@
+//go:build linux || (darwin && amd64)
+
+package mount2
+
+import (
+	"context"
+	"os"
+	"testing"
+)
+
+// TestDirStreamSeekdir checks that Seekdir repositions the snapshot directory
+// stream to an arbitrary offset. A kernel-NFS-exported mount needs this for
+// readdir continuation: the stateless server opens a fresh handle and seeks to
+// the last returned cookie on every batch. Offset 0 must still reset to the
+// start (the rewind / re-read case) and offsets at or past the end must clamp
+// to EOF.
+func TestDirStreamSeekdir(t *testing.T) {
+	ctx := context.Background()
+	// 3 real entries plus the synthesized "." and ".." => 5 entries total,
+	// occupying internal indices 0..4 (go-fuse offsets 1..5). HasNext is true
+	// while i < len(nodes)+2, i.e. i < 5.
+	ds := &dirStream{nodes: make([]os.FileInfo, 3)}
+
+	for _, tc := range []struct {
+		off      uint64
+		wantI    int
+		wantNext bool
+	}{
+		{0, 0, true},    // rewind to start (the re-read case)
+		{2, 2, true},    // resume at the first real entry
+		{4, 4, true},    // last entry
+		{5, 5, false},   // exactly at end => EOF
+		{99, 99, false}, // past end => clamped to EOF via HasNext
+	} {
+		if errno := ds.Seekdir(ctx, tc.off); errno != 0 {
+			t.Fatalf("Seekdir(%d) returned errno %v, want 0", tc.off, errno)
+		}
+		if ds.i != tc.wantI {
+			t.Errorf("Seekdir(%d): i = %d, want %d", tc.off, ds.i, tc.wantI)
+		}
+		if got := ds.HasNext(); got != tc.wantNext {
+			t.Errorf("Seekdir(%d): HasNext() = %v, want %v", tc.off, got, tc.wantNext)
+		}
+	}
+}