fs: fix Lseek whence=SEEK_HOLE default implementation

According to the lseek(2) manpage, seek(SEEK_HOLE) should behave as if
there is an implicit hole at the end of every file.

Add a test.

Fixes #417.

Change-Id: I169a803ea81e3c920b61edf8bb8b774870d042c5

fs/bridge.go

@@ -1153,23 +1153,35 @@ func (b *rawBridge) CopyFileRange(cancel <-chan struct{}, in *fuse.CopyFileRange
 func (b *rawBridge) Lseek(cancel <-chan struct{}, in *fuse.LseekIn, out *fuse.LseekOut) fuse.Status {
 	n, f := b.inode(in.NodeId, in.Fh)
 
+	ctx := &fuse.Context{Caller: in.Caller, Cancel: cancel}
+
 	ls, ok := n.ops.(NodeLseeker)
 	if ok {
-		off, errno := ls.Lseek(&fuse.Context{Caller: in.Caller, Cancel: cancel},
+		off, errno := ls.Lseek(ctx,
 			f.file, in.Offset, in.Whence)
 		out.Offset = off
 		return errnoToStatus(errno)
 	}
 	if fs, ok := f.file.(FileLseeker); ok {
-		off, errno := fs.Lseek(&fuse.Context{Caller: in.Caller, Cancel: cancel}, in.Offset, in.Whence)
+		off, errno := fs.Lseek(ctx, in.Offset, in.Whence)
 		out.Offset = off
 		return errnoToStatus(errno)
 	}
 
-	if in.Whence == _SEEK_DATA || in.Whence == _SEEK_HOLE {
+	if in.Whence == _SEEK_DATA {
 		out.Offset = in.Offset
 		return fuse.OK
 	}
 
+	if in.Whence == _SEEK_HOLE {
+		var attr fuse.AttrOut
+		if s := b.getattr(ctx, n, nil, &attr); s != 0 {
+			return errnoToStatus(s)
+		}
+
+		out.Offset = attr.Size
+		return fuse.OK
+	}
+
 	return fuse.ENOTSUP
 }

fs/mem_test.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/hanwen/go-fuse/v2/fuse"
 	"github.com/hanwen/go-fuse/v2/internal/testutil"
+	"github.com/hanwen/go-fuse/v2/posixtest"
 )
 
 func testMount(t *testing.T, root InodeEmbedder, opts *Options) (string, *fuse.Server) {
@@ -90,6 +91,28 @@ func TestRootInode(t *testing.T) {
 	}
 }
 
+func TestLseekDefault(t *testing.T) {
+	data := []byte("hello")
+	root := &Inode{}
+	mntDir, _ := testMount(t, root, &Options{
+		FirstAutomaticIno: 1,
+		OnAdd: func(ctx context.Context) {
+			n := root.EmbeddedInode()
+			ch := n.NewPersistentInode(
+				ctx,
+				&MemRegularFile{
+					Data: data,
+					Attr: fuse.Attr{
+						Mode: 0464,
+					},
+				}, StableAttr{})
+			n.AddChild("file.bin", ch, false)
+		},
+	})
+
+	posixtest.LseekHoleSeeksToEOF(t, mntDir)
+}
+
 func TestDataFile(t *testing.T) {
 	want := "hello"
 	root := &Inode{}

posixtest/test.go

@@ -33,6 +33,7 @@ var All = map[string]func(*testing.T, string){
 	"ParallelFileOpen":           ParallelFileOpen,
 	"Link":                       Link,
 	"LinkUnlinkRename":           LinkUnlinkRename,
+	"LseekHoleSeeksToEOF":        LseekHoleSeeksToEOF,
 	"RenameOverwriteDestNoExist": RenameOverwriteDestNoExist,
 	"RenameOverwriteDestExist":   RenameOverwriteDestExist,
 	"RenameOpenDir":              RenameOpenDir,
@@ -698,3 +699,24 @@ func FcntlFlockLocksFile(t *testing.T, mnt string) {
 		t.Errorf("FcntlFlock returned %v, expected EAGAIN", err)
 	}
 }
+
+func LseekHoleSeeksToEOF(t *testing.T, mnt string) {
+	fn := filepath.Join(mnt, "file.bin")
+	content := bytes.Repeat([]byte("abcxyz\n"), 1024)
+	if err := ioutil.WriteFile(fn, content, 0644); err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+
+	fd, err := syscall.Open(fn, syscall.O_RDONLY, 0644)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer syscall.Close(fd)
+
+	off, err := unix.Seek(fd, int64(len(content)/2), unix.SEEK_HOLE)
+	if err != nil {
+		t.Fatalf("Seek: %v", err)
+	} else if off != int64(len(content)) {
+		t.Errorf("got offset %d, want %d", off, len(content))
+	}
+}