fs: allow extending LoopbackNode

Make LoopbackNode and LoopbackRoot public, and add an example of how
to extend them.

Fixes #364.

Change-Id: Idcce1bf3765e93ece3bcf9d14de8a957b8c6b63a

fs/loopback_darwin.go

@@ -16,23 +16,23 @@ import (
 	"github.com/hanwen/go-fuse/v2/internal/utimens"
 )
 
-func (n *loopbackNode) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
+func (n *LoopbackNode) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
 	return 0, syscall.ENOSYS
 }
 
-func (n *loopbackNode) Setxattr(ctx context.Context, attr string, data []byte, flags uint32) syscall.Errno {
+func (n *LoopbackNode) Setxattr(ctx context.Context, attr string, data []byte, flags uint32) syscall.Errno {
 	return syscall.ENOSYS
 }
 
-func (n *loopbackNode) Removexattr(ctx context.Context, attr string) syscall.Errno {
+func (n *LoopbackNode) Removexattr(ctx context.Context, attr string) syscall.Errno {
 	return syscall.ENOSYS
 }
 
-func (n *loopbackNode) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
+func (n *LoopbackNode) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
 	return 0, syscall.ENOSYS
 }
 
-func (n *loopbackNode) renameExchange(name string, newparent InodeEmbedder, newName string) syscall.Errno {
+func (n *LoopbackNode) renameExchange(name string, newparent InodeEmbedder, newName string) syscall.Errno {
 	return syscall.ENOSYS
 }
 
@@ -111,7 +111,7 @@ func (f *loopbackFile) utimens(a *time.Time, m *time.Time) syscall.Errno {
 	return ToErrno(err)
 }
 
-func (n *loopbackNode) CopyFileRange(ctx context.Context, fhIn FileHandle,
+func (n *LoopbackNode) CopyFileRange(ctx context.Context, fhIn FileHandle,
 	offIn uint64, out *Inode, fhOut FileHandle, offOut uint64,
 	len uint64, flags uint64) (uint32, syscall.Errno) {
 	return 0, syscall.ENOSYS

fs/loopback_linux.go

@@ -14,27 +14,27 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func (n *loopbackNode) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
+func (n *LoopbackNode) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
 	sz, err := unix.Lgetxattr(n.path(), attr, dest)
 	return uint32(sz), ToErrno(err)
 }
 
-func (n *loopbackNode) Setxattr(ctx context.Context, attr string, data []byte, flags uint32) syscall.Errno {
+func (n *LoopbackNode) Setxattr(ctx context.Context, attr string, data []byte, flags uint32) syscall.Errno {
 	err := unix.Lsetxattr(n.path(), attr, data, int(flags))
 	return ToErrno(err)
 }
 
-func (n *loopbackNode) Removexattr(ctx context.Context, attr string) syscall.Errno {
+func (n *LoopbackNode) Removexattr(ctx context.Context, attr string) syscall.Errno {
 	err := unix.Lremovexattr(n.path(), attr)
 	return ToErrno(err)
 }
 
-func (n *loopbackNode) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
+func (n *LoopbackNode) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
 	sz, err := unix.Llistxattr(n.path(), dest)
 	return uint32(sz), ToErrno(err)
 }
 
-func (n *loopbackNode) renameExchange(name string, newparent InodeEmbedder, newName string) syscall.Errno {
+func (n *LoopbackNode) renameExchange(name string, newparent InodeEmbedder, newName string) syscall.Errno {
 	fd1, err := syscall.Open(n.path(), syscall.O_DIRECTORY, 0)
 	if err != nil {
 		return ToErrno(err)
@@ -69,7 +69,7 @@ func (n *loopbackNode) renameExchange(name string, newparent InodeEmbedder, newN
 	return ToErrno(unix.Renameat2(fd1, name, fd2, newName, unix.RENAME_EXCHANGE))
 }
 
-func (n *loopbackNode) CopyFileRange(ctx context.Context, fhIn FileHandle,
+func (n *LoopbackNode) CopyFileRange(ctx context.Context, fhIn FileHandle,
 	offIn uint64, out *Inode, fhOut FileHandle, offOut uint64,
 	len uint64, flags uint64) (uint32, syscall.Errno) {
 	lfIn, ok := fhIn.(*loopbackFile)

fs/windows_example_test.go

+// Copyright 2020 the Go-FUSE Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"sync"
+	"syscall"
+	"time"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+	"github.com/hanwen/go-fuse/v2/fuse"
+)
+
+// WindowsNode emulates Windows FS semantics, which forbids deleting open files.
+type WindowsNode struct {
+	// WindowsNode inherits most functionality from LoopbackNode.
+	fs.LoopbackNode
+
+	mu        sync.Mutex
+	openCount int
+}
+
+var _ = (fs.NodeOpener)((*WindowsNode)(nil))
+
+func (n *WindowsNode) Open(ctx context.Context, flags uint32) (fs.FileHandle, uint32, syscall.Errno) {
+	fh, flags, errno := n.LoopbackNode.Open(ctx, flags)
+	if errno == 0 {
+		n.mu.Lock()
+		defer n.mu.Unlock()
+
+		n.openCount++
+	}
+	return fh, flags, errno
+}
+
+var _ = (fs.NodeCreater)((*WindowsNode)(nil))
+
+func (n *WindowsNode) Create(ctx context.Context, name string, flags uint32, mode uint32, out *fuse.EntryOut) (*fs.Inode, fs.FileHandle, uint32, syscall.Errno) {
+	inode, fh, flags, errno := n.LoopbackNode.Create(ctx, name, flags, mode, out)
+	if errno == 0 {
+		wn := inode.Operations().(*WindowsNode)
+		wn.openCount++
+	}
+
+	return inode, fh, flags, errno
+}
+
+var _ = (fs.NodeReleaser)((*WindowsNode)(nil))
+
+// Release decreases the open count. The kernel doesn't wait with
+// returning from close(), so if the caller is too quick to
+// unlink/rename after calling close(), this may still trigger EBUSY.
+func (n *WindowsNode) Release(ctx context.Context, f fs.FileHandle) syscall.Errno {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	n.openCount--
+	if fr, ok := f.(fs.FileReleaser); ok {
+		return fr.Release(ctx)
+	}
+	return 0
+}
+
+func isBusy(parent *fs.Inode, name string) bool {
+	if ch := parent.GetChild(name); ch != nil {
+		if wn, ok := ch.Operations().(*WindowsNode); ok {
+			wn.mu.Lock()
+			defer wn.mu.Unlock()
+			if wn.openCount > 0 {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+var _ = (fs.NodeUnlinker)((*WindowsNode)(nil))
+
+func (n *WindowsNode) Unlink(ctx context.Context, name string) syscall.Errno {
+	if isBusy(n.EmbeddedInode(), name) {
+		return syscall.EBUSY
+	}
+
+	return n.LoopbackNode.Unlink(ctx, name)
+}
+
+func newWindowsNode(rootData *fs.LoopbackRoot, parent *fs.Inode, name string, st *syscall.Stat_t) fs.InodeEmbedder {
+	n := &WindowsNode{
+		LoopbackNode: fs.LoopbackNode{
+			RootData: rootData,
+		},
+	}
+	return n
+}
+
+// ExampleLoopbackReuse shows how to build a file system on top of the
+// loopback file system.
+func Example_loopbackReuse() {
+	mntDir := "/tmp/mnt"
+	origDir := "/tmp/orig"
+
+	rootData := &fs.LoopbackRoot{
+		NewNode: newWindowsNode,
+		Path:    origDir,
+	}
+
+	sec := time.Second
+	opts := &fs.Options{
+		AttrTimeout:  &sec,
+		EntryTimeout: &sec,
+	}
+
+	server, err := fs.Mount(mntDir, newWindowsNode(rootData, nil, "", nil), opts)
+	if err != nil {
+		log.Fatalf("Mount fail: %v\n", err)
+	}
+	fmt.Printf("files under %s cannot be deleted if they are opened", mntDir)
+	server.Wait()
+}

fs/windows_test.go

+// Copyright 2020 the Go-FUSE Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+	"github.com/hanwen/go-fuse/v2/internal/testutil"
+)
+
+func TestWindowsEmulations(t *testing.T) {
+	mntDir, err := ioutil.TempDir("", "ZipFS")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(mntDir)
+	origDir, err := ioutil.TempDir("", "ZipFS")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(origDir)
+
+	rootData := &fs.LoopbackRoot{
+		NewNode: newWindowsNode,
+		Path:    origDir,
+	}
+	opts := fs.Options{}
+	opts.Debug = testutil.VerboseTest()
+	server, err := fs.Mount(mntDir, newWindowsNode(rootData, nil, "", nil), &opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer server.Unmount()
+
+	data := []byte("hello")
+	nm := mntDir + "/file"
+	if err := ioutil.WriteFile(nm, data, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	if got, err := ioutil.ReadFile(nm); err != nil {
+		t.Fatal(err)
+	} else if bytes.Compare(got, data) != 0 {
+		t.Fatalf("got %q want %q", got, data)
+	}
+
+	f, err := os.Open(nm)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := syscall.Unlink(nm); err == nil {
+		t.Fatal("Unlink should have failed")
+	}
+
+	f.Close()
+	// Ugh - it may take a while for the RELEASE to be processed.
+	time.Sleep(10 * time.Millisecond)
+
+	if err := syscall.Unlink(nm); err != nil {
+		t.Fatalf("Unlink: %v", err)
+	}
+}