fs: add an example for file handles with direct I/O

Change-Id: I3464790cd8fd05a4f0e787a3895bf3e81569b29a

fs/directio_example_test.go

+// Copyright 2019 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"
+	"syscall"
+	"time"
+
+	"github.com/hanwen/go-fuse/fs"
+	"github.com/hanwen/go-fuse/fuse"
+)
+
+// bytesFileHandle is a file handle that carries separate content for
+// each Open call
+type bytesFileHandle struct {
+	content []byte
+}
+
+// bytesFileHandle allows reads
+var _ = (fs.FileReader)((*bytesFileHandle)(nil))
+
+func (fh *bytesFileHandle) Read(ctx context.Context, dest []byte, off int64) (fuse.ReadResult, syscall.Errno) {
+	end := off + int64(len(dest))
+	if end > int64(len(fh.content)) {
+		end = int64(len(fh.content))
+	}
+
+	// We could copy to the `dest` buffer, but since we have a
+	// []byte already, return that.
+	return fuse.ReadResultData(fh.content[off:end]), 0
+}
+
+// timeFile is a file that contains the wall clock time as ASCII.
+type timeFile struct {
+	fs.Inode
+}
+
+// timeFile implements Open
+var _ = (fs.NodeOpener)((*timeFile)(nil))
+
+func (f *timeFile) Open(ctx context.Context, openFlags uint32) (fh fs.FileHandle, fuseFlags uint32, errno syscall.Errno) {
+	// disallow writes
+	if fuseFlags&(syscall.O_RDWR|syscall.O_WRONLY) != 0 {
+		return nil, 0, syscall.EROFS
+	}
+
+	// capture open time
+	now := time.Now().Format(time.StampNano) + "\n"
+	fh = &bytesFileHandle{
+		content: []byte(now),
+	}
+
+	// Return FOPEN_DIRECT_IO so content is not cached.
+	return fh, fuse.FOPEN_DIRECT_IO, 0
+}
+
+// ExampleDirectIO shows how to create a file whose contents change on
+// every read.
+func Example_directIO() {
+	mntDir := "/tmp/x"
+	root := &fs.Inode{}
+
+	// Mount the file system
+	server, err := fs.Mount(mntDir, root, &fs.Options{
+		MountOptions: fuse.MountOptions{Debug: false},
+
+		// Setup the clock file.
+		OnAdd: func(ctx context.Context) {
+			ch := root.NewPersistentInode(
+				ctx,
+				&timeFile{},
+				fs.StableAttr{Mode: syscall.S_IFREG})
+			root.AddChild("clock", ch, true)
+		},
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("cat %s/clock to see the time\n", mntDir)
+	fmt.Printf("Unmount by calling 'fusermount -u %s'\n", mntDir)
+
+	// Serve the file system, until unmounted by calling fusermount -u
+	server.Wait()
+}