fusermount: Fix option escaping

Before this patch no commas were allowed in any fusermount option string.
But it is in fact possible to use commas in fusermount options: we only
need to escape them [1].

Now go-fuse users can use commas in fusermount options.

Related libfuse commits are [1] and [2].

This patch also adds a test which ensures neither go-fuse nor fusermount
crash if we pass options with commas.

---

[1] libfuse/libfuse@555d6b5
[2] libfuse/libfuse@5c094ac

Change-Id: Iadf8283e630dc1fb884d047d8765eeaaca90eabd

fuse/mount_linux_test.go

@@ -187,3 +187,31 @@ fusermount:        %#v`, o3, o1)
 		}
 	}
 }
+
+// TestEscapedMountOption tests that fusermount doesn't exit when when using commas or backslashs in options.
+// It also tests that commas or backslashs in options are correctly propagated to /proc/mounts.
+func TestEscapedMountOption(t *testing.T) {
+	fsname := `fsname,with\,many,comm\as,and\backsl\\ashs`
+	opts := &MountOptions{
+		FsName: fsname,
+	}
+	mnt := t.TempDir()
+	fs := NewDefaultRawFileSystem()
+	srv, err := NewServer(fs, mnt, opts)
+	if err != nil {
+		t.Error(err)
+	}
+	go srv.Serve()
+	defer srv.Unmount()
+	mounts, err := mountinfo.GetMounts(mountinfo.SingleEntryFilter(mnt))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(mounts) != 1 {
+		t.Fatalf("Could not find mountpoint %q in /proc/self/mountinfo", mnt)
+	}
+	m := *mounts[0]
+	if m.Source != fsname {
+		t.Errorf("mountinfo(%q): got %q want %q", mnt, m.Source, fsname)
+	}
+}

fuse/server.go

@@ -185,12 +185,6 @@ func NewServer(fs RawFileSystem, mountPoint string, opts *MountOptions) (*Server
 		o.Name = strings.Replace(name[:l], ",", ";", -1)
 	}
 
-	for _, s := range o.optionsStrings() {
-		if strings.Contains(s, ",") {
-			return nil, fmt.Errorf("found ',' in option string %q", s)
-		}
-	}
-
 	maxReaders := runtime.GOMAXPROCS(0)
 	if maxReaders < minMaxReaders {
 		maxReaders = minMaxReaders
@@ -251,6 +245,10 @@ func NewServer(fs RawFileSystem, mountPoint string, opts *MountOptions) (*Server
 	return ms, nil
 }
 
+func escape(optionValue string) string {
+	return strings.Replace(strings.Replace(optionValue, `\`, `\\`, -1), `,`, `\,`, -1)
+}
+
 func (o *MountOptions) optionsStrings() []string {
 	var r []string
 	r = append(r, o.Options...)
@@ -273,7 +271,15 @@ func (o *MountOptions) optionsStrings() []string {
 		r = append(r, "daemon_timeout=0")
 	}
 
-	return r
+	// Commas and backslashs in an option need to be escaped, because
+	// options are separated by a comma and backslashs are used to
+	// escape other characters.
+	var rEscaped []string
+	for _, s := range r {
+		rEscaped = append(rEscaped, escape(s))
+	}
+
+	return rEscaped
 }
 
 // DebugData returns internal status information for debugging