Enable StatFS on dynamicfs

MacOS apparently invokes StatFS on os.Lstat/os.Stat. Note that the
return fields are not actually populated here.

samples/dynamicfs/dynamic_fs.go

 package dynamicfs
 
 import (
-	"strings"
+	"fmt"
 	"io"
+	"log"
+	"os"
+	"strings"
+	"sync"
+	"time"
 
 	"golang.org/x/net/context"
 
-	"github.com/jacobsa/timeutil"
 	"github.com/jacobsa/fuse"
-	"github.com/jacobsa/fuse/fuseutil"
 	"github.com/jacobsa/fuse/fuseops"
-	"os"
-	"fmt"
-	"log"
-	"sync"
-	"time"
+	"github.com/jacobsa/fuse/fuseutil"
+	"github.com/jacobsa/timeutil"
 )
 
 // Create a file system that contains 2 files (`age` and `weekday`) and no directories. Every time
@@ -111,8 +111,8 @@ var gInodeInfo = map[fuseops.InodeID]inodeInfo{
 }
 
 func findChildInode(
-name string,
-children []fuseutil.Dirent) (inode fuseops.InodeID, err error) {
+	name string,
+	children []fuseutil.Dirent) (inode fuseops.InodeID, err error) {
 	for _, e := range children {
 		if e.Name == name {
 			inode = e.Inode
@@ -131,13 +131,12 @@ func (fs *dynamicFS) findUnusedHandle() fuseops.HandleID {
 		handle++
 	}
 	fs.nextHandle = handle + 1
-	log.Printf("Minting new handle: %d", handle)
 	return handle
 }
 
 func (fs *dynamicFS) GetInodeAttributes(
-ctx context.Context,
-op *fuseops.GetInodeAttributesOp) (err error) {
+	ctx context.Context,
+	op *fuseops.GetInodeAttributesOp) (err error) {
 	// Find the info for this inode.
 	info, ok := gInodeInfo[op.Inode]
 	if !ok {
@@ -150,8 +149,8 @@ op *fuseops.GetInodeAttributesOp) (err error) {
 }
 
 func (fs *dynamicFS) LookUpInode(
-ctx context.Context,
-op *fuseops.LookUpInodeOp) (err error) {
+	ctx context.Context,
+	op *fuseops.LookUpInodeOp) (err error) {
 	// Find the info for the parent.
 	parentInfo, ok := gInodeInfo[op.Parent]
 	if !ok {
@@ -173,15 +172,15 @@ op *fuseops.LookUpInodeOp) (err error) {
 }
 
 func (fs *dynamicFS) OpenDir(
-ctx context.Context,
-op *fuseops.OpenDirOp) (err error) {
+	ctx context.Context,
+	op *fuseops.OpenDirOp) (err error) {
 	// Allow opening directory.
 	return
 }
 
 func (fs *dynamicFS) ReadDir(
-ctx context.Context,
-op *fuseops.ReadDirOp) (err error) {
+	ctx context.Context,
+	op *fuseops.ReadDirOp) (err error) {
 	// Find the info for this inode.
 	info, ok := gInodeInfo[op.Inode]
 	if !ok {
@@ -218,8 +217,8 @@ op *fuseops.ReadDirOp) (err error) {
 }
 
 func (fs *dynamicFS) OpenFile(
-ctx context.Context,
-op *fuseops.OpenFileOp) (err error) {
+	ctx context.Context,
+	op *fuseops.OpenFileOp) (err error) {
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
 	var contents string
@@ -243,8 +242,8 @@ op *fuseops.OpenFileOp) (err error) {
 }
 
 func (fs *dynamicFS) ReadFile(
-ctx context.Context,
-op *fuseops.ReadFileOp) (err error) {
+	ctx context.Context,
+	op *fuseops.ReadFileOp) (err error) {
 	fs.mu.Lock()
 	defer fs.mu.Unlock()
 	contents, ok := fs.fileHandles[op.Handle]
@@ -275,3 +274,8 @@ func (fs *dynamicFS) ReleaseFileHandle(
 	delete(fs.fileHandles, op.Handle)
 	return
 }
+
+func (fs *dynamicFS) StatFS(ctx context.Context,
+	op *fuseops.StatFSOp) (err error) {
+	return
+}