fs: don't cause io errors on missing blobrefs

- Looking up missing blobrefs would cause fs errors. Instead, simply
  return fuse.ENOENT.

- Add tests for:
   * validating that the mountpoint/<blobref> lookups work

   * validating that the mountpount/<blobref> lookups don't fail when
     the blob does not exist.

pkg/fs/fs_test.go

@@ -90,6 +90,7 @@ type mountEnv struct {
 	t          *testing.T
 	mountPoint string
 	process    *os.Process
+	world      *test.World
 }
 
 func (e *mountEnv) Stat(s *stat) int64 {
@@ -200,6 +201,7 @@ func pkmountTest(t *testing.T, fn func(env *mountEnv)) {
 		t:          t,
 		mountPoint: mountPoint,
 		process:    mount.Process,
+		world:      w,
 	})
 
 }
@@ -224,6 +226,33 @@ func TestRoot(t *testing.T) {
 	})
 }
 
+func TestReadFileFromRoot(t *testing.T) {
+	condSkip(t)
+	pkmountTest(t, func(env *mountEnv) {
+		// pk-put a file
+		tmpFile, err := ioutil.TempFile(t.TempDir(), "camlitest")
+		if err != nil {
+			t.Fatal(err)
+		}
+		testContent := "some test content"
+		tmpFile.WriteString(testContent)
+		blobRef := env.world.PutFile(t, tmpFile.Name())
+
+		// Read it using the file's blobref
+		if contents, err := ioutil.ReadFile(filepath.Join(env.mountPoint, blobRef.String())); err != nil {
+			t.Fatal(err)
+		} else if got := string(contents); got != testContent {
+			t.Fatalf("Expected test content, got %q", got)
+		}
+
+		// Read a non-existing blobref, should return NotExist.
+		badRefPath := filepath.Join(env.mountPoint, "sha224-1853501438ffe541dd1e48b9efc4a230f67f7b98afe83df24bfbfa25")
+		if _, err := os.Stat(badRefPath); !os.IsNotExist(err) {
+			t.Fatalf("expected NotExist; got stat err = %v instead", err)
+		}
+	})
+}
+
 type testLog struct {
 	t *testing.T
 }

pkg/fs/root.go

@@ -135,7 +135,11 @@ func (n *root) Lookup(ctx context.Context, name string) (fs.Node, error) {
 
 	if br, ok := blob.Parse(name); ok {
 		Logger.Printf("Root lookup of blobref. %q => %v", name, br)
-		return &node{fs: n.fs, blobref: br}, nil
+		node := &node{fs: n.fs, blobref: br}
+		if _, err := node.schema(); os.IsNotExist(err) {
+			return nil, fuse.ENOENT
+		}
+		return node, nil
 	}
 	Logger.Printf("Bogus root lookup of %q", name)
 	return nil, fuse.ENOENT

pkg/test/world.go

@@ -311,6 +311,15 @@ func (w *World) NewPermanode(t *testing.T) blob.Ref {
 	return br
 }
 
+func (w *World) PutFile(t *testing.T, name string) blob.Ref {
+	out := MustRunCmd(t, w.Cmd("pk-put", "file", name))
+	br, ok := blob.Parse(strings.TrimSpace(out))
+	if !ok {
+		t.Fatalf("Expected blobref in pk-put stdout; got %q", out)
+	}
+	return br
+}
+
 func (w *World) Cmd(binary string, args ...string) *exec.Cmd {
 	return w.CmdWithEnv(binary, os.Environ(), args...)
 }