godoc/vfs/zipfs: join paths to get correct RootType

The logic was incorrectly written from vfs/os.go. zipfs filesystem
passes actual paths during dirTree building. So we need to join the paths
to determine whether they are under GOROOT or GOPATH.

Updates golang/go#27205

Change-Id: Ic4330fce02c6ebfc44ae21122e2412c33f0cd45a
Reviewed-on: https://go-review.googlesource.com/138435
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/vfs/zipfs/zipfs.go

@@ -86,7 +86,7 @@ func (fs *zipFS) String() string {
 func (fs *zipFS) RootType(abspath string) vfs.RootType {
 	var t vfs.RootType
 	switch {
-	case abspath == vfs.GOROOT:
+	case exists(path.Join(vfs.GOROOT, abspath)):
 		t = vfs.RootTypeGoRoot
 	case isGoPath(abspath):
 		t = vfs.RootTypeGoPath
@@ -94,15 +94,20 @@ func (fs *zipFS) RootType(abspath string) vfs.RootType {
 	return t
 }
 
-func isGoPath(path string) bool {
+func isGoPath(abspath string) bool {
 	for _, p := range filepath.SplitList(build.Default.GOPATH) {
-		if p == path {
+		if exists(path.Join(p, abspath)) {
 			return true
 		}
 	}
 	return false
 }
 
+func exists(path string) bool {
+	_, err := os.Stat(path)
+	return err == nil
+}
+
 func (fs *zipFS) Close() error {
 	fs.list = nil
 	return fs.ReadCloser.Close()

godoc/vfs/zipfs/zipfs_test.go

@@ -187,3 +187,20 @@ func TestZipFSOpenSeek(t *testing.T) {
 		}
 	}
 }
+
+func TestRootType(t *testing.T) {
+	tests := []struct {
+		path   string
+		fsType vfs.RootType
+	}{
+		{"/src/net/http", vfs.RootTypeGoRoot},
+		{"/src/badpath", ""},
+		{"/", vfs.RootTypeGoRoot},
+	}
+
+	for _, item := range tests {
+		if fs.RootType(item.path) != item.fsType {
+			t.Errorf("unexpected fsType. Expected- %v, Got- %v", item.fsType, fs.RootType(item.path))
+		}
+	}
+}