go/packages/packagestest: skip nested module directories in copying

MustCopyFileTree is used to construct a module representation based on
a real directory tree and its output consumers (e.g. Export) assume the
output contains only files that belong to the module. If the real
directory contains nested module directories, they need to be trimmed
to satisfy the assumption.

This CL implements this trimming logic - a directory containing go.mod
is excluded from copying if the directory is not the top-level directory.

Change-Id: Ib09d4d74e120b502e338a021042d4d5e00719f23
Reviewed-on: https://go-review.googlesource.com/c/tools/+/358037
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

go/packages/packagestest/export.go

@@ -582,7 +582,7 @@ func GroupFilesByModules(root string) ([]Module, error) {
 
 // MustCopyFileTree returns a file set for a module based on a real directory tree.
 // It scans the directory tree anchored at root and adds a Copy writer to the
-// map for every file found.
+// map for every file found. It skips copying files in nested modules.
 // This is to enable the common case in tests where you have a full copy of the
 // package in your testdata.
 // This will panic if there is any kind of error trying to walk the file tree.
@@ -593,6 +593,12 @@ func MustCopyFileTree(root string) map[string]interface{} {
 			return err
 		}
 		if info.IsDir() {
+			// skip nested modules.
+			if path != root {
+				if fi, err := os.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
+					return filepath.SkipDir
+				}
+			}
 			return nil
 		}
 		fragment, err := filepath.Rel(root, path)

go/packages/packagestest/export_test.go

@@ -5,8 +5,11 @@
 package packagestest_test
 
 import (
+	"io/ioutil"
 	"os"
 	"path/filepath"
+	"reflect"
+	"sort"
 	"testing"
 
 	"golang.org/x/tools/go/packages/packagestest"
@@ -180,3 +183,53 @@ func TestGroupFilesByModules(t *testing.T) {
 		})
 	}
 }
+
+func TestMustCopyFiles(t *testing.T) {
+	// Create the following test directory structure in a temporary directory.
+	src := map[string]string{
+		// copies all files under the specified directory.
+		"go.mod": "module example.com",
+		"m.go":   "package m",
+		"a/a.go": "package a",
+		// contents from a nested module shouldn't be copied.
+		"nested/go.mod": "module example.com/nested",
+		"nested/m.go":   "package nested",
+		"nested/b/b.go": "package b",
+	}
+
+	tmpDir, err := ioutil.TempDir("", t.Name())
+	if err != nil {
+		t.Fatalf("failed to create a temporary directory: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	for fragment, contents := range src {
+		fullpath := filepath.Join(tmpDir, filepath.FromSlash(fragment))
+		if err := os.MkdirAll(filepath.Dir(fullpath), 0755); err != nil {
+			t.Fatal(err)
+		}
+		if err := ioutil.WriteFile(fullpath, []byte(contents), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	copied := packagestest.MustCopyFileTree(tmpDir)
+	var got []string
+	for fragment := range copied {
+		got = append(got, filepath.ToSlash(fragment))
+	}
+	want := []string{"go.mod", "m.go", "a/a.go"}
+
+	sort.Strings(got)
+	sort.Strings(want)
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("packagestest.MustCopyFileTree = %v, want %v", got, want)
+	}
+
+	// packagestest.Export is happy.
+	exported := packagestest.Export(t, packagestest.Modules, []packagestest.Module{{
+		Name:  "example.com",
+		Files: packagestest.MustCopyFileTree(tmpDir),
+	}})
+	defer exported.Cleanup()
+}