relnote: CheckAPIFile: look in right directory

Have CheckAPIFile look for corresponding release note files in the
directory doc/next/*stdlib/*minor.

Previously, it was looking in doc/next.

For golang/go#64169.

Change-Id: I40fa31f0e95885648536ce74aa070f9f7fdf692b
Reviewed-on: https://go-review.googlesource.com/c/build/+/560295
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

relnote/relnote.go

@@ -381,7 +381,9 @@ func GroupAPIFeaturesByFile(fs []APIFeature) (map[string][]APIFeature, error) {
 
 // CheckAPIFile reads the api file at filename in apiFS, and checks the corresponding
 // release-note files under docFS. It checks that the files exist and that they have
-// some minimal content (see [CheckFragment]). The docRoot argument is used in error messages.
+// some minimal content (see [CheckFragment]).
+// The docRoot argument is the path from the repo or project root to the root of docFS.
+// It is used only for error messages.
 func CheckAPIFile(apiFS fs.FS, filename string, docFS fs.FS, docRoot string) error {
 	features, err := parseAPIFile(apiFS, filename)
 	if err != nil {
@@ -396,17 +398,42 @@ func CheckAPIFile(apiFS fs.FS, filename string, docFS fs.FS, docRoot string) err
 		filenames = append(filenames, fn)
 	}
 	slices.Sort(filenames)
+	mcDir, err := minorChangesDir(docFS)
+	if err != nil {
+		return err
+	}
 	var errs []error
 	for _, fn := range filenames {
+		// Use path.Join for consistency with io/fs pathnames.
+		fn = path.Join(mcDir, fn)
 		// TODO(jba): check that the file mentions each feature?
 		if err := checkFragmentFile(docFS, fn); err != nil {
-			// Use path.Join for consistency with io/fs pathnames.
 			errs = append(errs, fmt.Errorf("%s: %v\nSee doc/README.md for more information.", path.Join(docRoot, fn), err))
 		}
 	}
 	return errors.Join(errs...)
 }
 
+// minorChangesDir returns the unique directory in docFS that corresponds to the
+// "Minor changes to the standard library" section of the release notes.
+func minorChangesDir(docFS fs.FS) (string, error) {
+	dirs, err := fs.Glob(docFS, "*stdlib/*minor")
+	if err != nil {
+		return "", err
+	}
+	var bad string
+	if len(dirs) == 0 {
+		bad = "No"
+	} else if len(dirs) > 1 {
+		bad = "More than one"
+	}
+	if bad != "" {
+		return "", fmt.Errorf("%s directory matches *stdlib/*minor.\nThis shouldn't happen; please file a bug at https://go.dev/issues/new.",
+			bad)
+	}
+	return dirs[0], nil
+}
+
 func checkFragmentFile(fsys fs.FS, filename string) error {
 	f, err := fsys.Open(filename)
 	if err != nil {

relnote/relnote_test.go

@@ -252,7 +252,7 @@ func TestCheckAPIFile(t *testing.T) {
 				t.Fatal(err)
 			}
 			var got string
-			gotErr := CheckAPIFile(fsys, "api.txt", fsys, "doc/next/*stdlib/*minor")
+			gotErr := CheckAPIFile(fsys, "api.txt", fsys, "doc/next")
 			if gotErr != nil {
 				got = gotErr.Error()
 			}

relnote/testdata/checkAPIFile/fail.txt

@@ -4,10 +4,10 @@ errors:
 -- api.txt --
 pkg foo, type T #123
 pkg bar, type T #123
--- bar/123.md --
+-- 7-stdlib/99-minor/bar/123.md --
 Not a sentence
 -- want --
-doc/next/*stdlib/*minor/bar/123.md: File must contain a complete sentence or a TODO.
+doc/next/7-stdlib/99-minor/bar/123.md: File must contain a complete sentence or a TODO.
 See doc/README.md for more information.
-doc/next/*stdlib/*minor/foo/123.md: File does not exist. Every API change must have a corresponding release note file.
+doc/next/7-stdlib/99-minor/foo/123.md: File does not exist. Every API change must have a corresponding release note file.
 See doc/README.md for more information.

relnote/testdata/checkAPIFile/missing.txt

@@ -0,0 +1,8 @@
+Check the error when the *stdlib/*minor directory is missing.
+
+-- api.txt --
+pkg foo, type T #123
+pkg bar, type T #123
+-- want --
+No directory matches *stdlib/*minor.
+This shouldn't happen; please file a bug at https://go.dev/issues/new.

relnote/testdata/checkAPIFile/ok.txt

@@ -1,9 +1,9 @@
 -- api.txt --
 pkg foo, type T #123
 pkg bar, type T #123
--- foo/123.md --
+-- stdlib/minor/foo/123.md --
 TODO
--- bar/123.md --
+-- stdlib/minor/bar/123.md --
 A sentence.
 -- want --