internal/goroot: in IsStandardPackage check for go source files

Be more strict in IsStandardPackage: before this change we'd just
check for the existence of the directory, but now we check to see that
there's at least one .go file in the directory.

Also update some comments in the modindex package to reflect the fact
that an IndexPackage might represent a directory that does not contain
any source files.

Fixes #65406
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest

Change-Id: I82f0c0e7bfcd5bb4df0195c4c8c7fc7c67fae53e
Reviewed-on: https://go-review.googlesource.com/c/go/+/561338
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/modindex/read.go

@@ -124,7 +124,7 @@ var (
 	errNotFromModuleCache = fmt.Errorf("%w: not from module cache", ErrNotIndexed)
 )
 
-// GetPackage returns the IndexPackage for the package at the given path.
+// GetPackage returns the IndexPackage for the directory at the given path.
 // It will return ErrNotIndexed if the directory should be read without
 // using the index, for instance because the index is disabled, or the package
 // is not in a module.
@@ -669,11 +669,9 @@ func IsStandardPackage(goroot_, compiler, path string) bool {
 		reldir = str.TrimFilePathPrefix(reldir, "cmd")
 		modroot = filepath.Join(modroot, "cmd")
 	}
-	if _, err := GetPackage(modroot, filepath.Join(modroot, reldir)); err == nil {
-		// Note that goroot.IsStandardPackage doesn't check that the directory
-		// actually contains any go files-- merely that it exists. GetPackage
-		// returning a nil error is enough for us to know the directory exists.
-		return true
+	if pkg, err := GetPackage(modroot, filepath.Join(modroot, reldir)); err == nil {
+		hasGo, err := pkg.IsDirWithGoFiles()
+		return err == nil && hasGo
 	} else if errors.Is(err, ErrNotIndexed) {
 		// Fall back because package isn't indexable. (Probably because
 		// a file was modified recently)
@@ -786,8 +784,8 @@ func shouldBuild(sf *sourceFile, tags map[string]bool) bool {
 	return true
 }
 
-// IndexPackage holds the information needed to access information in the
-// index needed to load a package in a specific directory.
+// IndexPackage holds the information in the index
+// needed to load a package in a specific directory.
 type IndexPackage struct {
 	error error
 	dir   string // directory of the package relative to the modroot

src/cmd/go/testdata/script/list_testdata.txt

@@ -0,0 +1,11 @@
+# Issue 65406. The testdata directory in GOROOT/src
+# shouldn't be treated as a standard package.
+
+go list -f '{{.ImportPath}} {{.Dir}}' testdata
+! stderr 'found package testdata in multiple modules'
+stdout 'testdata '$WORK${/}'gopath'${/}'src'
+
+-- go.mod --
+module testdata
+-- p.go --
+package p

src/cmd/go/testdata/script/mod_list.txt

@@ -44,9 +44,9 @@ stderr '^go: module rsc.io/quote/buggy: not a known dependency'
 
 # Module loader does not interfere with list -e (golang.org/issue/24149).
 go list -e -f '{{.Error.Err}}' database
-stdout 'no Go files in '
+stdout 'package database is not in std'
 ! go list database
-stderr 'no Go files in '
+stderr 'package database is not in std'
 
 -- go.mod --
 module x

src/internal/goroot/gc.go

@@ -20,8 +20,16 @@ func IsStandardPackage(goroot, compiler, path string) bool {
 	switch compiler {
 	case "gc":
 		dir := filepath.Join(goroot, "src", path)
-		info, err := os.Stat(dir)
-		return err == nil && info.IsDir()
+		dirents, err := os.ReadDir(dir)
+		if err != nil {
+			return false
+		}
+		for _, dirent := range dirents {
+			if strings.HasSuffix(dirent.Name(), ".go") {
+				return true
+			}
+		}
+		return false
 	case "gccgo":
 		return gccgoSearch.isStandard(path)
 	default:

src/internal/goroot/gccgo.go

@@ -9,6 +9,7 @@ package goroot
 import (
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // IsStandardPackage reports whether path is a standard package,
@@ -17,8 +18,16 @@ func IsStandardPackage(goroot, compiler, path string) bool {
 	switch compiler {
 	case "gc":
 		dir := filepath.Join(goroot, "src", path)
-		_, err := os.Stat(dir)
-		return err == nil
+		dirents, err := os.ReadDir(dir)
+		if err != nil {
+			return false
+		}
+		for _, dirent := range dirents {
+			if strings.HasSuffix(dirent.Name(), ".go") {
+				return true
+			}
+		}
+		return false
 	case "gccgo":
 		return stdpkg[path]
 	default: