cmd/go: fix modload response for std-vendored packages

This fixes a failure when using Go 1.11 to build App Engine code.

Change-Id: I008e8cf5ad4c568676d904deddff031a166f2d5d
Reviewed-on: https://go-review.googlesource.com/130138
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/modload/build.go

@@ -25,15 +25,21 @@ var (
 )
 
 func isStandardImportPath(path string) bool {
+	return findStandardImportPath(path) != ""
+}
+
+func findStandardImportPath(path string) string {
 	if search.IsStandardImportPath(path) {
-		if _, err := os.Stat(filepath.Join(cfg.GOROOT, "src", path)); err == nil {
-			return true
+		dir := filepath.Join(cfg.GOROOT, "src", path)
+		if _, err := os.Stat(dir); err == nil {
+			return dir
 		}
-		if _, err := os.Stat(filepath.Join(cfg.GOROOT, "src/vendor", path)); err == nil {
-			return true
+		dir = filepath.Join(cfg.GOROOT, "src/vendor", path)
+		if _, err := os.Stat(dir); err == nil {
+			return dir
 		}
 	}
-	return false
+	return ""
 }
 
 func PackageModuleInfo(pkgpath string) *modinfo.ModulePublic {

src/cmd/go/internal/modload/load.go

@@ -399,11 +399,17 @@ func ModuleUsedDirectly(path string) bool {
 func Lookup(path string) (dir, realPath string, err error) {
 	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
 	if !ok {
-		if isStandardImportPath(path) {
-			dir := filepath.Join(cfg.GOROOT, "src", path)
-			if _, err := os.Stat(dir); err == nil {
-				return dir, path, nil
-			}
+		// The loader should have found all the relevant paths.
+		// There are a few exceptions, though:
+		//	- during go list without -test, the p.Resolve calls to process p.TestImports and p.XTestImports
+		//	  end up here to canonicalize the import paths.
+		//	- during any load, non-loaded packages like "unsafe" end up here.
+		//	- during any load, build-injected dependencies like "runtime/cgo" end up here.
+		//	- because we ignore appengine/* in the module loader,
+		//	  the dependencies of any actual appengine/* library end up here.
+		dir := findStandardImportPath(path)
+		if dir != "" {
+			return dir, path, nil
 		}
 		return "", "", errMissing
 	}

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

+env GO111MODULE=on
+
+go list -f '{{.TestImports}}'
+stdout net/http # from .TestImports
+
+go list -test -f '{{.Deps}}'
+stdout golang_org/x/crypto # dep of .TestImports
+
+-- go.mod --
+module m
+
+-- x.go --
+package x
+
+-- x_test.go --
+package x
+import "testing"
+import _ "net/http"
+func Test(t *testing.T) {}