go/packages: suppress go list errors when ad-hoc package doesn't exist

Updates golang/go#29280

Change-Id: Ie5a5dc1fef8f3d989b3a5fffb6c2ca66e97c143a
Reviewed-on: https://go-review.googlesource.com/c/154517
Reviewed-by: Ian Cottrell <iancottrell@google.com>

go/packages/golist.go

@@ -741,7 +741,10 @@ func invokeGo(cfg *Config, args ...string) (*bytes.Buffer, error) {
 		// If that build fails, errors appear on stderr
 		// (despite the -e flag) and the Export field is blank.
 		// Do not fail in that case.
-		if !usesExportData(cfg) {
+		// The same is true if an ad-hoc package given to go list doesn't exist.
+		// TODO(matloob): Remove these once we can depend on go list to exit with a zero status with -e even when
+		// packages don't exist or a build fails.
+		if !usesExportData(cfg) && !containsGoFile(args) {
 			return nil, fmt.Errorf("go %v: %s: %s", args, exitErr, stderr)
 		}
 	}
@@ -764,6 +767,15 @@ func invokeGo(cfg *Config, args ...string) (*bytes.Buffer, error) {
 	return stdout, nil
 }
 
+func containsGoFile(s []string) bool {
+	for _, f := range s {
+		if strings.HasSuffix(f, ".go") {
+			return true
+		}
+	}
+	return false
+}
+
 func cmdDebugStr(cfg *Config, args ...string) string {
 	env := make(map[string]string)
 	for _, kv := range cfg.Env {

go/packages/packages_test.go

@@ -1570,6 +1570,24 @@ func testBasicXTest(t *testing.T, exporter packagestest.Exporter) {
 	}
 }
 
+func TestErrorMissingFile(t *testing.T) { packagestest.TestAll(t, testErrorMissingFile) }
+func testErrorMissingFile(t *testing.T, exporter packagestest.Exporter) {
+	exported := packagestest.Export(t, exporter, []packagestest.Module{{
+		Name: "golang.org/fake",
+		Files: map[string]interface{}{
+			"a/a_test.go": `package a;`,
+		}}})
+	defer exported.Cleanup()
+
+	exported.Config.Mode = packages.LoadSyntax
+	exported.Config.Tests = false
+	pkgs, err := packages.Load(exported.Config, "missing.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Log(pkgs)
+}
+
 func errorMessages(errors []packages.Error) []string {
 	var msgs []string
 	for _, err := range errors {