cmd/api: fix no go files package panic

Fixes #29837

Change-Id: I7d57c24d2133932c076df6f41dd6589f777b65dd
Reviewed-on: https://go-review.googlesource.com/c/158877
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/api/goapi.go

@@ -169,7 +169,13 @@ func main() {
 					// w.Import(name) will return nil
 					continue
 				}
-				pkg, _ := w.Import(name)
+				pkg, err := w.Import(name)
+				if _, nogo := err.(*build.NoGoError); nogo {
+					continue
+				}
+				if err != nil {
+					log.Fatalf("Import(%q): %v", name, err)
+				}
 				w.export(pkg)
 			}
 		}
@@ -470,7 +476,7 @@ func (w *Walker) Import(name string) (*types.Package, error) {
 	info, err := context.ImportDir(dir, 0)
 	if err != nil {
 		if _, nogo := err.(*build.NoGoError); nogo {
-			return nil, nil
+			return nil, err
 		}
 		log.Fatalf("pkg %q, dir %q: ScanDir: %v", name, dir, err)
 	}

src/cmd/api/goapi_test.go

@@ -203,3 +203,16 @@ func TestIssue21181(t *testing.T) {
 		w.export(pkg)
 	}
 }
+
+func TestIssue29837(t *testing.T) {
+	for _, c := range contexts {
+		c.Compiler = build.Default.Compiler
+	}
+	for _, context := range contexts {
+		w := NewWalker(context, "testdata/src/issue29837")
+		_, err := w.Import("p")
+		if _, nogo := err.(*build.NoGoError); !nogo {
+			t.Errorf("expected *build.NoGoError, got %T", err)
+		}
+	}
+}

src/cmd/api/testdata/src/issue29837/p/README

+Empty directory for test, see https://golang.org/issues/29837.
\ No newline at end of file