cmd/go: stop early in disallowInternal on erroneous packages

Fixes #11201

Change-Id: I80d8fcfcb5c856aaf9d0e73d756d86018e2bec3b
Reviewed-on: https://go-review.googlesource.com/11110Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -875,6 +875,12 @@ func testMove(t *testing.T, vcs, url, base, config string) {
 	tg.grepStderr("validating server certificate|not found", "go get -d -f -u "+url+" failed for wrong reason")
 }
 
+func TestInternalPackageErrorsAreHandled(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.run("list", "./testdata/testinternal3")
+}
+
 func TestMoveGit(t *testing.T) {
 	testMove(t, "git", "rsc.io/pdf", "pdf", "rsc.io/pdf/.git/config")
 }

src/cmd/go/pkg.go

@@ -327,6 +327,11 @@ func disallowInternal(srcDir string, p *Package, stk *importStack) *Package {
 	// is disallowed if the importing code is outside the tree
 	// rooted at the parent of the “internal” directory.
 
+	// There was an error loading the package; stop here.
+	if p.Error != nil {
+		return p
+	}
+
 	// The stack includes p.ImportPath.
 	// If that's the only thing on the stack, we started
 	// with a name given on the command line, not an

src/cmd/go/testdata/testinternal3/t.go

+package t
+
+import _ "internal/does-not-exist"