cmd/go: fix vendor-related index out of range panic on bad file tree

Fixes #12156.

Change-Id: I2d71163b98bcc770147eb9e78dc551a9d0b5b817
Reviewed-on: https://go-review.googlesource.com/13674Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/pkg.go

@@ -416,7 +416,7 @@ func vendoredImportPath(parent *Package, path string) (found string, searched []
 		return path, nil
 	}
 	dir := filepath.Clean(parent.Dir)
-	root := filepath.Clean(parent.Root)
+	root := filepath.Join(parent.Root, "src")
 	if !hasFilePathPrefix(dir, root) || len(dir) <= len(root) || dir[len(root)] != filepath.Separator {
 		fatalf("invalid vendoredImportPath: dir=%q root=%q separator=%q", dir, root, string(filepath.Separator))
 	}

src/cmd/go/testdata/testvendor2/src/p/p.go

+package p
+
+import "x"

src/cmd/go/testdata/testvendor2/vendor/x/x.go

+package x

src/cmd/go/vendor_test.go

@@ -244,3 +244,15 @@ func TestVendorList(t *testing.T) {
 	tg.run("list", "-f", `{{join .XTestImports "\n"}}`, "github.com/rsc/go-get-issue-11864/vendor/vendor.org/tx3")
 	tg.grepStdout("go-get-issue-11864/vendor/vendor.org/tx3", "did not find vendor-expanded tx3")
 }
+
+func TestVendor12156(t *testing.T) {
+	// Former index out of range panic.
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata/testvendor2"))
+	tg.setenv("GO15VENDOREXPERIMENT", "1")
+	tg.cd(filepath.Join(tg.pwd(), "testdata/testvendor2/src/p"))
+	tg.runFail("build", "p.go")
+	tg.grepStderrNot("panic", "panicked")
+	tg.grepStderr(`cannot find package "x"`, "wrong error")
+}