cmd/go: referee another vendor vs symlink fight

Avoid crash in the specific case reported in #15201 but also
print more useful error message, avoiding slice panic.

Fixes #15201.
Fixes #16167.
Fixes #16566.

Change-Id: I66499621e9678a05bc9b12b0da77906cd7027bdd
Reviewed-on: https://go-review.googlesource.com/31665
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/go_test.go

@@ -1759,6 +1759,27 @@ func TestSymlinksVendor(t *testing.T) {
 	tg.run("install")
 }
 
+// Issue 15201.
+func TestSymlinksVendor15201(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9", "windows":
+		t.Skipf("skipping symlink test on %s", runtime.GOOS)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+
+	tg.tempDir("gopath/src/x/y/_vendor/src/x")
+	tg.must(os.Symlink("../../..", tg.path("gopath/src/x/y/_vendor/src/x/y")))
+	tg.tempFile("gopath/src/x/y/w/w.go", "package w\nimport \"x/y/z\"\n")
+	tg.must(os.Symlink("../_vendor/src", tg.path("gopath/src/x/y/w/vendor")))
+	tg.tempFile("gopath/src/x/y/z/z.go", "package z\n")
+
+	tg.setenv("GOPATH", tg.path("gopath/src/x/y/_vendor")+string(filepath.ListSeparator)+tg.path("gopath"))
+	tg.cd(tg.path("gopath/src"))
+	tg.run("list", "./...")
+}
+
 func TestSymlinksInternal(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9", "windows":

src/cmd/go/pkg.go

@@ -415,13 +415,26 @@ func vendoredImportPath(parent *Package, path string) (found string) {
 
 	dir := filepath.Clean(parent.Dir)
 	root := filepath.Join(parent.Root, "src")
-	if !hasFilePathPrefix(dir, root) {
+	if !hasFilePathPrefix(dir, root) || parent.ImportPath != "command-line-arguments" && filepath.Join(root, parent.ImportPath) != dir {
 		// Look for symlinks before reporting error.
 		dir = expandPath(dir)
 		root = expandPath(root)
 	}
-	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))
+
+	if !hasFilePathPrefix(dir, root) || len(dir) <= len(root) || dir[len(root)] != filepath.Separator || parent.ImportPath != "command-line-arguments" && filepath.Join(root, parent.ImportPath) != dir {
+		fatalf("unexpected directory layout:\n"+
+			"	import path: %s\n"+
+			"	root: %s\n"+
+			"	dir: %s\n"+
+			"	expand root: %s\n"+
+			"	expand dir: %s\n"+
+			"	separator: %s",
+			parent.ImportPath,
+			filepath.Join(parent.Root, "src"),
+			filepath.Clean(parent.Dir),
+			root,
+			dir,
+			string(filepath.Separator))
 	}
 
 	vpath := "vendor/" + path