cmd/go/internal/modload: ignore directories when looking for go.mod file in Plan 9

This fixes builds in Plan 9 under /n. Since directories in /n are
automatically created, /n/go.mod always exists.

Fixes #27074

Change-Id: Ie9a1155b7c316bdc27655f5b99172550b413838d
Reviewed-on: https://go-review.googlesource.com/129804Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modload/init.go

@@ -24,6 +24,7 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strconv"
 	"strings"
 )
@@ -379,7 +380,7 @@ func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string)
 
 	// Look for enclosing go.mod.
 	for {
-		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
+		if fi, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil && !(runtime.GOOS == "plan9" && fi.IsDir()) {
 			return dir, "go.mod"
 		}
 		if dir == limit {
@@ -397,7 +398,7 @@ func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string)
 		dir = dir1
 		for {
 			for _, name := range altConfigs {
-				if _, err := os.Stat(filepath.Join(dir, name)); err == nil {
+				if fi, err := os.Stat(filepath.Join(dir, name)); err == nil && !(runtime.GOOS == "plan9" && fi.IsDir()) {
 					return dir, name
 				}
 			}

src/cmd/go/internal/modload/init_plan9_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestFindModuleRootIgnoreDir(t *testing.T) {
+	// In Plan 9, directories are automatically created in /n.
+	// For example, /n/go.mod always exist, but it's a directory.
+	// Test that we ignore directories when trying to find go.mod and other config files.
+
+	dir, err := ioutil.TempDir("", "gotest")
+	if err != nil {
+		t.Fatalf("failed to create temporary directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+	if err := os.Mkdir(filepath.Join(dir, "go.mod"), os.ModeDir|0755); err != nil {
+		t.Fatalf("Mkdir failed: %v", err)
+	}
+	for _, name := range altConfigs {
+		if err := os.MkdirAll(filepath.Join(dir, name), os.ModeDir|0755); err != nil {
+			t.Fatalf("MkdirAll failed: %v", err)
+		}
+	}
+	p := filepath.Join(dir, "example")
+	if err := os.Mkdir(p, os.ModeDir|0755); err != nil {
+		t.Fatalf("Mkdir failed: %v", err)
+	}
+	if root, _ := FindModuleRoot(p, "", false); root != "" {
+		t.Errorf("FindModuleRoot(%q, \"\", false): %q, want empty string", p, root)
+	}
+	if root, _ := FindModuleRoot(p, "", true); root != "" {
+		t.Errorf("FindModuleRoot(%q, \"\", true): %q, want empty string", p, root)
+	}
+}