cmd/go/internal/cache: squelch cache init warnings when $HOME is /

Docker sets $HOME to / when running with a UID that doesn't exist within
the container.  This not  uncommon on CI servers.

Fixes #26280

Change-Id: Ic7ff62b41403fe6e7c0cef12814667ef73f6c954
Reviewed-on: https://go-review.googlesource.com/122487
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/cache/default.go

@@ -35,12 +35,14 @@ See golang.org to learn more about Go.
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
 func initDefaultCache() {
-	dir := DefaultDir()
+	dir, showWarnings := defaultDir()
 	if dir == "off" {
 		return
 	}
 	if err := os.MkdirAll(dir, 0777); err != nil {
+		if showWarnings {
 			fmt.Fprintf(os.Stderr, "go: disabling cache (%s) due to initialization failure: %s\n", dir, err)
+		}
 		return
 	}
 	if _, err := os.Stat(filepath.Join(dir, "README")); err != nil {
@@ -50,7 +52,9 @@ func initDefaultCache() {
 
 	c, err := Open(dir)
 	if err != nil {
+		if showWarnings {
 			fmt.Fprintf(os.Stderr, "go: disabling cache (%s) due to initialization failure: %s\n", dir, err)
+		}
 		return
 	}
 	defaultCache = c
@@ -59,14 +63,24 @@ func initDefaultCache() {
 // DefaultDir returns the effective GOCACHE setting.
 // It returns "off" if the cache is disabled.
 func DefaultDir() string {
+	dir, _ := defaultDir()
+	return dir
+}
+
+// defaultDir returns the effective GOCACHE setting.
+// It returns "off" if the cache is disabled.
+// The second return value reports whether warnings should
+// be shown if the cache fails to initialize.
+func defaultDir() (string, bool) {
 	dir := os.Getenv("GOCACHE")
 	if dir != "" {
-		return dir
+		return dir, true
 	}
 
 	// Compute default location.
 	// TODO(rsc): This code belongs somewhere else,
 	// like maybe ioutil.CacheDir or os.CacheDir.
+	showWarnings := true
 	switch runtime.GOOS {
 	case "windows":
 		dir = os.Getenv("LocalAppData")
@@ -76,20 +90,20 @@ func DefaultDir() string {
 			dir = os.Getenv("AppData")
 		}
 		if dir == "" {
-			return "off"
+			return "off", true
 		}
 
 	case "darwin":
 		dir = os.Getenv("HOME")
 		if dir == "" {
-			return "off"
+			return "off", true
 		}
 		dir += "/Library/Caches"
 
 	case "plan9":
 		dir = os.Getenv("home")
 		if dir == "" {
-			return "off"
+			return "off", true
 		}
 		// Plan 9 has no established per-user cache directory,
 		// but $home/lib/xyz is the usual equivalent of $HOME/.xyz on Unix.
@@ -101,10 +115,15 @@ func DefaultDir() string {
 		if dir == "" {
 			dir = os.Getenv("HOME")
 			if dir == "" {
-				return "off"
+				return "off", true
+			}
+			if dir == "/" {
+				// probably docker run with -u flag
+				// https://golang.org/issue/26280
+				showWarnings = false
 			}
 			dir += "/.cache"
 		}
 	}
-	return filepath.Join(dir, "go-build")
+	return filepath.Join(dir, "go-build"), showWarnings
 }

src/cmd/go/internal/cache/default_unix_test.go

@@ -0,0 +1,67 @@
+// 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.
+
+// +build !windows,!darwin,!plan9
+
+package cache
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestDefaultDir(t *testing.T) {
+	goCacheDir := "/tmp/test-go-cache"
+	xdgCacheDir := "/tmp/test-xdg-cache"
+	homeDir := "/tmp/test-home"
+
+	// undo env changes when finished
+	defer func(GOCACHE, XDG_CACHE_HOME, HOME string) {
+		os.Setenv("GOCACHE", GOCACHE)
+		os.Setenv("XDG_CACHE_HOME", XDG_CACHE_HOME)
+		os.Setenv("HOME", HOME)
+	}(os.Getenv("GOCACHE"), os.Getenv("XDG_CACHE_HOME"), os.Getenv("HOME"))
+
+	os.Setenv("GOCACHE", goCacheDir)
+	os.Setenv("XDG_CACHE_HOME", xdgCacheDir)
+	os.Setenv("HOME", homeDir)
+
+	dir, showWarnings := defaultDir()
+	if dir != goCacheDir {
+		t.Errorf("Cache DefaultDir %q should be $GOCACHE %q", dir, goCacheDir)
+	}
+	if !showWarnings {
+		t.Error("Warnings should be shown when $GOCACHE is set")
+	}
+
+	os.Unsetenv("GOCACHE")
+	dir, showWarnings = defaultDir()
+	if !strings.HasPrefix(dir, xdgCacheDir+"/") {
+		t.Errorf("Cache DefaultDir %q should be under $XDG_CACHE_HOME %q when $GOCACHE is unset", dir, xdgCacheDir)
+	}
+	if !showWarnings {
+		t.Error("Warnings should be shown when $XDG_CACHE_HOME is set")
+	}
+
+	os.Unsetenv("XDG_CACHE_HOME")
+	dir, showWarnings = defaultDir()
+	if !strings.HasPrefix(dir, homeDir+"/.cache/") {
+		t.Errorf("Cache DefaultDir %q should be under $HOME/.cache %q when $GOCACHE and $XDG_CACHE_HOME are unset", dir, homeDir+"/.cache")
+	}
+	if !showWarnings {
+		t.Error("Warnings should be shown when $HOME is not /")
+	}
+
+	os.Unsetenv("HOME")
+	if dir, _ := defaultDir(); dir != "off" {
+		t.Error("Cache not disabled when $GOCACHE, $XDG_CACHE_HOME, and $HOME are unset")
+	}
+
+	os.Setenv("HOME", "/")
+	if _, showWarnings := defaultDir(); showWarnings {
+		// https://golang.org/issue/26280
+		t.Error("Cache initalization warnings should be squelched when $GOCACHE and $XDG_CACHE_HOME are unset and $HOME is /")
+	}
+}