cli/config: prevent warning if HOME is not set

commit c2626a8270 replaced the use of
github.com/docker/docker/pkg/homedir with Golang's os.UserHomeDir().

This change was partially reverted in 7a279af43d
to account for situations where `$HOME` is not set.

In  situations where no configuration file is present in `~/.config/`, the CLI
falls back to looking for the (deprecated) `~/.dockercfg` configuration file,
which was still using `os.UserHomeDir()`, which produces an error/warning if
`$HOME` is not set.

This patch introduces a helper function and a global variable to get the user's
home-directory. The global variable is used to prevent repeatedly looking up
the user's information (which, depending on the setup can be a costly operation).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c85a37dbb4)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/config/config.go

@@ -26,15 +26,29 @@ const (
 var (
 	initConfigDir sync.Once
 	configDir     string
+	homeDir       string
 )
 
+// resetHomeDir is used in testing to resets the "homeDir" package variable to
+// force re-lookup of the home directory between tests.
+func resetHomeDir() {
+	homeDir = ""
+}
+
+func getHomeDir() string {
+	if homeDir == "" {
+		homeDir = homedir.Get()
+	}
+	return homeDir
+}
+
 func setConfigDir() {
 	if configDir != "" {
 		return
 	}
 	configDir = os.Getenv("DOCKER_CONFIG")
 	if configDir == "" {
-		configDir = filepath.Join(homedir.Get(), configFileDir)
+		configDir = filepath.Join(getHomeDir(), configFileDir)
 	}
 }
 
@@ -109,11 +123,7 @@ func Load(configDir string) (*configfile.ConfigFile, error) {
 	}
 
 	// Can't find latest config file so check for the old one
-	home, err := os.UserHomeDir()
-	if err != nil {
-		return configFile, errors.Wrap(err, oldConfigfile)
-	}
-	filename = filepath.Join(home, oldConfigfile)
+	filename = filepath.Join(getHomeDir(), oldConfigfile)
 	if file, err := os.Open(filename); err == nil {
 		defer file.Close()
 		if err := configFile.LegacyLoadFromReader(file); err != nil {

cli/config/config_test.go

@@ -115,6 +115,7 @@ password`: "Invalid Auth config file",
 email`: "Invalid auth configuration file",
 	}
 
+	resetHomeDir()
 	tmpHome, err := ioutil.TempDir("", "config-test")
 	assert.NilError(t, err)
 	defer os.RemoveAll(tmpHome)
@@ -131,6 +132,7 @@ email`: "Invalid auth configuration file",
 }
 
 func TestOldValidAuth(t *testing.T) {
+	resetHomeDir()
 	tmpHome, err := ioutil.TempDir("", "config-test")
 	assert.NilError(t, err)
 	defer os.RemoveAll(tmpHome)
@@ -165,6 +167,7 @@ func TestOldValidAuth(t *testing.T) {
 }
 
 func TestOldJSONInvalid(t *testing.T) {
+	resetHomeDir()
 	tmpHome, err := ioutil.TempDir("", "config-test")
 	assert.NilError(t, err)
 	defer os.RemoveAll(tmpHome)
@@ -184,6 +187,7 @@ func TestOldJSONInvalid(t *testing.T) {
 }
 
 func TestOldJSON(t *testing.T) {
+	resetHomeDir()
 	tmpHome, err := ioutil.TempDir("", "config-test")
 	assert.NilError(t, err)
 	defer os.RemoveAll(tmpHome)