cli/config/configfile: Atomically rewrite the config file when saving.

The config file was being truncated first, which created a window during
which it was empty, causing concurrent uses of the `docker` command to
potentially fail with:
  WARNING: Error loading config file: /var/lib/jenkins/.docker/config.json: EOF
  Error response from daemon: Get https://registry/v2/foo/manifests/latest: no basic auth credentials

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>

cli/config/configfile/file.go

@@ -175,15 +175,21 @@ func (configFile *ConfigFile) Save() error {
 		return errors.Errorf("Can't save config with empty filename")
 	}
 
-	if err := os.MkdirAll(filepath.Dir(configFile.Filename), 0700); err != nil {
+	dir := filepath.Dir(configFile.Filename)
+	if err := os.MkdirAll(dir, 0700); err != nil {
 		return err
 	}
-	f, err := os.OpenFile(configFile.Filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
+	temp, err := ioutil.TempFile(dir, filepath.Base(configFile.Filename))
 	if err != nil {
 		return err
 	}
-	defer f.Close()
-	return configFile.SaveToWriter(f)
+	err = configFile.SaveToWriter(temp)
+	temp.Close()
+	if err != nil {
+		os.Remove(temp.Name())
+		return err
+	}
+	return os.Rename(temp.Name(), configFile.Filename)
 }
 
 // ParseProxyConfig computes proxy configuration by retrieving the config for the provided host and

cli/config/configfile/file_test.go

@@ -2,6 +2,8 @@ package configfile
 
 import (
 	"fmt"
+	"io/ioutil"
+	"os"
 	"testing"
 
 	"github.com/docker/cli/cli/config/credentials"
@@ -413,3 +415,13 @@ func TestCheckKubernetesConfigurationRaiseAnErrorOnInvalidValue(t *testing.T) {
 		}
 	}
 }
+
+func TestSave(t *testing.T) {
+	configFile := New("test-save")
+	defer os.Remove("test-save")
+	err := configFile.Save()
+	assert.NilError(t, err)
+	cfg, err := ioutil.ReadFile("test-save")
+	assert.NilError(t, err)
+	assert.Check(t, is.Equal(string(cfg), "{\n	\"auths\": {}\n}"))
+}