Fix ConfigFile.Save() replacing symlink with file

In situations where `~/.docker/config.json` was a symlink, saving
the file would replace the symlink with a file, instead of updating
the target file location;

    mkdir -p ~/.docker
    touch ~/real-config.json
    ln -s ~/real-config.json ~/.docker/config.json

    ls -la ~/.docker/config.json
    # lrwxrwxrwx 1 root root 22 Jun 23 12:34 /root/.docker/config.json -> /root/real-config.json

    docker login
    # Username: thajeztah
    # Password:

    # Login Succeeded

    ls -la ~/.docker/config.json
    -rw-r--r-- 1 root root 229 Jun 23 12:36 /root/.docker/config.json

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/config/configfile/file.go

@@ -209,9 +209,15 @@ func (configFile *ConfigFile) Save() (retErr error) {
 		return errors.Wrap(err, "error closing temp file")
 	}
 
+	// Handle situation where the configfile is a symlink
+	cfgFile := configFile.Filename
+	if f, err := os.Readlink(cfgFile); err == nil {
+		cfgFile = f
+	}
+
 	// Try copying the current config file (if any) ownership and permissions
-	copyFilePermissions(configFile.Filename, temp.Name())
-	return os.Rename(temp.Name(), configFile.Filename)
+	copyFilePermissions(cfgFile, temp.Name())
+	return os.Rename(temp.Name(), cfgFile)
 }
 
 // ParseProxyConfig computes proxy configuration by retrieving the config for the provided host and

cli/config/configfile/file_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/docker/cli/cli/config/types"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
+	"gotest.tools/v3/fs"
 	"gotest.tools/v3/golden"
 )
 
@@ -468,6 +469,33 @@ func TestSave(t *testing.T) {
 	assert.Check(t, is.Equal(string(cfg), "{\n	\"auths\": {}\n}"))
 }
 
+func TestSaveWithSymlink(t *testing.T) {
+	dir := fs.NewDir(t, t.Name(), fs.WithFile("real-config.json", `{}`))
+	defer dir.Remove()
+
+	symLink := dir.Join("config.json")
+	realFile := dir.Join("real-config.json")
+	err := os.Symlink(realFile, symLink)
+	assert.NilError(t, err)
+
+	configFile := New(symLink)
+
+	err = configFile.Save()
+	assert.NilError(t, err)
+
+	fi, err := os.Lstat(symLink)
+	assert.NilError(t, err)
+	assert.Assert(t, fi.Mode()&os.ModeSymlink != 0, "expected %s to be a symlink", symLink)
+
+	cfg, err := ioutil.ReadFile(symLink)
+	assert.NilError(t, err)
+	assert.Check(t, is.Equal(string(cfg), "{\n	\"auths\": {}\n}"))
+
+	cfg, err = ioutil.ReadFile(realFile)
+	assert.NilError(t, err)
+	assert.Check(t, is.Equal(string(cfg), "{\n	\"auths\": {}\n}"))
+}
+
 func TestPluginConfig(t *testing.T) {
 	configFile := New("test-plugin")
 	defer os.Remove("test-plugin")