configFile: directly write changes to configFile

In some situations, the config.json is volumed from somewhere else and the file is busy, move can't be performed on this file and configFile.Save() will be failed because it's trying to move another file to this file. This will write changes directly to the file.
For concurrency issues, it will lock the file using flock and release after write.

Signed-off-by: Seena Fallah <seenafallah@gmail.com>

Author: clwluvw

cli/config/configfile/file.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/docker/cli/cli/config/credentials"
 	"github.com/docker/cli/cli/config/types"
@@ -194,37 +195,37 @@ func (configFile *ConfigFile) Save() (retErr error) {
 	if err := os.MkdirAll(dir, 0700); err != nil {
 		return err
 	}
-	temp, err := ioutil.TempFile(dir, filepath.Base(configFile.Filename))
+
+	// Handle situation where the configfile is a symlink
+	cfgFile := configFile.Filename
+	if f, err := os.Readlink(cfgFile); err == nil {
+		cfgFile = f
+	}
+
+	f, err := os.OpenFile(cfgFile, os.O_CREATE|os.O_WRONLY, 0600)
 	if err != nil {
 		return err
 	}
 	defer func() {
-		temp.Close()
-		if retErr != nil {
-			if err := os.Remove(temp.Name()); err != nil {
-				logrus.WithError(err).WithField("file", temp.Name()).Debug("Error cleaning up temp file")
-			}
+		err = syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
+		if err != nil {
+			logrus.WithError(err).WithField("file", cfgFile).Error("Unable to unlock config file")
 		}
+		f.Close()
 	}()
 
-	err = configFile.SaveToWriter(temp)
+	err = syscall.Flock(int(f.Fd()), syscall.LOCK_EX)
 	if err != nil {
+		logrus.WithError(err).WithField("file", cfgFile).Error("Unable to lock config file")
 		return err
 	}
 
-	if err := temp.Close(); err != nil {
-		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
+	err = f.Truncate(0)
+	if err != nil {
+		return err
 	}
 
-	// Try copying the current config file (if any) ownership and permissions
-	copyFilePermissions(cfgFile, temp.Name())
-	return os.Rename(temp.Name(), cfgFile)
+	return configFile.SaveToWriter(f)
 }
 
 // ParseProxyConfig computes proxy configuration by retrieving the config for the provided host and