Reload daemon even if "/etc/docker/daemon.json" does not exist

Before this commit if "--config-file" wasn't set the daemon would use
the default configuration file which is "/etc/docker/daemon.json".

When attempting to reload the daemon if that file didn't exist
and error message would display.

This behaviour is changed in a way that if the default configuration
file does not exist and no other configuration file is set
the daemon uses an empty configuration which later will be updated
and reloaded using the "reload" function given as an argument in Reload.

However, if the "--config-file" is set and the file is removed
or renamed an error message will be displayed and no reload will be done.

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

daemon/config/config.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 	"reflect"
 	"runtime"
 	"strings"
@@ -244,7 +245,10 @@ func Reload(configFile string, flags *pflag.FlagSet, reload func(*Config)) error
 	logrus.Infof("Got signal to reload configuration, reloading from: %s", configFile)
 	newConfig, err := getConflictFreeConfiguration(configFile, flags)
 	if err != nil {
-		return err
+		if flags.Changed("config-file") || !os.IsNotExist(err) {
+			return fmt.Errorf("unable to configure the Docker daemon with file %s: %v", configFile, err)
+		}
+		newConfig = New()
 	}
 
 	if err := Validate(newConfig); err != nil {

daemon/config/config_test.go

@@ -389,3 +389,49 @@ func discoveryConfig(backendAddr, advertiseAddr string, opts map[string]string)
 		},
 	}
 }
+
+// TestReloadSetConfigFileNotExist tests that when `--config-file` is set
+// and it doesn't exist the `Reload` function returns an error.
+func TestReloadSetConfigFileNotExist(t *testing.T) {
+	configFile := "/tmp/blabla/not/exists/config.json"
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flags.String("config-file", "", "")
+	flags.Set("config-file", configFile)
+
+	err := Reload(configFile, flags, func(c *Config) {})
+	assert.Error(t, err)
+	testutil.ErrorContains(t, err, "unable to configure the Docker daemon with file")
+}
+
+// TestReloadDefaultConfigNotExist tests that if the default configuration file
+// doesn't exist the daemon still will be reloaded.
+func TestReloadDefaultConfigNotExist(t *testing.T) {
+	reloaded := false
+	configFile := "/etc/docker/daemon.json"
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flags.String("config-file", configFile, "")
+	err := Reload(configFile, flags, func(c *Config) {
+		reloaded = true
+	})
+	assert.Nil(t, err)
+	assert.True(t, reloaded)
+}
+
+// TestReloadBadDefaultConfig tests that when `--config-file` is not set
+// and the default configuration file exists and is bad return an error
+func TestReloadBadDefaultConfig(t *testing.T) {
+	f, err := ioutil.TempFile("", "docker-config-")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	configFile := f.Name()
+	f.Write([]byte(`{wrong: "configuration"}`))
+	f.Close()
+
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flags.String("config-file", configFile, "")
+	err = Reload(configFile, flags, func(c *Config) {})
+	assert.Error(t, err)
+	testutil.ErrorContains(t, err, "unable to configure the Docker daemon with file")
+}