config: Allow overrides from the command line

This patch allows the configuration values to be overridden from the
command-line, with a new -config_overrides flag.

There is a fairly specific use case for this, when editing the
configuration file is not feasible or convenient (e.g. running an
user-supplied configuration in a managed environment).

internal/config/config.go

@@ -33,8 +33,8 @@ var defaultConfig = &Config{
 	MailLogPath: "<syslog>",
 }
 
-// Load the config from the given file.
-func Load(path string) (*Config, error) {
+// Load the config from the given file, with the given overrides.
+func Load(path, overrides string) (*Config, error) {
 	// Start with a copy of the default config.
 	c := proto.Clone(defaultConfig).(*Config)
 
@@ -51,6 +51,14 @@ func Load(path string) (*Config, error) {
 	}
 	override(c, fromFile)
 
+	// Handle command line overrides.
+	fromOverrides := &Config{}
+	err = prototext.Unmarshal([]byte(overrides), fromOverrides)
+	if err != nil {
+		return nil, fmt.Errorf("parsing override: %v", err)
+	}
+	override(c, fromOverrides)
+
 	// Handle hostname separate, because if it is set, we don't need to call
 	// os.Hostname which can fail.
 	if c.Hostname == "" {