chore(master): run the master as a non-root user (#25) (#27)

cmd/master/main.go

@@ -48,19 +48,29 @@ const pollFirstDelay = time.Minute
 const pollWorkers = 4
 
 // registryCredentialsDir returns the directory the registry client reads
-// credentials from (DOCKER_CONFIG, else ~/.docker) and whether a config.json
-// is in it. Only the path is logged, never the file.
-func registryCredentialsDir() (dir string, found bool) {
+// credentials from (DOCKER_CONFIG, else ~/.docker), whether a config.json is
+// in it, and the error from opening it when it exists but cannot be read (for
+// example because the master runs as another user than the file owner). The
+// file is opened and closed, never read; only the path is logged.
+func registryCredentialsDir() (dir string, found bool, readErr error) {
 	dir = os.Getenv("DOCKER_CONFIG")
 	if dir == "" {
 		home, err := os.UserHomeDir()
 		if err != nil {
-			return "", false
+			return "", false, nil
 		}
 		dir = filepath.Join(home, ".docker")
 	}
-	_, err := os.Stat(filepath.Join(dir, "config.json"))
-	return dir, err == nil
+	path := filepath.Join(dir, "config.json")
+	if _, err := os.Stat(path); err != nil {
+		return dir, false, nil
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return dir, true, err
+	}
+	_ = f.Close()
+	return dir, true, nil
 }
 
 func run() error {
@@ -154,8 +164,12 @@ func run() error {
 		BaseContext: baseContext,
 	}
 
-	credDir, credFound := registryCredentialsDir()
-	logger.Info("registry_credentials", "docker_config", credDir, "config_json_found", credFound)
+	credDir, credFound, credReadErr := registryCredentialsDir()
+	logger.Info("registry_credentials", "docker_config", credDir, "config_json_found", credFound,
+		"config_json_readable", credFound && credReadErr == nil)
+	if credReadErr != nil {
+		logger.Warn("registry_credentials_unreadable", "docker_config", credDir, "error", credReadErr)
+	}
 	pollCtx, stopPolling := context.WithCancel(ctx)
 	defer stopPolling()
 	registryPoller := poller.New(st, jobService, registry.New("package-updater-master/"+version, logger), broker, poller.Config{