Reject non-regular explicit config files

config/user_config.go

@@ -364,8 +364,8 @@ func RequireConfigFile(fn string) error {
 	if err != nil {
 		return fmt.Errorf("config file %q: %w", path, err)
 	}
-	if info.IsDir() {
-		return fmt.Errorf("config file %q is a directory", path)
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("config file %q is not a regular file", path)
 	}
 	return nil
 }

config/user_config_test.go

@@ -17,6 +17,16 @@ func TestRequireConfigFileRejectsMissingExplicitPath(t *testing.T) {
 	}
 }
 
+func TestRequireConfigFileRejectsNonRegularFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.fifo")
+	if err := os.Mkdir(path, 0700); err != nil {
+		t.Fatal(err)
+	}
+	if err := RequireConfigFile(path); err == nil {
+		t.Fatal("directory was accepted as an explicit config file")
+	}
+}
+
 func TestConfigBackfillsRecordingDefaults(t *testing.T) {
 	dir := t.TempDir()
 	configPath := filepath.Join(dir, "barnard.toml")