Add config from environment

Author: ptman

main.go

@@ -103,6 +103,17 @@ func readConfig(cfile io.Reader, conf *config) {
 	}
 }
 
+// applyEnvOverrides updates config fields from environment variables when set.
+func applyEnvOverrides(conf *config) {
+	if port := os.Getenv("PORT"); port != "" {
+		conf.Listen = ":" + port
+	}
+
+	if databaseURL := os.Getenv("DATABASE_URL"); databaseURL != "" {
+		conf.DB = databaseURL
+	}
+}
+
 // setupServeMux returns a set up http.Handler.
 func setupServeMux(db *sql.DB) http.Handler {
 	mux := http.NewServeMux()
@@ -187,6 +198,7 @@ func main() {
 	var err error
 
 	readConfigFile("config.json", &conf)
+	applyEnvOverrides(&conf)
 
 	if conf.Debug {
 		logLevel.Set(slog.LevelDebug)

main_test.go

@@ -78,6 +78,48 @@ func TestConfigFromFile(t *testing.T) {
 	readConfigFile("config.json.sample", conf)
 }
 
+func TestApplyEnvOverrides(t *testing.T) {
+	t.Run("overrides when env vars are set", func(t *testing.T) {
+		t.Setenv("PORT", "1234")
+		t.Setenv("DATABASE_URL", "postgres://db.example/urlredir")
+
+		c := &config{
+			Listen: ":9999",
+			DB:     "host=/run/postgresql dbname=urlredir",
+		}
+
+		applyEnvOverrides(c)
+
+		if c.Listen != ":1234" {
+			t.Fatalf("Listen = %q, want %q", c.Listen, ":1234")
+		}
+
+		if c.DB != "postgres://db.example/urlredir" {
+			t.Fatalf("DB = %q, want %q", c.DB, "postgres://db.example/urlredir")
+		}
+	})
+
+	t.Run("keeps config when env vars are unset", func(t *testing.T) {
+		t.Setenv("PORT", "")
+		t.Setenv("DATABASE_URL", "")
+
+		c := &config{
+			Listen: ":8080",
+			DB:     "host=/run/postgresql dbname=urlredir",
+		}
+
+		applyEnvOverrides(c)
+
+		if c.Listen != ":8080" {
+			t.Fatalf("Listen = %q, want %q", c.Listen, ":8080")
+		}
+
+		if c.DB != "host=/run/postgresql dbname=urlredir" {
+			t.Fatalf("DB = %q, want %q", c.DB, "host=/run/postgresql dbname=urlredir")
+		}
+	})
+}
+
 func TestSetupServeMux(t *testing.T) {
 	t.Parallel()