fix: modules losing data after HTTP 304 responses (#4180)

When a server responds with 304 (nothing changed since last fetch), the
response has no body. Several modules were trying to parse that empty
body anyway - which either cleared their cached data or threw an
exception. The result: a blank calendar, empty newsfeed, or missing
weather data after the next refresh cycle.

This was reported in the forum:
https://forum.magicmirror.builders/topic/20250/calendar-events-broadcasting-nothing-showing

The bug was "introduced" by #4120, which correctly started forwarding
304s to consumers - but not all were ready for it.

### Fix

Skip parsing on 304 and keep the existing data as-is:

- **calendar** - re-broadcasts cached events
- **newsfeed** - re-broadcasts cached items
- **buienradar, openmeteo, weatherflow, weathergov** - return early
before calling `response.json()`

defaultmodules/calendar/calendarfetcher.js

@@ -51,6 +51,13 @@ class CalendarFetcher {
 	 */
 	async #handleResponse (response) {
 		try {
+			// 304 Not Modified has no body: keep previously fetched events and just re-broadcast them.
+			if (response.status === 304) {
+				this.lastFetch = Date.now();
+				this.broadcastEvents();
+				return;
+			}
+
 			const responseData = await response.text();
 			const parsed = await ical.async.parseICS(responseData);
 

defaultmodules/newsfeed/newsfeedfetcher.js

@@ -68,6 +68,12 @@ class NewsfeedFetcher {
 	 * @param {Response} response - The fetch Response object
 	 */
 	async #handleResponse (response) {
+		// 304 Not Modified has no body: keep previously fetched items and re-broadcast them.
+		if (response.status === 304) {
+			this.broadcastItems();
+			return;
+		}
+
 		this.items = [];
 		const parser = new FeedMe();
 

defaultmodules/weather/providers/buienradar.js

@@ -111,6 +111,7 @@ class BuienradarProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/envcanada.js

@@ -69,6 +69,7 @@ class EnvCanadaProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				// Check if hour changed - restart fetcher with new URL
 				const newHour = new Date().toISOString().substring(11, 13);

defaultmodules/weather/providers/openmeteo.js

@@ -172,6 +172,7 @@ class OpenMeteoProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/openweathermap.js

@@ -73,6 +73,7 @@ class OpenWeatherMapProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/pirateweather.js

@@ -52,6 +52,7 @@ class PirateweatherProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/smhi.js

@@ -95,6 +95,7 @@ class SMHIProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/ukmetofficedatahub.js

@@ -64,6 +64,7 @@ class UkMetOfficeDataHubProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

defaultmodules/weather/providers/weatherflow.js

@@ -74,6 +74,7 @@ class WeatherFlowProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				const processed = this.#processData(data);

defaultmodules/weather/providers/weathergov.js

@@ -207,6 +207,7 @@ class WeatherGovProvider {
 		});
 
 		this.fetcher.on("response", async (response) => {
+			if (response.status === 304) return;
 			try {
 				const data = await response.json();
 				this.#handleResponse(data);

tests/unit/modules/default/calendar/calendar_fetcher_spec.js

@@ -0,0 +1,130 @@
+global.moment = require("moment-timezone");
+
+const ical = require("node-ical");
+const moment = require("moment-timezone");
+const defaults = require("../../../../../js/defaults");
+
+const CalendarFetcherUtils = require(`../../../../../${defaults.defaultModulesDir}/calendar/calendarfetcherutils`);
+
+const CalendarFetcher = require(`../../../../../${defaults.defaultModulesDir}/calendar/calendarfetcher`);
+
+const makeFetcher = (options = {}) => new CalendarFetcher(
+	options.url ?? "http://test.example.com/cal.ics",
+	options.reloadInterval ?? 60000,
+	options.excludedEvents ?? [],
+	options.maximumEntries ?? 10,
+	options.maximumNumberOfDays ?? 365,
+	options.auth ?? null,
+	options.includePastEvents ?? false,
+	options.selfSignedCert ?? false
+);
+
+// Triggers a fetch and resolves once the fetcher finishes (success or error).
+// On error, resolves with the errorInfo object so tests can inspect it.
+const emitResponse = (fetcher, response) => new Promise((resolve) => {
+	fetcher.onReceive(resolve);
+	fetcher.onError((_, errorInfo) => resolve(errorInfo));
+	fetcher.httpFetcher.emit("response", response);
+});
+
+const futureEventICS = () => {
+	const start = moment().add(1, "hour");
+	const end = moment().add(2, "hours");
+	return [
+		"BEGIN:VCALENDAR",
+		"BEGIN:VEVENT",
+		`DTSTART:${start.utc().format("YYYYMMDDTHHmmss")}Z`,
+		`DTEND:${end.utc().format("YYYYMMDDTHHmmss")}Z`,
+		"UID:future-1@test",
+		"SUMMARY:Future Event",
+		"END:VEVENT",
+		"END:VCALENDAR"
+	].join("\r\n");
+};
+
+describe("CalendarFetcher", () => {
+	afterEach(() => {
+		vi.restoreAllMocks();
+	});
+
+	describe("304 handling", () => {
+		it("keeps previously fetched events when a 304 Not Modified response arrives", async () => {
+			const fetcher = makeFetcher();
+
+			await emitResponse(fetcher, new Response(futureEventICS(), { status: 200 }));
+			expect(fetcher.events).toHaveLength(1);
+
+			// 304 Not Modified has an empty body: events must be preserved
+			await emitResponse(fetcher, new Response(null, { status: 304 }));
+			expect(fetcher.events).toHaveLength(1);
+		});
+	});
+
+	describe("error handling", () => {
+		it("forwards HTTP fetch errors to onError callback", () => {
+			const fetcher = makeFetcher();
+			const onError = vi.fn();
+			const errorInfo = { errorType: "NETWORK_ERROR", message: "boom" };
+
+			fetcher.onError(onError);
+			fetcher.httpFetcher.emit("error", errorInfo);
+
+			expect(onError).toHaveBeenCalledWith(fetcher, errorInfo);
+		});
+
+		it("keeps existing events and reports PARSE_ERROR when parsing fails", async () => {
+			const fetcher = makeFetcher();
+
+			await emitResponse(fetcher, new Response(futureEventICS(), { status: 200 }));
+			expect(fetcher.events).toHaveLength(1);
+
+			vi.spyOn(ical.async, "parseICS").mockRejectedValueOnce(new Error("invalid ics"));
+			const error = await emitResponse(fetcher, new Response("BROKEN", { status: 200 }));
+
+			expect(fetcher.events).toHaveLength(1);
+			expect(error).toMatchObject({
+				errorType: "PARSE_ERROR",
+				translationKey: "MODULE_ERROR_UNSPECIFIED",
+				url: "http://test.example.com/cal.ics"
+			});
+		});
+	});
+
+	describe("delegation and refetch", () => {
+		it("delegates fetchCalendar to HTTPFetcher.startPeriodicFetch", () => {
+			const fetcher = makeFetcher();
+			const startSpy = vi.spyOn(fetcher.httpFetcher, "startPeriodicFetch");
+
+			fetcher.fetchCalendar();
+
+			expect(startSpy).toHaveBeenCalledTimes(1);
+		});
+
+		it("shouldRefetch respects reload interval boundaries", () => {
+			const fetcher = makeFetcher();
+
+			expect(fetcher.shouldRefetch()).toBe(true);
+
+			fetcher.lastFetch = Date.now() - 59999;
+			expect(fetcher.shouldRefetch()).toBe(false);
+
+			fetcher.lastFetch = Date.now() - 60000;
+			expect(fetcher.shouldRefetch()).toBe(true);
+		});
+
+		it("passes configured filter options to CalendarFetcherUtils.filterEvents", async () => {
+			const excludedEvents = ["Do not show me"];
+			const filterSpy = vi.spyOn(CalendarFetcherUtils, "filterEvents");
+			const fetcher = makeFetcher({ excludedEvents, maximumEntries: 7, maximumNumberOfDays: 30, includePastEvents: true });
+
+			await emitResponse(fetcher, new Response(futureEventICS(), { status: 200 }));
+
+			expect(filterSpy).toHaveBeenCalledWith(expect.any(Object), {
+				excludedEvents,
+				includePastEvents: true,
+				maximumEntries: 7,
+				maximumNumberOfDays: 30
+			});
+		});
+	});
+});