jcoffey-dev
4d18d94b63
A timetable, a rota, a public holiday list: the calendars people are given as a link, which ihasmail could not show at all. Nothing is stored. The document is fetched when the calendar is opened and parsed in the browser; the server keeps no copy, no cache and no schedule, which is what lets an immutable container serve this. There is no timer either -- there is nowhere to run one -- so the guarantee is that a subscription is as current as the last time somebody looked, which is also when it matters. That is said plainly rather than implied. The fetch has to happen on the server: a calendar URL belongs to whoever published it and almost none of them send CORS headers. That makes it the second place this app knocks on a door somebody else chose, so the guard the image proxy has always had was lifted out and both now call it. A second SSRF implementation is how one of them ends up missing a case; this way there is one, and the extraction is covered by the image proxy's own tests still passing unchanged. webcal: is understood, because that is how these are published, and it is read as https: rather than waved past the checks -- a webcal URL pointing at loopback is refused exactly like an http one. Recurrence is deliberately not expanded. RRULE is a small language with a lot of edge cases, and a subscription quietly showing the wrong dates would be worse than one showing the first occurrence and saying so. The parser is a subscription parser rather than an importer: a subscribed calendar is read-only and redrawn from scratch each refresh, so nothing has to round-trip or survive an edit, which is most of what makes a full iCalendar implementation large. What it does have to do is never mis-state a time -- a DATE is built in local time rather than at UTC midnight, which would land on the day before for anyone west of Greenwich -- and never hang on a document somebody else wrote. Events go through instancesIn like the birthdays, so no view has to know they are not real calendars, and the calendar they hang off reports no write rights, so everything that asks before offering an edit declines on its own. A subscription that cannot be read says so in the sidebar rather than drawing an empty calendar, which looks like a calendar with nothing in it.
86 lines
3.3 KiB
TypeScript
86 lines
3.3 KiB
TypeScript
import type { Context } from "hono";
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import { safeFetch, safeFetchStatus } from "./imageproxy.js";
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/**
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* Fetching a calendar somebody has subscribed to.
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*
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* The browser cannot do this itself: a calendar URL belongs to whoever
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* published it and almost none of them send CORS headers, so the request has
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* to be made from here. That makes it the second place ihasmail reaches out to
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* an address a stranger chose, and it goes through exactly the same guard as
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* the first — `safeFetch` resolves the name, refuses private space on every
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* answer, pins the connection to the address it checked, and re-checks each
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* redirect. There is deliberately no second implementation of that.
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*
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* **Nothing is stored.** The text goes straight back to the browser, which
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* parses it and holds the result in memory for as long as the tab is open. The
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* server keeps no copy, no cache and no schedule, which is what lets an
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* immutable container serve this at all.
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*/
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/** Generous for a calendar, small enough that nobody can post a film through it. */
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const MAX_ICS_BYTES = 4 * 1024 * 1024;
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/**
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* Types a calendar is served as in practice. `text/plain` and the octet-stream
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* are here because a great many servers get this wrong, and refusing a real
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* calendar over a header the publisher chose badly helps nobody -- the parser
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* checks the content itself, which is the claim that actually matters.
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*/
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const ACCEPTABLE = new Set(["text/calendar", "text/plain", "application/octet-stream", "application/ics", ""]);
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export async function icsProxyHandler(c: Context) {
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const got = await safeFetch(c.req.query("url") ?? "", 20_000);
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if (typeof got === "string") return c.json({ error: got }, safeFetchStatus(got) as 400);
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const { res, done } = got;
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if (!res.statusCode || res.statusCode < 200 || res.statusCode >= 300) {
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done();
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res.resume();
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return c.json({ error: "fetch_failed", status: res.statusCode ?? 0 }, 502);
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}
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const type = (res.headers["content-type"] ?? "").split(";")[0]!.trim().toLowerCase();
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if (!ACCEPTABLE.has(type)) {
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done();
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res.resume();
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return c.json({ error: "not_calendar", type }, 415);
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}
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const declared = Number(res.headers["content-length"] ?? "0");
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if (declared > MAX_ICS_BYTES) {
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done();
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res.resume();
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return c.json({ error: "too_large" }, 413);
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}
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// Read it here rather than streaming: the browser needs the whole document
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// to parse it, and the cap has to hold whether or not a length was declared.
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let total = 0;
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const chunks: Buffer[] = [];
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try {
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await new Promise<void>((resolve, reject) => {
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res.on("data", (chunk: Buffer) => {
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total += chunk.byteLength;
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if (total > MAX_ICS_BYTES) {
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res.destroy();
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reject(new Error("too_large"));
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return;
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}
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chunks.push(chunk);
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});
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res.on("end", () => resolve());
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res.on("error", reject);
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});
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} catch (err) {
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done();
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return c.json({ error: (err as Error).message === "too_large" ? "too_large" : "fetch_failed" }, 502);
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}
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done();
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return c.body(Buffer.concat(chunks).toString("utf8"), 200, {
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"Content-Type": "text/calendar; charset=utf-8",
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// Never stored on disk, and never held by anything in between either.
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"Cache-Control": "no-store",
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"X-Content-Type-Options": "nosniff",
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});
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}
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