Stop a 404 or a rate limit being read as the offsets data

[greluc]

A 404 or a rate limit from the offsets mirror can be read as the offsets data.

Why

Both fetchers in src/main/offsetStore.ts hand the response straight to response.json():

return fetch(`${url}/lookup.json`)
	.then((response) => response.json())
	.then((data) => {
		return data as IOffsetsLookup;
	})

Nothing looks at the status, so a non-200 whose body happens to be JSON is returned as if it were the payload. Whether that throws depends on what the host chose to send back: raw.githubusercontent.com answers a missing file with the plain text 404: Not Found, which fails to parse and is caught — but an error or rate-limit page carrying JSON parses cleanly and is accepted. The offsets decide where the client reads inside the game's memory, so what happens next is not a clean failure.

Two smaller things sit alongside it:

• One attempt per host. raw.githubusercontent.com rate limits per IP. A household where several people start the game at once, or anyone behind a shared address, can be turned away by both hosts and told to check their internet connection.
• No timeout. A request that hangs holds up the start with nothing to cut it off.

What changed

fetchOffsetLookupJson and fetchOffsetsJson were the same function with a different path and a different error, so they are folded into one:

async function fetchJsonFromMirrors<T>(path: string, failure: string): Promise<T>

It checks response.ok, tries both hosts, tries again twice after a growing backoff, and gives each request ten seconds. The thrown value is unchanged — still Errors.LOOKUP_FETCH_ERROR / Errors.OFFSETS_FETCH_ERROR — so fetchOffsetLookup's fallback to the cached lookup still sees exactly what it expects. Net: 39 insertions, 31 deletions, one file, no new dependencies.

Verification

There is no test runner in this repository, so the check is a harness that extracts the real fetcher from offsetStore.ts and drives it against a scripted fetch. The same script runs against either revision, which is what makes the before column meaningful. The timeout and backoff constants are shrunk from seconds to milliseconds so it finishes; the control flow around them is untouched.

	nightly	this branch
both hosts healthy	ok	ok
first host 404s with a JSON body	FAIL — returns {"error":"Not Found"} as the lookup	ok, falls to the second host
both hosts rate-limited, healthy next round	FAIL — returns {"message":"rate limited"} as the lookup	ok, succeeds on the retry
a host hangs	FAIL — never returns	ok, times out and uses the other host
every host and round fails	ok — throws LOOKUP_FETCH_ERROR	ok — same error

eslint src and tsc --noEmit are clean, and prettier reports the file unchanged.

Limits, stated plainly

The harness drives the fetcher with a fake fetch; nothing here was run against the real mirror or a real rate limit, and the app was not started. The retry budget is a judgement call — three rounds over two hosts, backing off 1.5s then 3s, so a completely dead mirror now costs about 4.5 seconds before falling back to the cache rather than failing immediately. If you would rather that be shorter, or the timeout longer, say so and I will change it; the constants are together at the top.

I have not touched the caching or validation of what comes back, which is a separate question from whether the fetch itself is sound.

Disclaimer

This change was written with AI assistance (Claude Code). The author reviewed it, and every claim above was checked by running code rather than reasoned about — the before/after table is real output from the harness against both revisions. Please review it as you would any patch from a stranger; if something here is wrong I would rather hear it than have it merged.

🤖 Generated with Claude Code

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