Function dispatchProxyLatenciesGuarded

site/src/contexts/useProxyLatency.ts:148–199

(entry: PerformanceEntry)

		// via the reducer. But it will only do so if the performance entry is
		// a resource entry that we care about.
		const dispatchProxyLatenciesGuarded = (entry: PerformanceEntry) => {
			if (entry.entryType !== "resource") {
				// We should never get these, but just in case.
				return;
			}

			// The entry.name is the url of the request.
			const check = proxyChecks[entry.name];
			if (!check) {
				// This is not a proxy request, so ignore it.
				return;
			}

			// These docs are super useful.
			// https://developer.mozilla.org/en-US/docs/Web/API/Performance_API/Resource_timing
			let latencyMS = 0;
			let accurate = false;
			let nextHopProtocol: string | undefined;
			if (
				"requestStart" in entry &&
				(entry as PerformanceResourceTiming).requestStart !== 0
			) {
				// This is the preferred logic to get the latency.
				const timingEntry = entry as PerformanceResourceTiming;
				latencyMS = timingEntry.responseStart - timingEntry.requestStart;
				accurate = true;
				nextHopProtocol = timingEntry.nextHopProtocol;
			} else {
				// This is the total duration of the request and will be off by a good margin.
				// This is a fallback if the better timing is not available.
				// We can remove this when we display the "accurate" bool on the UI
				console.warn(
					`Using fallback latency calculation for "${entry.name}". Latency will be incorrect and larger then actual.`,
				);
				latencyMS = entry.duration;
			}
			const update = {
				proxyID: check.id,
				cached: false,
				report: {
					latencyMS,
					accurate,
					at: new Date(),
					nextHopProtocol: nextHopProtocol,
				} as ProxyLatencyReport,
			};
			dispatchProxyLatencies(update);
			// Also save to local storage to persist the latency across page refreshes.
			updateStoredLatencies(update);

			return;
		};

		// Start a new performance observer to record of all the requests
		// to the proxies.