fix issue 210 slow source search reliability

lib/api/source-latency.ts

@@ -0,0 +1,69 @@
+export interface SourceLatencyResult {
+  latency: number;
+  success: boolean;
+  timeout: boolean;
+  method: 'HEAD' | 'GET';
+}
+
+type ProbeAttemptResult = SourceLatencyResult;
+
+interface ProbeSourceLatencyOptions {
+  fetcher?: typeof fetch;
+  timeoutMs?: number;
+  now?: () => number;
+}
+
+async function probeAttempt(
+  url: string,
+  method: 'HEAD' | 'GET',
+  fetcher: typeof fetch,
+  timeoutMs: number,
+  now: () => number,
+): Promise<ProbeAttemptResult> {
+  const controller = new AbortController();
+  let timedOut = false;
+  const timeoutId = setTimeout(() => {
+    timedOut = true;
+    controller.abort();
+  }, timeoutMs);
+  const startedAt = now();
+
+  try {
+    await fetcher(url, {
+      method,
+      signal: controller.signal,
+      redirect: 'follow',
+      ...(method === 'GET' ? { headers: { Range: 'bytes=0-0' } } : {}),
+    });
+
+    return {
+      latency: Math.max(0, Math.round(now() - startedAt)),
+      success: true,
+      timeout: false,
+      method,
+    };
+  } catch (error) {
+    return {
+      latency: Math.max(0, Math.round(now() - startedAt)),
+      success: false,
+      timeout: timedOut || (error instanceof Error && error.name === 'AbortError'),
+      method,
+    };
+  } finally {
+    clearTimeout(timeoutId);
+  }
+}
+
+export async function probeSourceLatency(
+  url: string,
+  options: ProbeSourceLatencyOptions = {},
+): Promise<SourceLatencyResult> {
+  const fetcher = options.fetcher ?? fetch;
+  const timeoutMs = options.timeoutMs ?? 5000;
+  const now = options.now ?? (() => performance.now());
+
+  const headResult = await probeAttempt(url, 'HEAD', fetcher, timeoutMs, now);
+  if (headResult.success) return headResult;
+
+  return probeAttempt(url, 'GET', fetcher, timeoutMs, now);
+}