fix: fall back to firefox, then plain fetch, when impers refuses a fingerprint

impers resolves the chrome alias to its newest fingerprint (chrome150 as
of impers 0.1.1), but the native library it loads can be an older system
copy of libcurl-impersonate found before its own pinned download. Such a
copy refuses the fingerprint with an ImpersonateError before any request
leaves, and oc died on the spot with "Impersonating chrome150 is not
supported" (#40).

A refused identity now downgrades the same way a 403 already did: chrome
falls back to firefox, and when both are refused the plain fetch
transport still gets the page. Any other impers failure propagates
unchanged.

src/fetch.js

@@ -555,9 +555,24 @@ function captureSetCookie(jar, url, res) {
   jar.storeFromResponse(url, getSetCookieHeaders(res));
 }
 
-async function viaImpers(impers, target, jar) {
+/**
+ * Fetch a page through impers, downgrading identity when one is refused.
+ * Exported so the downgrade chain can be proven against a fake impers; the
+ * real entry point is fetchPage.
+ * @param {any} impers - the impers module (or a stand-in with a get method)
+ * @param {string} target
+ * @param {object} [jar]
+ * @returns {Promise<{url: string, html: string, status: number, via: string}>}
+ */
+export async function viaImpers(impers, target, jar) {
   // Some sites (Reddit) 403 the chrome fingerprint but accept firefox, so a
-  // blocked first attempt gets one cheap retry with a second identity.
+  // blocked first attempt gets one cheap retry with a second identity. An
+  // ImpersonateError is the same story one layer down: impers resolves the
+  // 'chrome' alias to its newest fingerprint, but the native library it loads
+  // can be an older system copy of libcurl-impersonate that predates that
+  // fingerprint and refuses it before any request leaves. Firefox aliases to
+  // an older target that such a library usually still knows, and when both
+  // identities are refused the plain fetch transport still gets the page.
   const asking = (impersonate) => (url) =>
     impers.get(url, {
       impersonate,
@@ -566,14 +581,23 @@ async function viaImpers(impers, target, jar) {
       headers: jarHeaders(jar, url, {}),
     });
   const onResponse = (url, res) => captureSetCookie(jar, url, res);
+  const attempt = async (impersonate) => {
+    try {
+      const { res } = await followRedirects(asking(impersonate), target, { onResponse });
+      return { res, status: res.status ?? res.statusCode ?? 0 };
+    } catch (err) {
+      if (err?.name !== 'ImpersonateError') throw err;
+      return null;
+    }
+  };
   let via = 'impers:chrome';
-  let { res } = await followRedirects(asking('chrome'), target, { onResponse });
-  let status = res.status ?? res.statusCode ?? 0;
-  if (status >= 400) {
+  let got = await attempt('chrome');
+  if (!got || got.status >= 400) {
     via = 'impers:firefox';
-    ({ res } = await followRedirects(asking('firefox'), target, { onResponse }));
-    status = res.status ?? res.statusCode ?? 0;
+    got = (await attempt('firefox')) ?? got;
   }
+  if (!got) return viaFetch(target, jar);
+  const { res, status } = got;
   if (status >= 400) throw new Error(`fetch failed: ${status} for ${target}`);
   assertReadableType(res.headers.get('content-type'));
   assertBodySize(Number(res.headers.get('content-length')) || 0, target);

tests/fetch.test.js

@@ -5,7 +5,7 @@ import https from 'node:https';
 import net from 'node:net';
 import tls from 'node:tls';
 
-const { fetchPage, followRedirects, resolveProxy, proxyGet } = await import('../src/fetch.js');
+const { fetchPage, followRedirects, resolveProxy, proxyGet, viaImpers } = await import('../src/fetch.js');
 
 const BLOCKED_MESSAGE = 'blocked: private or internal URL';
 
@@ -727,3 +727,73 @@ test('the proxy transport counts the body against the same cap', async () => {
     proxy.close();
   }
 });
+
+// A stand-in for the impers module whose get() either answers with a minimal
+// 200 page or refuses the identity the way impers does when the loaded native
+// library does not know the fingerprint an alias resolves to: it throws an
+// ImpersonateError before any request leaves the process (issue #40, where a
+// stale system libcurl-impersonate predating chrome150 broke oc outright).
+const fakeImpers = (refuse, html = '<html>ok</html>') => {
+  const identities = [];
+  const get = (url, opts) => {
+    identities.push(opts.impersonate);
+    if (refuse.includes(opts.impersonate)) {
+      const err = new Error(`Impersonating ${opts.impersonate} is not supported`);
+      err.name = 'ImpersonateError';
+      return Promise.reject(err);
+    }
+    return Promise.resolve({
+      status: 200,
+      headers: new Map([['content-type', 'text/html']]),
+      text: () => Promise.resolve(html),
+      url,
+    });
+  };
+  return { get, identities };
+};
+
+test('a refused chrome fingerprint falls back to the firefox identity', () => withoutProxyEnv(async () => {
+  const impers = fakeImpers(['chrome']);
+  const page = await viaImpers(impers, 'https://public.example/page');
+  assert.deepEqual(impers.identities, ['chrome', 'firefox']);
+  assert.equal(page.via, 'impers:firefox');
+  assert.equal(page.status, 200);
+  assert.equal(page.html, '<html>ok</html>');
+}));
+
+test('when both identities are refused the page still arrives via plain fetch', async () => {
+  // The proxy is only here to give viaFetch somewhere real to land without
+  // leaving the machine, same shape as the HTTP_PROXY wiring test above.
+  const seen = [];
+  const proxy = http.createServer((req, res) => {
+    seen.push(req.url);
+    res.writeHead(200, { 'content-type': 'text/html' });
+    res.end('<html><title>via fetch</title></html>');
+  });
+  const port = await listen(proxy);
+  const prev = Object.fromEntries(PROXY_ENV_KEYS.map((k) => [k, process.env[k]]));
+  for (const k of PROXY_ENV_KEYS) delete process.env[k];
+  process.env.HTTP_PROXY = `http://127.0.0.1:${port}`;
+  try {
+    const impers = fakeImpers(['chrome', 'firefox']);
+    const page = await viaImpers(impers, 'http://1.1.1.1/page');
+    assert.deepEqual(impers.identities, ['chrome', 'firefox']);
+    assert.equal(page.via, 'fetch');
+    assert.equal(page.status, 200);
+    assert.equal(page.html, '<html><title>via fetch</title></html>');
+    assert.deepEqual(seen, ['http://1.1.1.1/page']);
+  } finally {
+    proxy.close();
+    for (const k of PROXY_ENV_KEYS) {
+      if (prev[k] === undefined) delete process.env[k];
+      else process.env[k] = prev[k];
+    }
+  }
+});
+
+test('only an ImpersonateError downgrades; other impers failures propagate', () => withoutProxyEnv(async () => {
+  const impers = {
+    get: () => Promise.reject(new Error('connection reset')),
+  };
+  await assert.rejects(() => viaImpers(impers, 'https://public.example/page'), /connection reset/);
+}));