feat: add stdio-based function discovery mode

Adds an alternative discovery mechanism that outputs function manifests via stderr
instead of starting an HTTP server. This improves reliability by avoiding issues
where module loading blocks the HTTP endpoint from becoming available.

When FUNCTIONS_DISCOVERY_MODE=stdio is set:
- Outputs base64-encoded manifest to stderr with __FIREBASE_FUNCTIONS_MANIFEST__: prefix
- Outputs errors to stderr with __FIREBASE_FUNCTIONS_MANIFEST_ERROR__: prefix
- Exits immediately without starting HTTP server
- Maintains backward compatibility (HTTP remains default)

Author: taeold

src/bin/firebase-functions.ts

@@ -49,34 +49,53 @@ if (args.length > 1) {
   functionsDir = args[0];
 }
 
-let server: http.Server = undefined;
-const app = express();
-
-function handleQuitquitquit(req: express.Request, res: express.Response) {
-  res.send("ok");
-  server.close();
+async function runStdioDiscovery() {
+  try {
+    const stack = await loadStack(functionsDir);
+    const wireFormat = stackToWire(stack);
+    const manifestJson = JSON.stringify(wireFormat);
+    const base64 = Buffer.from(manifestJson).toString("base64");
+    process.stderr.write(`__FIREBASE_FUNCTIONS_MANIFEST__:${base64}\n`);
+    process.exit(0);
+  } catch (e) {
+    console.error("Failed to generate manifest from function source:", e);
+    process.stderr.write(`__FIREBASE_FUNCTIONS_MANIFEST_ERROR__:${e.message}\n`);
+    process.exit(1);
+  }
 }
 
-app.get("/__/quitquitquit", handleQuitquitquit);
-app.post("/__/quitquitquit", handleQuitquitquit);
+if (process.env.FUNCTIONS_CONTROL_API === "true" && process.env.FUNCTIONS_DISCOVERY_MODE === "stdio") {
+  runStdioDiscovery();
+} else {
+  let server: http.Server = undefined;
+  const app = express();
 
-if (process.env.FUNCTIONS_CONTROL_API === "true") {
-  app.get("/__/functions.yaml", async (req, res) => {
-    try {
-      const stack = await loadStack(functionsDir);
-      res.setHeader("content-type", "text/yaml");
-      res.send(JSON.stringify(stackToWire(stack)));
-    } catch (e) {
-      console.error(e);
-      res.status(400).send(`Failed to generate manifest from function source: ${e}`);
-    }
-  });
-}
+  function handleQuitquitquit(req: express.Request, res: express.Response) {
+    res.send("ok");
+    server.close();
+  }
 
-let port = 8080;
-if (process.env.PORT) {
-  port = Number.parseInt(process.env.PORT);
-}
+  app.get("/__/quitquitquit", handleQuitquitquit);
+  app.post("/__/quitquitquit", handleQuitquitquit);
 
-console.log("Serving at port", port);
-server = app.listen(port);
+  if (process.env.FUNCTIONS_CONTROL_API === "true") {
+    app.get("/__/functions.yaml", async (req, res) => {
+      try {
+        const stack = await loadStack(functionsDir);
+        res.setHeader("content-type", "text/yaml");
+        res.send(JSON.stringify(stackToWire(stack)));
+      } catch (e) {
+        console.error(e);
+        res.status(400).send(`Failed to generate manifest from function source: ${e}`);
+      }
+    });
+  }
+
+  let port = 8080;
+  if (process.env.PORT) {
+    port = Number.parseInt(process.env.PORT);
+  }
+
+  console.log("Serving at port", port);
+  server = app.listen(port);
+}