fix(emulator): stringify send() errors before writing them to stderr

CHANGELOG.md

@@ -2,3 +2,4 @@
 - Added `appcheck:apps:list` to show every app with its configured App Check providers.
 - Added web app support for Crashlytics MCP tools and prompts.
 - Added support for forwarding custom HTTP headers (`Mcp-Param-*`) to remote MCP tools when defined in tool parameter input schemas (`x-mcp-header`), per [SEP-2243](https://modelcontextprotocol.io/seps/2243-http-standardization).
+- Fixed the Functions emulator replacing an IPC failure with an unrelated `TypeError` about stream chunk types, hiding why the runtime became unreachable (#10876).

src/emulator/types.spec.ts

@@ -0,0 +1,73 @@
+import { expect } from "chai";
+import * as sinon from "sinon";
+
+import { EmulatorLog } from "./types";
+
+type SendCallback = (err: unknown) => void;
+
+describe("EmulatorLog", () => {
+  describe("flush()", () => {
+    let stderrWrite: sinon.SinonStub;
+    // eslint-disable-next-line @typescript-eslint/unbound-method
+    const originalSend = process.send;
+
+    beforeEach(() => {
+      // Enforce the same chunk types the real stream does, so a regression throws here too.
+      stderrWrite = sinon.stub(process.stderr, "write").callsFake((chunk: unknown): boolean => {
+        if (typeof chunk !== "string" && !ArrayBuffer.isView(chunk)) {
+          throw new TypeError(
+            `The "chunk" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received ${typeof chunk}`,
+          );
+        }
+        return true;
+      });
+    });
+
+    afterEach(() => {
+      sinon.restore();
+      process.send = originalSend;
+    });
+
+    // Reports `err` to the send callback, the way Node does when the IPC channel has failed.
+    function stubSend(err: unknown): void {
+      process.send = ((
+        _message: unknown,
+        _sendHandle: unknown,
+        _options: unknown,
+        callback: SendCallback,
+      ): boolean => {
+        callback(err);
+        return true;
+      }) as unknown as typeof process.send;
+    }
+
+    it("writes the stack of an Error reported by process.send()", () => {
+      stubSend(new Error("channel closed"));
+
+      expect(() => new EmulatorLog("INFO", "system", "hello").log()).to.not.throw();
+
+      expect(stderrWrite.calledOnce).to.be.true;
+      const written = stderrWrite.firstCall.args[0] as string;
+      expect(written).to.be.a("string");
+      expect(written).to.contain("channel closed");
+      expect(written).to.match(/\n$/);
+    });
+
+    it("writes a non-Error reported by process.send()", () => {
+      stubSend("ERR_IPC_CHANNEL_CLOSED");
+
+      expect(() => new EmulatorLog("INFO", "system", "hello").log()).to.not.throw();
+
+      expect(stderrWrite.calledOnce).to.be.true;
+      expect(stderrWrite.firstCall.args[0]).to.contain("ERR_IPC_CHANNEL_CLOSED");
+    });
+
+    it("writes nothing when process.send() succeeds", () => {
+      stubSend(null);
+
+      new EmulatorLog("INFO", "system", "hello").log();
+
+      expect(stderrWrite.called).to.be.false;
+    });
+  });
+});

src/emulator/types.ts

@@ -1,6 +1,8 @@
 import { ChildProcess } from "child_process";
 import { EventEmitter } from "events";
 
+import { getErrStack } from "../error";
+
 export enum Emulators {
   AUTH = "auth",
   HUB = "hub",
@@ -357,12 +359,15 @@ export class EmulatorLog {
 
     EmulatorLog.WAITING_FOR_FLUSH = true;
     if (process.send) {
-      // For some reason our node.d.ts file does not include the version of subprocess.send() with a callback
-      // but the node docs assert that it has an optional callback.
       // https://nodejs.org/api/child_process.html#child_process_subprocess_send_message_sendhandle_options_callback
-      (process.send as any)(nextMsg, undefined, {}, (err: any) => {
+      process.send(nextMsg, undefined, {}, (err: Error | null) => {
         if (err) {
-          process.stderr.write(err);
+          // process.send() hands the callback an Error object, which stream.write()
+          // rejects -- writing it directly throws and destroys the original error.
+          process.stderr.write(`${getErrStack(err)}\n`);
+          // Clear the buffer to prevent flooding stderr with duplicate stack traces
+          // for subsequent messages when the IPC channel is permanently broken.
+          EmulatorLog.LOG_BUFFER = [];
         }
 
         EmulatorLog.WAITING_FOR_FLUSH = EmulatorLog.LOG_BUFFER.length > 0;