Ensuring stdio transport closure despite server hanging. Fixing #271

src/client/stdio.ts

@@ -170,6 +170,13 @@ export class StdioClientTransport implements Transport {
     return this._process?.stderr ?? null;
   }
 
+  /**
+   * The process id of the child process, if it has been started.
+   */
+  get pid(): number | undefined {
+    return this._process?.pid;
+  }
+
   private processReadBuffer() {
     while (true) {
       try {
@@ -185,8 +192,34 @@ export class StdioClientTransport implements Transport {
     }
   }
 
+  private gracefullyExitProcess(): void {
+    const pid = this.pid;
+
+    if (!pid) {
+      return;
+    }
+
+    // Killing after 3 seconds to ensure the process has time to exit gracefully.
+    setTimeout(() => {
+      try {
+        process.kill(pid, "SIGKILL");
+
+      } catch (err) {
+        if (typeof err === "object" && err !== null && "code" in err && err.code === "ESRCH") {
+          // Ignoring ESRCH error, which means the process is already dead.
+          return;
+        }
+
+        console.warn(`Failed to kill process ${pid}: ${err}`);
+      }
+    }, 3000);
+
+    process.kill(pid, "SIGINT");
+  }
+
   async close(): Promise<void> {
     this._abortController.abort();
+    this.gracefullyExitProcess();
     this._process = undefined;
     this._readBuffer.clear();
   }

src/integration-tests/process-cleanup.test.ts

@@ -1,10 +1,13 @@
+import { execSync } from "node:child_process";
 import { Server } from "../server/index.js";
 import { StdioServerTransport } from "../server/stdio.js";
+import { Client } from "../client/index.js";
+import { StdioClientTransport } from "../client/stdio.js";
 
 describe("Process cleanup", () => {
-  jest.setTimeout(5000); // 5 second timeout
+  jest.setTimeout(10 * 1000); // 10 second timeout
 
-  it("should exit cleanly after closing transport", async () => {
+  it("server should exit cleanly after closing transport", async () => {
     const server = new Server(
       {
         name: "test-server",
@@ -25,4 +28,36 @@ describe("Process cleanup", () => {
     // The test runner will fail if the process hangs
     expect(true).toBe(true);
   });
-});
+
+  it("client should exit cleanly after closing transport", async () => {  
+    const isProcessRunning = (pid: number) => {
+      try {
+        execSync(`ps -p ${pid}`, { stdio: 'ignore' });
+        return true;
+
+      /* eslint-disable @typescript-eslint/no-unused-vars */
+      } catch (error) {
+        return false;
+      }
+    }
+
+    const client = new Client({
+      name: "test-client",
+      version: "1.0.0",
+    });
+
+    const transport = new StdioClientTransport({
+      command: "node",
+      args: ["server-that-hangs.js"],
+      cwd: __dirname
+    });
+
+    await client.connect(transport);
+    const pid = transport.pid;
+
+    await client.close();
+    await new Promise(resolve => setTimeout(resolve, 5000));
+
+    expect(isProcessRunning(pid!)).toBe(false);
+  });
+});

src/integration-tests/server-that-hangs.js

@@ -0,0 +1,21 @@
+import { setTimeout } from 'node:timers'
+import process from 'node:process'
+import { McpServer } from "../../dist/esm/server/mcp.js";
+import { StdioServerTransport } from "../../dist/esm/server/stdio.js";
+
+const transport = new StdioServerTransport();
+
+const server = new McpServer({
+  name: "server-that-hangs",
+  version: "1.0.0"
+});
+
+await server.connect(transport);
+
+const doNotExitImmediately = async () => {
+  setTimeout(() => process.exit(0), 30 * 1000);
+};
+
+process.stdin.on('close', doNotExitImmediately);
+process.on('SIGINT', doNotExitImmediately);
+process.on('SIGTERM', doNotExitImmediately);