Add regression test for SIGINT cleanup handling

NOTE: this test FAIL without #555

This test verifies that stdio_client properly handles processes that
ignore SIGTERM but respond to SIGINT, preventing hanging issues with
Node.js servers and other interactive tools that expect Ctrl+C signals.

Author: felixweinberger

tests/client/test_stdio.py

@@ -1,7 +1,9 @@
 import shutil
 import sys
+import textwrap
 import time
 
+import anyio
 import pytest
 
 from mcp.client.session import ClientSession
@@ -178,3 +180,67 @@ async def test_stdio_client_immediate_completion():
         elapsed = end_time - start_time
         print(f"❌ Test failed after {elapsed:.1f} seconds: {e}")
         raise
+
+
+@pytest.mark.anyio
+@pytest.mark.skipif(sys.platform == "win32", reason="Windows signal handling is different")
+async def test_stdio_client_sigint_only_process():
+    """
+    Test cleanup with a process that ignores SIGTERM but responds to SIGINT.
+
+    This tests Cristipufu's concern: processes that expect SIGINT (like many
+    Node.js servers or interactive tools) may not respond to SIGTERM, causing
+    hanging during cleanup.
+    """
+    # Create a Python script that ignores SIGTERM but handles SIGINT
+    script_content = textwrap.dedent(
+        """
+        import signal
+        import sys
+        import time
+
+        # Ignore SIGTERM (what process.terminate() sends)
+        signal.signal(signal.SIGTERM, signal.SIG_IGN)
+
+        # Handle SIGINT (Ctrl+C signal) by exiting cleanly
+        def sigint_handler(signum, frame):
+            sys.exit(0)
+
+        signal.signal(signal.SIGINT, sigint_handler)
+
+        # Keep running until SIGINT received
+        while True:
+            time.sleep(0.1)
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command="python",
+        args=["-c", script_content],
+    )
+
+    start_time = time.time()
+
+    try:
+        # Use anyio timeout to prevent test from hanging forever
+        with anyio.fail_after(5.0):
+            async with stdio_client(server_params) as (read_stream, write_stream):
+                # Let the process start and begin ignoring SIGTERM
+                await anyio.sleep(0.5)
+                # Exit context triggers cleanup - this should not hang
+                pass
+
+        end_time = time.time()
+        elapsed = end_time - start_time
+
+        # Should complete quickly even with SIGTERM-ignoring process
+        # This will fail if cleanup only uses process.terminate() without fallback
+        assert elapsed < 5.0, (
+            f"stdio_client cleanup took {elapsed:.1f} seconds with SIGTERM-ignoring process. "
+            f"Expected < 5.0 seconds. This suggests the cleanup needs SIGINT/SIGKILL fallback."
+        )
+    except TimeoutError:
+        pytest.fail(
+            "stdio_client cleanup timed out after 5.0 seconds with SIGTERM-ignoring process. "
+            "This confirms the cleanup needs SIGINT/SIGKILL fallback for processes that ignore SIGTERM."
+        )