fix: prevent credential_process hang on Windows in stdio transport mode (#293)

* fix: prevent credential_process hang on Windows in stdio transport mode

Monkey-patch botocore's ProcessProvider to pass stdin=subprocess.DEVNULL
when spawning credential_process subprocesses. Without this, the child
inherits the MCP JSON-RPC pipe as stdin, causing Popen.communicate() to
hang indefinitely on Windows (IOCP) due to the open pipe handle.

Fixes D454977820

* refactor: patch _popen instead of copying _retrieve_credentials_using

Wrap ProcessProvider._popen via __init__ to inject stdin=DEVNULL,
rather than replacing the entire _retrieve_credentials_using method.
This is less fragile as it doesn't duplicate botocore's credential
parsing logic.

* chore: trigger PR sync

* fix: suppress pyright type error for mock popen argument

* test: simplify credential_process stdin tests to focus on the patch

Author: rshevchuk-git

mcp_proxy_for_aws/sigv4_helper.py

@@ -18,9 +18,10 @@
 import httpx
 import json
 import logging
+import subprocess
 from botocore.auth import SigV4Auth
 from botocore.awsrequest import AWSRequest
-from botocore.credentials import Credentials
+from botocore.credentials import Credentials, ProcessProvider
 from functools import partial
 from httpx import __version__ as httpx_version
 from mcp_proxy_for_aws import __version__
@@ -30,6 +31,33 @@
 
 logger = logging.getLogger(__name__)
 
+
+def _patch_credential_process_stdin():
+    """Patch botocore ProcessProvider to pass stdin=subprocess.DEVNULL.
+
+    When mcp-proxy-for-aws runs in stdio transport mode, its stdin is the MCP
+    JSON-RPC pipe. Botocore's ProcessProvider spawns credential_process
+    subprocesses without specifying stdin, so the child inherits the MCP pipe.
+    On Windows this causes the subprocess to hang indefinitely because it holds
+    an open handle to the pipe, blocking Popen.communicate() from completing.
+    """
+    original_init = ProcessProvider.__init__
+
+    def _patched_init(self, *args, **kwargs):
+        original_init(self, *args, **kwargs)
+        original_popen = self._popen
+
+        def _popen_with_devnull_stdin(*popen_args, **popen_kwargs):
+            popen_kwargs.setdefault('stdin', subprocess.DEVNULL)
+            return original_popen(*popen_args, **popen_kwargs)
+
+        self._popen = _popen_with_devnull_stdin
+
+    ProcessProvider.__init__ = _patched_init
+
+
+_patch_credential_process_stdin()
+
 # Headers that should be redacted when logging to prevent credential exposure
 SENSITIVE_HEADERS = frozenset({'authorization', 'x-amz-security-token', 'x-amz-date'})
 

tests/unit/test_credential_process_stdin_inheritance.py

@@ -0,0 +1,96 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for the credential_process stdin isolation patch in sigv4_helper.
+
+When mcp-proxy-for-aws runs in stdio transport mode, its stdin is the MCP
+JSON-RPC pipe. Without the patch, botocore's ProcessProvider would spawn
+credential_process subprocesses that inherit the pipe, causing hangs on Windows.
+"""
+
+import mcp_proxy_for_aws.sigv4_helper  # noqa: F401 - ensure patch is applied
+import subprocess
+from botocore.credentials import ProcessProvider
+from unittest.mock import MagicMock
+
+
+class TestCredentialProcessStdinPatch:
+    """Verify the sigv4_helper patch injects stdin=DEVNULL into ProcessProvider."""
+
+    def test_popen_receives_stdin_devnull(self):
+        """ProcessProvider._popen passes stdin=subprocess.DEVNULL after the patch."""
+        popen_kwargs = {}
+
+        def mock_popen(*args, **kwargs):
+            popen_kwargs.update(kwargs)
+            mock = MagicMock()
+            mock.returncode = 0
+            mock.communicate.return_value = (
+                b'{"Version":1,"AccessKeyId":"A","SecretAccessKey":"B"}',
+                b'',
+            )
+            return mock
+
+        provider = ProcessProvider(
+            profile_name='test',
+            load_config=lambda: {'profiles': {'test': {'credential_process': 'echo hi'}}},
+            popen=mock_popen,  # type: ignore[arg-type]
+        )
+        provider.load()
+        assert popen_kwargs.get('stdin') == subprocess.DEVNULL
+
+    def test_explicit_stdin_is_not_overridden(self):
+        """If caller explicitly sets stdin, the patch does not override it."""
+        popen_kwargs = {}
+
+        def mock_popen(*args, **kwargs):
+            popen_kwargs.update(kwargs)
+            mock = MagicMock()
+            mock.returncode = 0
+            mock.communicate.return_value = (
+                b'{"Version":1,"AccessKeyId":"A","SecretAccessKey":"B"}',
+                b'',
+            )
+            return mock
+
+        provider = ProcessProvider(
+            profile_name='test',
+            load_config=lambda: {'profiles': {'test': {'credential_process': 'echo hi'}}},
+            popen=mock_popen,  # type: ignore[arg-type]
+        )
+
+        # Call _popen directly with an explicit stdin to verify setdefault behavior
+        provider._popen('echo', stdin=subprocess.PIPE)
+        assert popen_kwargs.get('stdin') == subprocess.PIPE
+
+    def test_credential_process_error_propagates(self):
+        """The patch does not swallow errors from credential_process."""
+        from botocore.exceptions import CredentialRetrievalError
+
+        def mock_popen(*args, **kwargs):
+            mock = MagicMock()
+            mock.returncode = 1
+            mock.communicate.return_value = (b'', b'access denied')
+            return mock
+
+        provider = ProcessProvider(
+            profile_name='test',
+            load_config=lambda: {'profiles': {'test': {'credential_process': 'fail'}}},
+            popen=mock_popen,  # type: ignore[arg-type]
+        )
+
+        import pytest
+
+        with pytest.raises(CredentialRetrievalError):
+            provider.load()