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[WIP] fix(auth): set default max_metadata_size to 32KB in gRPC channels #17661

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[WIP] fix(auth): set default max_metadata_size to 32KB in gRPC channels #17661

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19 changes: 19 additions & 0 deletions packages/google-auth/google/auth/transport/grpc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -274,6 +274,25 @@ def my_client_cert_callback():
ssl_credentials, google_auth_credentials
)

# Set default max_metadata_size to 32768 (32 KB) to prevent gRPC C-core metadata
# limit rejections (default 8 KB) when Post-Quantum Cryptography (PQC) bearer tokens
# (ML-DSA-65 is 4.5 KB) or large trace headers are used.
options = kwargs.pop("options", None)
if options is None:
kwargs["options"] = [("grpc.max_metadata_size", 32768)]
elif isinstance(options, (list, tuple)):
opts = list(options)
if not any(
isinstance(opt, (list, tuple))
and len(opt) >= 2
and opt[0] == "grpc.max_metadata_size"
for opt in opts
):
opts.append(("grpc.max_metadata_size", 32768))
kwargs["options"] = opts
else:
kwargs["options"] = options

return grpc.secure_channel(target, composite_credentials, **kwargs)


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22 changes: 22 additions & 0 deletions packages/google-auth/tests/transport/test_grpc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -275,6 +275,28 @@ def test_secure_authorized_channel_explicit_ssl(
ssl_credentials, metadata_call_credentials.return_value
)

def test_secure_authorized_channel_pqc_max_metadata_size(
self,
secure_channel,
ssl_channel_credentials,
metadata_call_credentials,
composite_channel_credentials,
get_client_ssl_credentials,
):
credentials = mock.Mock()
request = mock.Mock()
target = "example.com:80"
ssl_credentials = mock.Mock()

# Call without explicit options; should automatically add grpc.max_metadata_size=32768
google.auth.transport.grpc.secure_authorized_channel(
credentials, request, target, ssl_credentials=ssl_credentials
)

_, kwargs = secure_channel.call_args
assert "options" in kwargs
assert ("grpc.max_metadata_size", 32768) in kwargs["options"]
Comment on lines +278 to +298

@gemini-code-assist gemini-code-assist Bot Jul 8, 2026

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medium

The current test only verifies the case where options is not provided. To ensure full coverage and robust behavior, we should also test when options is explicitly passed (both with and without grpc.max_metadata_size already set).

    def test_secure_authorized_channel_pqc_max_metadata_size(
        self,
        secure_channel,
        ssl_channel_credentials,
        metadata_call_credentials,
        composite_channel_credentials,
        get_client_ssl_credentials,
    ):
        credentials = mock.Mock()
        request = mock.Mock()
        target = "example.com:80"
        ssl_credentials = mock.Mock()

        # Call without explicit options; should automatically add grpc.max_metadata_size=32768
        google.auth.transport.grpc.secure_authorized_channel(
            credentials, request, target, ssl_credentials=ssl_credentials
        )

        _, kwargs = secure_channel.call_args
        assert "options" in kwargs
        assert ("grpc.max_metadata_size", 32768) in kwargs["options"]

        # Call with explicit options not containing grpc.max_metadata_size
        google.auth.transport.grpc.secure_authorized_channel(
            credentials,
            request,
            target,
            ssl_credentials=ssl_credentials,
            options=[("some_option", 1)],
        )
        _, kwargs = secure_channel.call_args
        assert ("some_option", 1) in kwargs["options"]
        assert ("grpc.max_metadata_size", 32768) in kwargs["options"]

        # Call with explicit options already containing grpc.max_metadata_size
        google.auth.transport.grpc.secure_authorized_channel(
            credentials,
            request,
            target,
            ssl_credentials=ssl_credentials,
            options=[("grpc.max_metadata_size", 1024)],
        )
        _, kwargs = secure_channel.call_args
        assert ("grpc.max_metadata_size", 1024) in kwargs["options"]
        assert ("grpc.max_metadata_size", 32768) not in kwargs["options"]


def test_secure_authorized_channel_mutual_exclusive(
self,
secure_channel,
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