Summary

• Fix bidirectional streaming RPCs raising AttributeError: 'generator' object has no attribute 'add_done_callback' by routing bidi streams through _intercept instead of the generator-based _intercept_server_stream (issue Thread-ConsumeBidirectionalStream caught unexpected exception 'generator' object has no attribute 'add_done_callback' and will exit. #1180)
• Add regression test test_stream_stream_preserves_call_interface verifying the bidi stream response preserves the grpc.Call interface (add_done_callback, cancel, is_active)
• Add CHANGELOG entry under Unreleased/Fixed

Root Cause

In _client.py, intercept_stream() routed all RPCs with is_server_stream=True into _intercept_server_stream(), including bidirectional streams (where both is_client_stream and is_server_stream are True). _intercept_server_stream is a generator function (uses yield from), so calling it returns a bare Python generator that lacks the grpc.Call interface (add_done_callback, cancel, is_active, etc.).

Downstream code — notably google.api_core.bidi.BidiRpc.open() used by Google Cloud Pub/Sub — expects a real gRPC call object and crashes:

AttributeError: 'generator' object has no attribute 'add_done_callback'

The Fix

One condition change in intercept_stream():

# Before:
if client_info.is_server_stream:

# After:
if client_info.is_server_stream and not client_info.is_client_stream:

This ensures only unary-stream RPCs use the generator-based _intercept_server_stream. Bidi streams fall through to _intercept, which preserves the grpc.Call/grpc.Future interface.

Files Changed

Test plan

• Existing test_stream_stream continues to pass (span correctness)
• New test_stream_stream_preserves_call_interface passes — verifies grpc.Call attributes on bidi response
• Existing unary-stream tests pass (no regression from condition change)
• Existing stream-unary tests pass

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