fix: don't use threads for gRPC AuthMetadataPlugin, reverts #390 #467
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lidizheng
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LGTM since gRPC Python already spawning a thread for plugin callbacks: https://github.com/grpc/grpc/blob/master/src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx.pxi#L16
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🤖 I have created a release \*beep\* \*boop\* --- ## [1.12.0](https://www.github.com/googleapis/google-auth-library-python/compare/v1.11.3...v1.12.0) (2020-03-25) ### Features * add mTLS ADC support for HTTP ([#457](https://www.github.com/googleapis/google-auth-library-python/issues/457)) ([bb9215a](https://www.github.com/googleapis/google-auth-library-python/commit/bb9215ad6dee6c1dc7f255a2e4ed7011b85bd6cf)) * add SslCredentials class for mTLS ADC ([#448](https://www.github.com/googleapis/google-auth-library-python/issues/448)) ([dafb41f](https://www.github.com/googleapis/google-auth-library-python/commit/dafb41fae3f513ea9a4f93404f6148bee7dda202)) * fetch id token from GCE metadata server ([#462](https://www.github.com/googleapis/google-auth-library-python/issues/462)) ([97e7700](https://www.github.com/googleapis/google-auth-library-python/commit/97e7700da031bfd80b63b1a3d2abc29c500936ef)) ### Bug Fixes * don't use threads for gRPC AuthMetadataPlugin ([#467](https://www.github.com/googleapis/google-auth-library-python/issues/467)) ([ee373f8](https://www.github.com/googleapis/google-auth-library-python/commit/ee373f88b512a38e791a1c085452c6c6da501eb6)) * make ThreadPoolExecutor a class var ([#461](https://www.github.com/googleapis/google-auth-library-python/issues/461)) ([b526473](https://www.github.com/googleapis/google-auth-library-python/commit/b5264730603947295cc97ecff2f6aef84aa3d6e9)) --- This PR was generated with [Release Please](https://github.com/googleapis/release-please).
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Reverts part of #390.
grpc's documentation states AuthMetadataPlugin's
__call__should be non-blocking. https://grpc.github.io/grpc/python/grpc.html#grpc.AuthMetadataPlugin.__call__. However, @lidiz of gRPC Python confirms that a separate thread is already spun up for the plugin callback so it will not block the gRPC call (see comment below).The current implementation of downstream clients results in threads being leaked.
Clients create gRPC channels but do not explicitly close them. See googleapis/google-cloud-python#9457 and googleapis/google-cloud-python#9790. This means associated resources do not get cleaned up.
See googleapis/google-cloud-python#9894 for an example of the changes that need to be made to downstream libraries. Also see google-api-core's
create_channelmethod for resources created with a channel. Googlers, there is a detailed design doc here.Issues: