Observed here:
https://travis-ci.org/github/mongodb/mongo-cxx-driver/jobs/703266901
With logs:
------------------------------------------------------------------------------- session pool ------------------------------------------------------------------------------- /home/travis/build/mongodb/mongo-cxx-driver/src/mongocxx/test/client_session.cpp:125 ............................................................................... /home/travis/build/mongodb/mongo-cxx-driver/src/mongocxx/test/client_session.cpp:137: FAILED: REQUIRE( session_c->id() == b_id ) with expansion: { "id" : { "$binary" : { "base64": "FqitkZCWSPq4O5kMipwlRQ==", "subType" : "04" } } } == {?} -------------------------------------------------------------------------------
After CDRIVER-3322, libmongoc no longer pools unused sessions. A test in the C++ driver was relying on this behavior. The fix is to simply use the session before destroying it so it gets back onto the pool.