When running mongo-sim with 300 client threads on OS X, I'm getting the following error from each thread:

Failure to buffer 4 bytes: Failed to buffer 4 bytes within 300000 milliseconds.

Note that each thread reports this almost immediately; it's not timing out after 300 seconds or anything. You can reproduce by checking out 10gen/mongo-sim.git, building, and running the following from the build directory:

./src/simulation/mongo-sim -w workloads/simple_read -t 300 -e 10000

This spawns 300 client threads, one mongoc client per thread and executes a query 10,000 times total. Each thread executes a query operation 33 or 34 times, then iterates over every document in a tight loop (using mongoc_cursor_error(), mongoc_cursor_more() and mongoc_cursor_next() as outlined in cursor.hpp's Cursor::itcount() method.

Note that the same error occurs when running an insert-only workload (change -w to insert_only). I have not been able to reproduce this issue on Linux.

Sorry I haven't been able to narrow this down to a minimal reproducible case yet. I don't have many free cycles, but I'm happy to help anywhere I can.