In Python, multithreading is ineffective at concurrency for CPU bound tasks due to the GIL (global interpreter lock). Extension modules can release the GIL in CPU bound tasks, but that isn‘t an option in pure Python. Users use libraries such as multiprocessing, subprocess, concurrent.futures.ProcessPoolExecutor, etc, to work around the GIL. These modules call fork() underneath the hood. Various issues have been reported when using these modules with gRPC Python. gRPC Python wraps gRPC core, which uses multithreading for performance, and hence doesn’t support fork(). Historically, we didn't support forking in gRPC, but some users seemed to be doing fine until their code started to break on version 1.6. This was likely caused by the addition of background c-threads and a background Python thread.
The background Python thread was removed entirely. This allows forking after creating a channel. However, the channel must not have issued any RPCs prior to the fork. Attempting to fork with an active channel that has been used can result in deadlocks/corrupted wire data.
A regression was noted in cases where users are doing fork/exec. This was due to pthread_atfork() handler that was added in 1.7 to partially support forking in gRPC. A deadlock can happen when pthread_atfork handler is running, and an application thread is calling into gRPC. We have provided a workaround for this issue by allowing users to turn off the handler using env flag GRPC_ENABLE_FORK_SUPPORT=False. This should be set whenever a user expects to always call exec immediately following fork. It will disable the fork handlers.
A pthread_atfork() handler was added in 1.7 to automatically shut down the background c-threads when fork was called. This does not shut down the background Python thread, so users could not have any open channels when forking.
The workaround when using fork/exec by setting GRPC_ENABLE_FORK_SUPPORT=False should no longer be needed. Following this PR, fork handlers will not automatically run when multiple threads are calling into gRPC.