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Disable gRPC fork support in the server - #4365
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gRPC's fork handlers restart its threads in every forked child, which can crash subprocesses the server starts before they exec and stall the server (#4362).
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Fixes #4362. On uvloop, every subprocess the server starts (e.g.
ssh) is a plainfork(), which runs gRPC's fork handlers in the child. They restart gRPC's threads, and the child can crash before it execs, stalling the server while it waits (for minutes withsystemd-coredump). In a standalone repro on EC2 (grpcio 1.84.0, uvloop 0.23.0), about 4 in 10,000 spawns crashed; withGRPC_ENABLE_FORK_SUPPORT=falsethere were 0 in about 290,000.bootstrap.set_up_process(), which server entry points call before other imports. It defaultsGRPC_ENABLE_FORK_SUPPORT=falseandGRPC_VERBOSITY=ERROR(moved from the CLI), and warns ifgrpcwas already imported, since gRPC reads the setting on import.set_up_process()wasn't called, so applications that extend the server notice the missing call.