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sim: fix macOS MetaDrive support #36875

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sim: fix macOS MetaDrive support #36875

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4 changes: 3 additions & 1 deletion system/manager/manager.py
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Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,9 @@ def main() -> None:


if __name__ == "__main__":
unblock_stdout()
# Skip unblock_stdout on macOS - os.forkpty() causes crashes with raylib/AppKit
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@adeebshihadeh adeebshihadeh Dec 15, 2025

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why? i believe this used to work

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@zgohr zgohr Dec 15, 2025

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The issue is described here: #36785

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@adeebshihadeh adeebshihadeh Dec 15, 2025

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oh i see the issue, but why does it happen now?

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@zgohr zgohr Dec 15, 2025 •
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MetaDrive's raylib initializes AppKit before manager.py runs. unblock_stdout() then calls os.forkpty(), which triggers macOS CoreFoundation's fork safety crash (can't fork after Cocoa init). Doesn't affect on-device since no GUI frameworks are loaded before manager starts.

The Objective-C runtime can't be both thread-safe and fork-safe. After AppKit/CoreFoundation initializes, it may have threads running or state that can't be safely duplicated via fork(). Apple's solution: crash with the message:

The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().

The rule is: after fork(), you must immediately exec() a new program (which replaces the process image). But os.forkpty() in unblock_stdout() forks and continues running Python in the child — no exec().

Workarounds:

  • spawn instead of fork (what multiprocessing uses by default on macOS now)
  • OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES env var (must be set before process starts)
  • Or just skip the fork entirely on macOS (what my fix does)

My fix is cleanest approach since unblock_stdout() isn't critical, from what I can tell. Even from looking back at the git history, it's not clear why it's there in the first place. But take for instance it's there to potentially speed up on-device logging pipelines; since the sim runs in terminal, blocking shouldn't be a concern.

if sys.platform != 'darwin':
unblock_stdout()

try:
main()
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