[mypyc] Ensure a generator can't be entered while it's being executed - #21939
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This only applies to coroutines. This allows using unsynchronized attribute access, which is a bug win on free-threaded builds. This matches Python semantics. We should perhaps do this in non-FT builds as well, but this is left as a follow-up task, as there it won't directly help with performance.
# Conflicts: # mypyc/ir/class_ir.py
p-sawicki
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Sep 4, 2026
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| try: | ||
| next(g) | ||
| except ValueError as e: | ||
| assert str(e) == "generator already executing", str(e) | ||
| else: | ||
| assert False |
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could use assertRaises from testutil, here and in other tests.
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This matches Python semantics -- an exception is now raised if there is an attempt to enter generator/coroutine while it's running.
This also allows unsynchronized access of generator attributes in free-threaded builds, since there can't be concurrent accesses (in simple cases where we use a merged generator and environment). This has a big performance impact on free-threaded-builds. Some microbenchmarks were 1.5x+ faster with this optimization, as synchronized attribute access is quite inefficient, and it was being used for all registers in generators and async defs.
I used coding agent assist but reviewed changes manually.