[issue28254] Add C API for gc.enable, gc.disable, and gc.isenabled

[Stefan Behnel]

Stefan Behnel <stefan_ml at behnel.de> added the comment:

I support that there should be a simple way to do this from C. The way via importing the "gc" module and then looking up an attribute (possibly building a Unicode string) and calling a function in it involves several operations that can take some time and require useless error handling since each of them will practically never fail but may. An operation as simple as changing the GC status shouldn't require that.

A use case is building large data structures, or critical rearrangements of data structures that involve object operations that might trigger a GC run, maybe even including temporarily invalid object states and graphs. When larger data structures are involved but no collectable cycles, then the GC will probably not do anything useful except slowing down the creation process and/or running arbitrary code in between.

I consider the need to disable garbage collection in critical sections of C code similar to locking and GIL handling. It isn't quite the same as locking, since other code can enable it again when it runs, which isn't the case for a lock, but especially in that case, being able to detect quickly whether it was re-enabled when my own code gains back control seems beneficial. Having to call a Python function for that and taking care of the object result is way too much overhead for this case.

The fact that this is a rare request may not necessarily mean that it's rarely needed. There is certainly a bunch of C code out there that would benefit from temporarily disabling the GC in critical sections. I would imagine that people simply don't think of doing it and fail to notice any resulting slow-downs or even crashes since those often require elaborate circumstances to occur, and thus may not become visible at all in test or benchmark scenarios.

Note that the GC state is now part of the PyInterpreterState, so a patch would need to do what "gc_enable_impl" and "gc_disable_impl" do in Modules/gcmodule.c, or, rather, become their implementation to share code.

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components: +C API -Extension Modules
nosy: +scoder
resolution: rejected -> 
stage:  -> needs patch
status: closed -> open
type:  -> resource usage
versions: +Python 3.10 -Python 3.7

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<https://bugs.python.org/issue28254>
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