[issue43760] The DISPATCH() macro is not as efficient as it could be (move PyThreadState.use_tracing)

[Mark Shannon]

Mark Shannon <mark at hotpy.org> added the comment:

IMO those failures are bugs in the projects listed not in CPython.


Relying on the exact meaning, or even the existence of an undocumented field of a C struct is not, nor ever has been, safe.
The user of the field is assuming a meaning that is not known to the developers of the library/application, so there is no way for them to preserve that meaning.
This is not specific to CPython, but applies to any C API.

The code in the examples given above are using `tstate->use_tracing` assuming that its meaning is to determine whether tracing is turned on or not.
However, it has no such meaning. It is an internal performance optimization to avoid checking both `tstate->c_tracefunc` and `tstate->c_profilefunc`.
It is `tstate->tracing` that determines whether tracing is active or not.

I propose adding back `tstate->use_tracing` as a copy of `tstate->cframe->us_tracing`. Any writes to `tstate->use_tracing` will be ignored, but any code that also sets `tstate->tracing`, as the Cython code does, will work correctly. Any code that reads `tstate->use_tracing` will work correctly.

I'm minded to prefix all the names of all fields in all C structs that happen to be included by Python.h with "if_you_use_this_then_we_will_break_your_code_in_some_way_that_will_ruin_your_reputation_as_a_library_developer__maybe_not_tomorrow_maybe_not_next_year_but_someday"
Although that might prove tricky with a 80 character line limit :)

My attempts to avoid this happening again next year, and the year after that, and...
https://bugs.python.org/issue45247
https://github.com/cython/cython/issues/4382

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