[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build

[Victor Stinner]

Le mer. 10 avr. 2019 à 21:45, Brett Cannon <brett at python.org> a écrit :
>> Can anyone explain what pydebug builds are... for? Confession: I've never used them myself, and don't know why I would want to.
>
> There is a bunch of extra things done in a debug build, e.g. all freed memory is blanked out with a known pattern so it's easy to tell when you're reading from freed memory (and thus probably messed up your refcounts).

Since the debug build ABI is incompatible, it's not easy to use a
debug build. For that reasons, I'm working for a few years to add such
debugging features into regular release build. For example, you can
now get this debugger on memory allocations using PYTHONMALLOC=debug
environment variable since Python 3.6.

Since such debug feature is not easy to discover (especially if you
don't read closely What's New In Python 3.x), I added a generic "-X
dev" command line option to enable a "development mode". It enables
various similar features to debug code:
https://docs.python.org/dev/using/cmdline.html#id5

Effect of the developer mode:

* Add default warning filter, as -W default.
* Install debug hooks on memory allocators: see the
PyMem_SetupDebugHooks() C function.
* Enable the faulthandler module to dump the Python traceback on a crash.
* Enable asyncio debug mode.
* Set the dev_mode attribute of sys.flags to True

See also https://pythondev.readthedocs.io/debug_tools.html where I
started to document these debug tools and how to use them.


> And then various extras are tossed on to the sys module to help with things. Basically anything people have found useful and require being compiled in typically get clumped in under the debug build.

The debug build still contains many features which are useful to debug
C extensions. For example, it adds sys.gettotalrefcnt() which is a
convenient way to detect reference leaks. This funtion require
Py_REF_DEBUG which modifies Py_INCREF() to add "_Py_RefTotal++;". Iit
is has an impact on overall Python performance and should not be
enabled in release build.

Victor
-- 
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