bpo-29683 - Fixes to _PyCode_SetExtra when co_extra->ce->extras is allocated

[brianfcoleman]

On PyMem_Malloc failure, _PyCode_SetExtra should set co_extra->ce_size = 0.
On PyMem_Realloc failure, _PyCode_SetExtra should set co_extra->ce_size = 0.
On PyMem_Realloc success, _PyCode_SetExtra should set all unused slots in co_extra->ce_extras to NULL.

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[mention-bot]

@brianfcoleman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @brettcannon, @avassalotti, @gvanrossum, @1st1 and @vadmium to be potential reviewers.

[serhiy-storchaka]

Previously allocated co_extra->ce_extras is leaked if PyMem_Realloc() returns NULL. It would be better to save the result of PyMem_Realloc() to the variable and check it for NULL first than assign it to co_extra->ce_extras.

[brianfcoleman]

I have made this change

[serhiy-storchaka]

Would not be better to deallocate o->co_extra and set it to NULL rather of setting co_extra->ce_size to 0?

[serhiy-storchaka]

Nice catch.

[serhiy-storchaka]

@brianfcoleman, please sign the CLA and add an entry in Misc/NEWS.

[serhiy-storchaka]

LGTM except minor details of Misc/NEWS entry.

[serhiy-storchaka]

New entries usually added are added at the start of corresponding section, so that entries are list in reversed chronological order.

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[brianfcoleman]

I've made this change

[vstinner]

Thanks @brianfcoleman for your contribution! Would you mind to backport the change to Python 3.6?

[brianfcoleman]

@Haypo I have created a pull request to backport the fix to 3.6 here:
#402