MemoryView implementation incompatible with Options.cache_builtins = False

[thirtythreeforty]

Disabling the cache_builtins option in setup.py:

Options.cache_builtins = False

causes code that uses MemoryView to fail to compile:

Error compiling Cython file:
------------------------------------------------------------
...
    if not is_slice:

        if start < 0:
            start += shape
        if not 0 <= start < shape:
            _err_dim(IndexError, "Index out of bounds (axis %d)", dim)
                    ^
------------------------------------------------------------

View.MemoryView:832:21: Accessing Python global or builtin not allowed without gil

(There are several such errors building the file, all stemming from doing a lookup on the exception type without the GIL held.)

To fix this, the GIL should be locked before calling _err_dim.

[da-woods]

I've done this a tiny bit differently to how you've suggested, since I think the whole point of _err_dim was avoiding writing duplicate with gil: code but the basic diagnosis was good.

[thirtythreeforty]

Great, thanks for the quick turnaround! Your way will prevent someone from mocking ValueError, but on the other hand I can't see anyone wanting to actually do that, so it should work!

[da-woods]

Your way will prevent someone from mocking ValueError, but on the other hand I can't see anyone wanting to actually do that, so it should work

I was just adding a comment to that effect in the PR description when you replied:

I guess this slightly breaks the contract of the cache_builtins option, that you should be able to monkey-patch builtin stuff (including exceptions). With that said - it's an implementation detail that it's written in Cython. If it was written in C then it'd just use the standard exception definitions in the headers and the user wouldn't be able to change them anyway.

I think the "implementation detail" excuse makes it OK but we'll see if it makes it past review...