fix #11482: random crash in large closure allocation

[damiendoligez]

Note: this is still a draft, I still want to check that it does make the bug disappear.

Fixes #11482

[damiendoligez]

Checked on 5.0.0~alpha0, it does fix the segfault. Now ready for review.

[damiendoligez]

Note: will have to move the Changes entry and cherry-pick to 5.0.

[xavierleroy]

This looks like a good fix for #11482. Two minor remarks:

• I'm not super happy about the function name caml_alloc_shr_with_safepoint. Is check_urgent_gc equivalent to the safe points that ocamlopt implements? caml_alloc_shr_check_urgent might be more descriptive. caml_alloc_shr_check would be shorter :-)
• I wondered about performance. We can probably assume that ocamlopt-generated code rarely allocates directly in the major heap, in which case performance should be OK.

[damiendoligez]

I've renamed the function to caml_alloc_shr_check_gc. About performance, we are not doing more checks that before, but we're removing the double initialization, so this is a strict improvement.

[xavierleroy]

Looks good to me, thanks. Could you please move the Changes entry to 5.0, merge in trunk, and cherry-pick -x in 5.0 ? Thanks.

[damiendoligez]

Cherry-picked to 5.0 (as 43754a5). Thanks for the review.