Major GC hooks are no longer allowed to interact with the GC heap.

[jhjourdan]

As discussed in #8691 (comment) (by @stedolan):

BTW, the fact that the major GC is calling hooks that are allowed to allocate and execute arbitrary OCaml code (see comment in misc.h) completely kills the point of this PR. I did not know about them!

Would it be possible to add the restriction that these functions should not call an OCaml callback? Where are these functions used, in practice?

I didn't know about these functions either!

I think it would be possible to add this restriction. I can only find two uses of this feature: one in flow and one in Jane Street's internal code. Neither allocates, performs callbacks, or indeed calls any OCaml runtime functions at all.

Since this is nominally a breaking change, I think there should be a seperate PR which just edits the comment in misc.h.

[stedolan]

In fact, the original PR proposing these hooks (#6675) says this:

I imagine there will be constraints on the C callbacks -- for example, the callbacks must not modify or allocate OCaml memory.

I can't find any uses of this feature that don't already conform to this restriction.

[damiendoligez]

I don't think allowing allocations in these hooks was a good idea.

[xavierleroy]

I merged in trunk after light editing.