[ruby-core:82138] [Ruby trunk Bug#13595] rb_alloc_tmp_buffer2 broken when: elsize % sizeof(VALUE) == 0

Issue #13595 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.2: DONTNEED, 2.3: DONTNEED, 2.4: REQUIRED to 2.2: DONTNEED, 2.3: DONTNEED, 2.4: DONE

ruby_2_4 r59408 merged revision(s) 58902.

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Bug #13595: rb_alloc_tmp_buffer2 broken when: elsize % sizeof(VALUE) == 0
https://bugs.ruby-lang.org/issues/13595#change-65896

* Author: normalperson (Eric Wong)
* Status: Closed
* Priority: Normal
* Assignee: naruse (Yui NARUSE)
* Target version: 
* ruby -v: 
* Backport: 2.2: DONTNEED, 2.3: DONTNEED, 2.4: DONE
----------------------------------------
Here is the function in full as of current trunk (r58863):

~~~c
static inline void *
rb_alloc_tmp_buffer2(volatile VALUE *store, long count, size_t elsize)
{
    size_t cnt = (size_t)count;
    if (elsize % sizeof(VALUE) == 0) {
	if (RB_UNLIKELY(cnt > LONG_MAX / sizeof(VALUE))) {
	    ruby_malloc_size_overflow(cnt, elsize);
	}
    }
    else {
	size_t size, max = LONG_MAX - sizeof(VALUE) + 1;
	if (RB_UNLIKELY(rb_mul_size_overflow(cnt, elsize, max, &size))) {
	    ruby_malloc_size_overflow(cnt, elsize);
	}
	cnt = (size + sizeof(VALUE) - 1) / sizeof(VALUE);
    }
    return rb_alloc_tmp_buffer_with_count(store, cnt * sizeof(VALUE), cnt);
}
~~~
Notice that elsize is completely ignored in the top branch when
"`(elsize % sizeof(VALUE) == 0)`" is true; this gives me problems
when attempting to use `ALLOCV_N`.

I am terrible at arithmetic and this function is too complicated for me,
so I'll let naruse or someone else fix this.  But please do.  Thanks




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