rebis-dev: Fix UB detected by miri in tests #2729
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This one was a toughie: it turns out that using `ptr::align_of()`` was a bad idea, since the buffer in `Heap` itself is not aligned to `Heap::heap_cell_alignment()`, so `ptr::align_of()` would sometimes return lower values than expected. That made for an heisenbug: if the alignment of the heap happened to be 4, then the bug wouldn't trigger.
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After seeing one incredible contribution after another from people who literally see this project for the first time, I have learned that "took me a few hours to track down" from new contributors usually, and also in this case, translates to: "casually corrects a mistake that has haunted us for months or years". |
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This fixes two sources of undefined behavior:
Heap::drop()could be called whileInnerHeap::ptris null, but it did not account for it, causing it to attempt to free a null pointer.ReservedHeapSection::push_pstr_segment()andHeap::copy_str_within()were usingptr::align_ofto compute the number of bytes needed to pad the string with zeroes, but since the underlying buffer (InnerHeap::ptr) itself isn't aligned tosize_of::<HeapCellValue>(), the returned values would vary randomly between 0 and 3, sometimes causing those two functions to not mark the cells as allocated.That second one took me a few hours to track down, since the tests would randomly work again should the stars (literally) align ^^'