[Issue 24860] array appending can create stale memory references

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24860

--- Comment #3 from Steven Schveighoffer <schveiguy at gmail.com> ---
So the test case is a bit more convoluted for older compilers (i.e. the release
on run.dlang.io), because the builtin append operation uses the exact size for
appending for blocks less than PAGE size.

And for more than page size, the GC is already scanning only the "used" array
elements. This means, it shouldn't be happening in the wild with the
non-appender array runtime. There is one byte that is not zeroed, but that's
because of the array metadata size (which isn't exactly correct, but one byte
won't be mistaken for a pointer).

However, in the next release I have modified the growth factor for small blocks
to still use the algorithm for smaller blocks (similar to Appender), so
ironically, this will introduce the problem.

It does mean that for large blocks, we don't need to zero the array data
exactly. But this really is GC dependent, and I'd prefer to have this properly
handled by the GC.

Really, we need a GC API to allocate N bytes, but notify we will only be using
M of those bytes, so the rest should be zeroed by the GC *if it will be
scanned*.

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