bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Pip Cet]

On Thu, May 28, 2020 at 2:28 PM Pip Cet <pipcet@gmail.com> wrote:
> My suggestion is instead to put MEM_TYPE_SYMBOL blocks into the rbtree
> twice, once at their proper address and once at the lispsym-based
> offset.
>
> We could then look up each pointer precisely once, though sometimes
> the blocks might overlap and we'd end up marking two objects for one
> pointer.
>
> But that would lead to overlapping rbtree entries, and that requires
> some extra code which wouldn't be exercised very often... still, I
> think it might be worth doing, particularly since there are relatively
> few symbol blocks on most systems.

Okay, here's some initial code that does that. It's a little tricky,
because real addresses and symbol offsets can overlap arbitrarily and
become mapped and unmapped in any order. The basic idea is that symbol
offsets are marked two ways:
1. an overlaps_with_symbols flag on a "normal" memory node
2. a mem node type of MEM_TYPE_SYMBOL_ADJUSTED

(2) implies (1), but not the other way around. There's only one flag
per normal memory node, which is true if any of the addresses in the
node are also valid symbol offsets. MEM_TYPE_SYMBOL_ADJUSTED nodes
have start and end addresses that do not necessarily correspond to
symbol blocks or even symbols; their length is arbitrary.

When we insert or delete memory nodes, we perform the obvious
operations to keep MEM_TYPE_SYMBOL_ADJUSTED blocks accurate: i.e.,
when a MEM_TYPE_SYMBOL_ADJUSTED node is split by an
intervening/overlapping normal node, we insert one or two new
MEM_TYPE_SYMBOL_ADJUSTED nodes to cover the remaining offsets, and set
the overlaps_with_symbols flag on the normal node, to cover those,
etc.

As I said, the code is tricky (i.e. might contain bugs that can only
be discovered through extensive testing on 32-bit systems), and it
complicates what should be generic functions for the rbtree
implementation, so this is probably a 32-bit optimization that is too
late because 32-bit systems are no longer that relevant...

0001-snapshot.patch
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