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bug#36447: 27.0.50; New "Unknown keyword" errors


From: Pip Cet
Subject: bug#36447: 27.0.50; New "Unknown keyword" errors
Date: Wed, 3 Jul 2019 11:57:52 +0000

On Tue, Jul 2, 2019 at 10:50 PM Pip Cet <address@hidden> wrote:
> > The compiler translates repeated `eq' in a cond like that into a hash
> > and jump.  See byte-compile-cond-use-jump-table.
>
> I think I found the problem. It's a bit tricky.

I'm attaching a patch which modifies standard Emacs in what should be
harmless ways, but demonstrates the bug.

As far as I can tell, this is a serious issue that will pop up
seemingly at random. We really should fix it.

However, I thought hash_table_rehash was called more often than it
actually is, so the fix should be simple: just copy-sequence a hash
table's ->next, ->hash, and ->index vectors before rehashing.
(->key_and_value isn't actually modified, so it's safe to share
between (read-only) hash tables.

What's happening is this:

Purecopied hash tables share structure, particularly the ->next
vector, with similar but different purecopied hash tables. Some
purecopied hash tables dumped with pdumper are rehashed upon first
access after loading the dump (in the example, this is the tables
which map 'dummy-symbol to t), while others are not (the tables with
just fixnums as keys). If a hash table is rehashed, it's ->next vector
will change to reflect the changed hash (of 'dummy-symbol); however,
the non-rehashed table that shares the ->next vector will be confused:
its key_and_value array will stay the same, and valid, but the ->next
vector will no longer match it. In practice, this means (gethash
valid-key hash-table) will return nil even though the key is valid.

While the attached patch appears to work, I would prefer simply
dumping hash tables with nil values for ->next, ->hash, and ->index,
and rebuilding the entire hash table upon first access. This would
also allow us to switch to secure randomized hashes should the need
arise.

Attachment: 0001-Test-bug-36447.patch
Description: Text Data

Attachment: 0002-Don-t-alter-shared-structure-in-dumped-purecopied-ha.patch
Description: Text Data


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