Fix two issues in TOAST decompression.
pglz_maximum_compressed_size() potentially underestimated the amount
of compressed data required to produce N bytes of decompressed data;
this is a fault in commit
11a078cf8.
Separately from that, pglz_decompress() failed to protect itself
against corrupt compressed data, particularly off == 0 in a match
tag. Commit
c60e520f6 turned such a situation into an infinite loop,
where before it'd just have resulted in garbage output.
The combination of these two bugs seems like it may explain bug #16694
from Tom Vijlbrief, though it's impossible to be quite sure without
direct inspection of the failing session. (One needs to assume that
the pglz_maximum_compressed_size() bug caused us to fail to fetch the
second byte of a match tag, and what happened to be there instead was
a zero. The reported infinite loop is hard to explain without off == 0,
though.)
Aside from fixing the bugs, rewrite associated comments for more
clarity.
Back-patch to v13 where both these commits landed.
Discussion: https://postgr.es/m/16694-
f107871e499ec114@postgresql.org