pgsql: Fix datum_image_*()'s inability to detect sign-extension variati

Fix datum_image_*()'s inability to detect sign-extension variations

Functions such as hash_numeric() are not careful to use the correct
PG_RETURN_*() macro according to the return type of that function as
defined in pg_proc.  Because that function is meant to return int32,
when the hashed value exceeds 2^31, the 64-bit Datum value won't wrap to
a negative number, which means the Datum won't have the same value as it
would have had it been cast to int32 on a two's complement machine.  This
isn't harmless as both datum_image_eq() and datum_image_hash() may receive
a Datum that's been formed and deformed from a tuple in some cases, and
not in other cases.  When formed into a tuple, the Datum value will be
coerced into an integer according to the attlen as specified by the
TupleDesc.  This can result in two Datums that should be equal being
classed as not equal, which could result in (but not limited to) an error
such as:

ERROR:  could not find memoization table entry

Here we fix this by ensuring we cast the Datum value to a signed integer
according to the typLen specified in the datum_image_eq/datum_image_hash
function call before comparing or hashing.

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: Tender Wang <tndrwang@gmail.com>
Backpatch-through: 14
Discussion: https://postgr.es/m/CAHewXNmcXVFdB9_WwA8Ez0P+m_TQy_KzYk5Ri5dvg+fuwjD_yw@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1bd90c8879b6bb00284401f3eb2f5911abb13dca

Modified Files
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src/backend/utils/adt/datum.c | 50 +++++++++++++++++++++++++++++++++++++------
1 file changed, 43 insertions(+), 7 deletions(-)