pgsql: Fix re-execution of a failed SQLFunctionCache entry.

Fix re-execution of a failed SQLFunctionCache entry.

If we error out during execution of a SQL-language function, we will
often leave behind non-null pointers in its SQLFunctionCache's cplan
and eslist fields. This is problematic if the SQLFunctionCache is
re-used, because those pointers will point at resources that were
released during error cleanup. This problem escaped detection so far
because ordinarily we won't re-use an FmgrInfo+SQLFunctionCache struct
after a query error. However, in the rather improbable case that
someone implements an opclass support function in SQL language, there
will be long-lived FmgrInfos for it in the relcache, and then the
problem is reachable after the function throws an error.

To fix, add a flag to SQLFunctionCache that tracks whether execution
escapes out of fmgr_sql, and clear out the relevant fields during
init_sql_fcache if so. (This is going to need more thought if we ever
try to share FMgrInfos across threads; but it's very far from being
the only problem such a project will encounter, since many functions
regard fn_extra as being query-local state.)

This broke at commit 0313c5dc6; before that we did not try to re-use
SQLFunctionCache state across calls. Hence, back-patch to v18.

Bug: #19026
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19026-90aed5e71d0c8af3@postgresql.org
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/906b68217a1e756a49acd0864773680d1eb8a10d

Modified Files
--------------
src/backend/executor/functions.c | 29 +++++++++++++++++++++++
src/test/regress/expected/create_function_sql.out | 21 +++++++++++++++-
src/test/regress/sql/create_function_sql.sql | 17 +++++++++++++
3 files changed, 66 insertions(+), 1 deletion(-)