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Allow "internal" subtransactions in parallel mode.

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Allow use of BeginInternalSubTransaction() in parallel mode, so long
as the subtransaction doesn't attempt to acquire an XID or increment
the command counter.  Given those restrictions, the other parallel
processes don't need to know about the subtransaction at all, so
this should be safe.  The benefit is that it allows subtransactions
intended for error recovery, such as pl/pgsql exception blocks,
to be used in PARALLEL SAFE functions.

Another reason for doing this is that the API of
BeginInternalSubTransaction() doesn't allow reporting failure.
pl/python for one, and perhaps other PLs, copes very poorly with an
error longjmp out of BeginInternalSubTransaction().  The headline
feature of this patch removes the only easily-triggerable failure
case within that function.  There remain some resource-exhaustion
and similar cases, which we now deal with by promoting them to FATAL
errors, so that callers need not try to clean up.  (It is likely
that such errors would leave us with corrupted transaction state
inside xact.c, making recovery difficult if not impossible anyway.)

Although this work started because of a report of a pl/python crash,
we're not going to do anything about that in the back branches.
Back-patching this particular fix is obviously not very wise.
While we could contemplate some narrower band-aid, pl/python is
already an untrusted language, so it seems okay to classify this
as a "so don't do that" case.

Patch by me, per report from Hao Zhang.  Thanks to Robert Haas for
review.

Discussion: https://postgr.es/m/CALY6Dr-2yLVeVPhNMhuBnRgOZo1UjoTETgtKBx1B2gUi8yy+3g@mail.gmail.com
This commit is contained in:
Tom Lane
2024-03-28 12:43:10 -04:00
parent e2395cdbe8
commit 0075d78947
7 changed files with 180 additions and 87 deletions
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8
doc/src/sgml/parallel.sgml
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@@ -545,10 +545,10 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</para>
<para>
Functions and aggregates must be marked <literal>PARALLEL UNSAFE</literal> if
they write to the database, access sequences, change the transaction state
even temporarily (e.g., a PL/pgSQL function that establishes an
<literal>EXCEPTION</literal> block to catch errors), or make persistent changes to
Functions and aggregates must be marked <literal>PARALLEL UNSAFE</literal>
if they write to the database, change the transaction state (other than by
using a subtransaction for error recovery), access sequences, or make
persistent changes to
settings. Similarly, functions must be marked <literal>PARALLEL
RESTRICTED</literal> if they access temporary tables, client connection state,
cursors, prepared statements, or miscellaneous backend-local state that