Fix performance issue in deadlock-parallel isolation test.

With debug_discard_caches = 1, the runtime of this test script
increased by about a factor of 10 after commit 0dca5d68d.  That's
causing some of our buildfarm animals to fail with a timeout.

The reason for the increased time is that now we are re-planning
some intentionally-non-inlineable SQL functions on every execution,
where the previous coding held onto the original plans throughout
the outer query.  The previous behavior was arguably quite buggy,
so I don't think 0dca5d68d deserves blame here.  But we would
like this test script to not take so long.

To fix, instead of forcing a "parallel safe" label via a
non-inlineable SQL function, apply it directly to the advisory-lock
functions by making internal-language aliases for them.  A small
problem is that the advisory-lock functions return void but this
test would really like them to return integer 1.  I cheated here by
declaring the aliases as returning "int".  That's perhaps undue
familiarity with the implementation of PG_RETURN_VOID(), but that
hasn't changed in twenty years and is unlikely to do so in the next
twenty.  That gets us an integer 0 result, and then an inline-able
wrapper to convert that to an integer 1 allows the rest of the script
to remain unchanged.

For me, this reduces the runtime with debug_discard_caches = 1
by about 100x, making the test comfortably faster than before
instead of slower.

Discussion: https://postgr.es/m/136163.1744179562@sss.pgh.pa.us

src/test/isolation/specs/deadlock-parallel.spec

@@ -2,8 +2,9 @@
 
 # It's fairly hard to get parallel worker processes to block on locks,
 # since generally they don't want any locks their leader didn't already
-# take.  We cheat like mad here by making a function that takes a lock,
-# and is incorrectly marked parallel-safe so that it can execute in a worker.
+# take.  We cheat like mad here by creating aliases for advisory-lock
+# functions that are incorrectly marked parallel-safe so that they can
+# execute in a worker.
 
 # Note that we explicitly override any global settings of isolation level
 # or debug_parallel_query, to ensure we're testing what we intend to.
@@ -36,11 +37,21 @@
 
 setup
 {
+-- The alias functions themselves.  Really these return "void", but
+-- the implementation is such that we can declare them to return "int",
+-- and we will get a zero result.
+  create function lock_share(bigint) returns int language internal as
+  'pg_advisory_xact_lock_shared_int8' strict parallel safe;
+
+  create function lock_excl(bigint) returns int language internal as
+  'pg_advisory_xact_lock_int8' strict parallel safe;
+
+-- Inline-able wrappers that will produce an integer "1" result:
   create function lock_share(int,int) returns int language sql as
-  'select pg_advisory_xact_lock_shared($1); select 1;' parallel safe;
+  'select 1 - lock_share($1)' parallel safe;
 
   create function lock_excl(int,int) returns int language sql as
-  'select pg_advisory_xact_lock($1); select 1;' parallel safe;
+  'select 1 - lock_excl($1)' parallel safe;
 
   create table bigt as select x from generate_series(1, 10000) x;
   analyze bigt;