Further refine _SPI_execute_plan's rule for atomic execution.

Commit 2dc1deaea turns out to have been still a brick shy of a load,
because CALL statements executing within a plpgsql exception block
could still pass the wrong snapshot to stable functions within the
CALL's argument list.  That happened because standard_ProcessUtility
forces isAtomicContext to true if IsTransactionBlock is true, which
it always will be inside a subtransaction.  Then ExecuteCallStmt
would think it does not need to push a new snapshot --- but
_SPI_execute_plan didn't do so either, since it thought it was in
nonatomic mode.

The best fix for this seems to be for _SPI_execute_plan to operate
in atomic execution mode if IsSubTransaction() is true, even when the
SPI context as a whole is non-atomic.  This makes _SPI_execute_plan
have the same rules about when non-atomic execution is allowed as
_SPI_commit/_SPI_rollback have about when COMMIT/ROLLBACK are allowed,
which seems appropriately symmetric.  (If anyone ever tries to allow
COMMIT/ROLLBACK inside a subtransaction, this would all need to be
rethought ... but I'm unconvinced that such a thing could be logically
consistent at all.)

For further consistency, also check IsSubTransaction() in
SPI_inside_nonatomic_context.  That does not matter for its
one present-day caller StartTransaction, which can't be reached
inside a subtransaction.  But if any other callers ever arise,
they'd presumably want this definition.

Per bug #18656 from Alexander Alehin.  Back-patch to all
supported branches, like previous fixes in this area.

Discussion: https://postgr.es/m/18656-cade1780866ef66c@postgresql.org

src/backend/executor/spi.c

@@ -328,13 +328,13 @@ _SPI_rollback(bool chain)
 {
 	MemoryContext oldcontext = CurrentMemoryContext;
 
-	/* see under SPI_commit() */
+	/* see comments in _SPI_commit() */
 	if (_SPI_current->atomic)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
 				 errmsg("invalid transaction termination")));
 
-	/* see under SPI_commit() */
+	/* see comments in _SPI_commit() */
 	if (IsSubTransaction())
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
@@ -587,8 +587,11 @@ SPI_inside_nonatomic_context(void)
 {
 	if (_SPI_current == NULL)
 		return false;			/* not in any SPI context at all */
+	/* these tests must match _SPI_commit's opinion of what's atomic: */
 	if (_SPI_current->atomic)
 		return false;			/* it's atomic (ie function not procedure) */
+	if (IsSubTransaction())
+		return false;			/* if within subtransaction, it's atomic */
 	return true;
 }
 
@@ -2212,9 +2215,12 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
 
 	/*
 	 * We allow nonatomic behavior only if plan->no_snapshots is set
-	 * *and* the SPI_OPT_NONATOMIC flag was given when connecting.
+	 * *and* the SPI_OPT_NONATOMIC flag was given when connecting and we are
+	 * not inside a subtransaction.  The latter two tests match whether
+	 * _SPI_commit() would allow a commit; see there for more commentary.
 	 */
-	allow_nonatomic = plan->no_snapshots && !_SPI_current->atomic;
+	allow_nonatomic = plan->no_snapshots &&
+		!_SPI_current->atomic && !IsSubTransaction();
 
 	/*
 	 * Setup error traceback support for ereport()

src/pl/plpgsql/src/expected/plpgsql_call.out

@@ -490,6 +490,26 @@ NOTICE:  f_get_x(1)
 NOTICE:  f_print_x(1)
 NOTICE:  f_get_x(2)
 NOTICE:  f_print_x(2)
+-- test in non-atomic context, except exception block is locally atomic
+DO $$
+BEGIN
+ BEGIN
+  UPDATE t_test SET x = x + 1;
+  RAISE NOTICE 'f_get_x(%)', f_get_x();
+  CALL f_print_x(f_get_x());
+  UPDATE t_test SET x = x + 1;
+  RAISE NOTICE 'f_get_x(%)', f_get_x();
+  CALL f_print_x(f_get_x());
+ EXCEPTION WHEN division_by_zero THEN
+   RAISE NOTICE '%', SQLERRM;
+ END;
+  ROLLBACK;
+END
+$$;
+NOTICE:  f_get_x(1)
+NOTICE:  f_print_x(1)
+NOTICE:  f_get_x(2)
+NOTICE:  f_print_x(2)
 -- test in atomic context
 BEGIN;
 DO $$

src/pl/plpgsql/src/sql/plpgsql_call.sql

@@ -460,6 +460,23 @@ BEGIN
 END
 $$;
 
+-- test in non-atomic context, except exception block is locally atomic
+DO $$
+BEGIN
+ BEGIN
+  UPDATE t_test SET x = x + 1;
+  RAISE NOTICE 'f_get_x(%)', f_get_x();
+  CALL f_print_x(f_get_x());
+  UPDATE t_test SET x = x + 1;
+  RAISE NOTICE 'f_get_x(%)', f_get_x();
+  CALL f_print_x(f_get_x());
+ EXCEPTION WHEN division_by_zero THEN
+   RAISE NOTICE '%', SQLERRM;
+ END;
+  ROLLBACK;
+END
+$$;
+
 -- test in atomic context
 BEGIN;