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@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19026-90aed5e71d0c8af3@postgresql.org
Backpatch-through: 18

src/backend/executor/functions.c

@@ -143,6 +143,7 @@ typedef struct SQLFunctionCache
 {
 	SQLFunctionHashEntry *func; /* associated SQLFunctionHashEntry */
 
+	bool		active;			/* are we executing this cache entry? */
 	bool		lazyEvalOK;		/* true if lazyEval is safe */
 	bool		shutdown_reg;	/* true if registered shutdown callback */
 	bool		lazyEval;		/* true if using lazyEval for result query */
@@ -556,6 +557,28 @@ init_sql_fcache(FunctionCallInfo fcinfo, bool lazyEvalOK)
 		finfo->fn_extra = fcache;
 	}
 
+	/*
+	 * If the SQLFunctionCache is marked as active, we must have errored out
+	 * of a prior execution.  Reset state.  (It might seem that we could also
+	 * reach this during recursive invocation of a SQL function, but we won't
+	 * because that case won't involve re-use of the same FmgrInfo.)
+	 */
+	if (fcache->active)
+	{
+		/*
+		 * In general, this stanza should clear all the same fields that
+		 * ShutdownSQLFunction would.  Note we must clear fcache->cplan
+		 * without doing ReleaseCachedPlan, because error cleanup from the
+		 * prior execution would have taken care of releasing that plan.
+		 * Likewise, if tstore is still set then it is pointing at garbage.
+		 */
+		fcache->cplan = NULL;
+		fcache->eslist = NULL;
+		fcache->tstore = NULL;
+		fcache->shutdown_reg = false;
+		fcache->active = false;
+	}
+
 	/*
 	 * If we are resuming execution of a set-returning function, just keep
 	 * using the same cache.  We do not ask funccache.c to re-validate the
@@ -1597,6 +1620,9 @@ fmgr_sql(PG_FUNCTION_ARGS)
 	 */
 	fcache = init_sql_fcache(fcinfo, lazyEvalOK);
 
+	/* Mark fcache as active */
+	fcache->active = true;
+
 	/* Remember info that we might need later to construct tuplestore */
 	fcache->tscontext = tscontext;
 	fcache->randomAccess = randomAccess;
@@ -1853,6 +1879,9 @@ fmgr_sql(PG_FUNCTION_ARGS)
 	if (es == NULL)
 		fcache->eslist = NULL;
 
+	/* Mark fcache as inactive */
+	fcache->active = false;
+
 	error_context_stack = sqlerrcontext.previous;
 
 	return result;

src/test/regress/expected/create_function_sql.out

@@ -733,6 +733,22 @@ SELECT double_append(array_append(ARRAY[q1], q2), q3)
  {4,5,6,4,5,6}
 (2 rows)
 
+-- Check that we can re-use a SQLFunctionCache after a run-time error.
+-- This function will fail with zero-divide at run time (not plan time).
+CREATE FUNCTION part_hashint4_error(value int4, seed int8) RETURNS int8
+LANGUAGE SQL STRICT IMMUTABLE PARALLEL SAFE AS
+$$ SELECT value + seed + random()::int/0 $$;
+-- Put it into an operator class so that FmgrInfo will be cached in relcache.
+CREATE OPERATOR CLASS part_test_int4_ops_bad FOR TYPE int4 USING hash AS
+  FUNCTION 2 part_hashint4_error(int4, int8);
+CREATE TABLE pt(i int) PARTITION BY hash (i part_test_int4_ops_bad);
+CREATE TABLE p1 PARTITION OF pt FOR VALUES WITH (modulus 4, remainder 0);
+INSERT INTO pt VALUES (1);
+ERROR:  division by zero
+CONTEXT:  SQL function "part_hashint4_error" statement 1
+INSERT INTO pt VALUES (1);
+ERROR:  division by zero
+CONTEXT:  SQL function "part_hashint4_error" statement 1
 -- Things that shouldn't work:
 CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
     AS 'SELECT ''not an integer'';';
@@ -773,7 +789,7 @@ CONTEXT:  SQL function "test1" during startup
 RESET check_function_bodies;
 -- Cleanup
 DROP SCHEMA temp_func_test CASCADE;
-NOTICE:  drop cascades to 35 other objects
+NOTICE:  drop cascades to 38 other objects
 DETAIL:  drop cascades to function functest_a_1(text,date)
 drop cascades to function functest_a_2(text[])
 drop cascades to function functest_a_3()
@@ -808,6 +824,9 @@ drop cascades to function create_and_insert()
 drop cascades to table ddl_test
 drop cascades to function alter_and_insert()
 drop cascades to function double_append(anyarray,anyelement)
+drop cascades to function part_hashint4_error(integer,bigint)
+drop cascades to operator family part_test_int4_ops_bad for access method hash
+drop cascades to table pt
 drop cascades to function test1(anyelement)
 DROP USER regress_unpriv_user;
 RESET search_path;

src/test/regress/sql/create_function_sql.sql

@@ -432,6 +432,23 @@ $$ SELECT array_append($1, $2) || array_append($1, $2) $$;
 SELECT double_append(array_append(ARRAY[q1], q2), q3)
   FROM (VALUES(1,2,3), (4,5,6)) v(q1,q2,q3);
 
+-- Check that we can re-use a SQLFunctionCache after a run-time error.
+
+-- This function will fail with zero-divide at run time (not plan time).
+CREATE FUNCTION part_hashint4_error(value int4, seed int8) RETURNS int8
+LANGUAGE SQL STRICT IMMUTABLE PARALLEL SAFE AS
+$$ SELECT value + seed + random()::int/0 $$;
+
+-- Put it into an operator class so that FmgrInfo will be cached in relcache.
+CREATE OPERATOR CLASS part_test_int4_ops_bad FOR TYPE int4 USING hash AS
+  FUNCTION 2 part_hashint4_error(int4, int8);
+
+CREATE TABLE pt(i int) PARTITION BY hash (i part_test_int4_ops_bad);
+CREATE TABLE p1 PARTITION OF pt FOR VALUES WITH (modulus 4, remainder 0);
+
+INSERT INTO pt VALUES (1);
+INSERT INTO pt VALUES (1);
+
 -- Things that shouldn't work:
 
 CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL