Free SQLSTATE and SQLERRM no earlier than other PL/pgSQL variables.

"RETURN SQLERRM" prompted plpgsql_exec_function() to read from freed
memory.  Back-patch to 9.0 (all supported versions).  Little code ran
between the premature free and the read, so non-assert builds are
unlikely to witness user-visible consequences.

Author: nmisch

src/pl/plpgsql/src/pl_exec.c

@@ -1220,8 +1220,9 @@ exec_stmt_block(PLpgSQL_execstate *estate, PLpgSQL_stmt_block *block)
 				{
 					/*
 					 * Initialize the magic SQLSTATE and SQLERRM variables for
-					 * the exception block. We needn't do this until we have
-					 * found a matching exception.
+					 * the exception block; this also frees values from any
+					 * prior use of the same exception. We needn't do this
+					 * until we have found a matching exception.
 					 */
 					PLpgSQL_var *state_var;
 					PLpgSQL_var *errm_var;
@@ -1245,13 +1246,6 @@ exec_stmt_block(PLpgSQL_execstate *estate, PLpgSQL_stmt_block *block)
 
 					rc = exec_stmts(estate, exception->action);
 
-					free_var(state_var);
-					state_var->value = (Datum) 0;
-					state_var->isnull = true;
-					free_var(errm_var);
-					errm_var->value = (Datum) 0;
-					errm_var->isnull = true;
-
 					break;
 				}
 			}

src/test/regress/expected/plpgsql.out

@@ -2644,9 +2644,21 @@ NOTICE:  P0001 user exception
 
 (1 row)
 
+create function excpt_test4() returns text as $$
+begin
+	begin perform 1/0;
+	exception when others then return sqlerrm; end;
+end; $$ language plpgsql;
+select excpt_test4();
+   excpt_test4    
+------------------
+ division by zero
+(1 row)
+
 drop function excpt_test1();
 drop function excpt_test2();
 drop function excpt_test3();
+drop function excpt_test4();
 -- parameters of raise stmt can be expressions
 create function raise_exprs() returns void as $$
 declare

src/test/regress/sql/plpgsql.sql

@@ -2241,11 +2241,19 @@ begin
 	    raise notice '% %', sqlstate, sqlerrm;
     end;
 end; $$ language plpgsql;
-
 select excpt_test3();
+
+create function excpt_test4() returns text as $$
+begin
+	begin perform 1/0;
+	exception when others then return sqlerrm; end;
+end; $$ language plpgsql;
+select excpt_test4();
+
 drop function excpt_test1();
 drop function excpt_test2();
 drop function excpt_test3();
+drop function excpt_test4();
 
 -- parameters of raise stmt can be expressions
 create function raise_exprs() returns void as $$