Fix corner-case uninitialized-variable issues in plpgsql.

If an error was raised during our initial attempt to check whether
a successfully-compiled expression is "simple", subsequent calls of
exec_stmt_execsql would suppose that stmt->mod_stmt was already computed
when it had not been.  This could lead to assertion failures in debug
builds; in production builds the effect would typically be to act as
if INTO STRICT had been specified even when it had not been.  Of course
that only matters if the subsequent attempt to execute the expression
succeeds, so that the problem can only be reached by fixing a failure
in some referenced, inline-able SQL function and then retrying the
calling plpgsql function in the same session.

(There might be even-more-obscure ways to change the expression's
behavior without changing the plpgsql function, but that one seems
like the only one people would be likely to hit in practice.)

The most foolproof way to fix this would be to arrange for
exec_prepare_plan to not set expr->plan until we've finished the
subsidiary simple-expression check.  But it seems hard to do that
without creating reference-count leak issues.  So settle for documenting
the hazard in a comment and fixing exec_stmt_execsql to test separately
for whether it's computed stmt->mod_stmt.  (That adds a test-and-branch
per execution, but hopefully that's negligible in context.)  In v11 and
up, also fix exec_stmt_call which had a variant of the same issue.

Per bug #17113 from Alexander Lakhin.  Back-patch to all
supported branches.

Discussion: https://postgr.es/m/17113-077605ce00e0e7ec@postgresql.org

src/pl/plpgsql/src/pl_exec.c

@@ -3384,6 +3384,22 @@ exec_eval_cleanup(PLpgSQL_execstate *estate)
 
 /* ----------
  * Generate a prepared plan
+ *
+ * CAUTION: it is possible for this function to throw an error after it has
+ * built a SPIPlan and saved it in expr->plan.  Therefore, be wary of doing
+ * additional things contingent on expr->plan being NULL.  That is, given
+ * code like
+ *
+ *	if (query->plan == NULL)
+ *	{
+ *		// okay to put setup code here
+ *		exec_prepare_plan(estate, query, ...);
+ *		// NOT okay to put more logic here
+ *	}
+ *
+ * extra steps at the end are unsafe because they will not be executed when
+ * re-executing the calling statement, if exec_prepare_plan failed the first
+ * time.  This is annoyingly error-prone, but the alternatives are worse.
  * ----------
  */
 static void
@@ -3427,15 +3443,15 @@ exec_prepare_plan(PLpgSQL_execstate *estate,
 	SPI_keepplan(plan);
 	expr->plan = plan;
 
-	/* Check to see if it's a simple expression */
-	exec_simple_check_plan(expr);
-
 	/*
 	 * Mark expression as not using a read-write param.  exec_assign_value has
 	 * to take steps to override this if appropriate; that seems cleaner than
 	 * adding parameters to all other callers.
 	 */
 	expr->rwparam = -1;
+
+	/* Check to see if it's a simple expression */
+	exec_simple_check_plan(expr);
 }
 
 
@@ -3457,10 +3473,12 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 	 * whether the statement is INSERT/UPDATE/DELETE
 	 */
 	if (expr->plan == NULL)
+		exec_prepare_plan(estate, expr, 0);
+
+	if (!stmt->mod_stmt_set)
 	{
 		ListCell   *l;
 
-		exec_prepare_plan(estate, expr, 0);
 		stmt->mod_stmt = false;
 		foreach(l, SPI_plan_get_plan_sources(expr->plan))
 		{
@@ -3481,6 +3499,7 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 				break;
 			}
 		}
+		stmt->mod_stmt_set = true;
 	}
 
 	/*
@@ -4181,6 +4200,14 @@ exec_assign_expr(PLpgSQL_execstate *estate, PLpgSQL_datum *target,
 	 * if we can pass the target variable as a read-write parameter to the
 	 * expression.  (This is a bit messy, but it seems cleaner than modifying
 	 * the API of exec_eval_expr for the purpose.)
+	 *
+	 * NOTE: this coding ignores the advice given in exec_prepare_plan's
+	 * comments, that one should not do additional setup contingent on
+	 * expr->plan being NULL.  This means that if we get an error while trying
+	 * to check for the expression being simple, we won't check for a
+	 * read-write parameter either, so that neither optimization will be
+	 * applied in future executions.  Nothing will fail though, so we live
+	 * with that bit of messiness too.
 	 */
 	if (expr->plan == NULL)
 	{
@@ -6544,6 +6571,10 @@ exec_simple_check_node(Node *node)
  * exec_simple_check_plan -		Check if a plan is simple enough to
  *								be evaluated by ExecEvalExpr() instead
  *								of SPI.
+ *
+ * Note: the refcount manipulations in this function assume that expr->plan
+ * is a "saved" SPI plan.  That's a bit annoying from the caller's standpoint,
+ * but it's otherwise difficult to avoid leaking the plan on failure.
  * ----------
  */
 static void

src/pl/plpgsql/src/pl_gram.y

@@ -2934,7 +2934,7 @@ make_execsql_stmt(int firsttoken, int location)
 
 	check_sql_expr(expr->query, location, 0);
 
-	execsql = palloc(sizeof(PLpgSQL_stmt_execsql));
+	execsql = palloc0(sizeof(PLpgSQL_stmt_execsql));
 	execsql->cmd_type = PLPGSQL_STMT_EXECSQL;
 	execsql->lineno  = plpgsql_location_to_lineno(location);
 	execsql->sqlstmt = expr;

src/pl/plpgsql/src/plpgsql.h

@@ -659,9 +659,9 @@ typedef struct
 	int			lineno;
 	PLpgSQL_expr *sqlstmt;
 	bool		mod_stmt;		/* is the stmt INSERT/UPDATE/DELETE? */
-	/* note: mod_stmt is set when we plan the query */
 	bool		into;			/* INTO supplied? */
 	bool		strict;			/* INTO STRICT flag */
+	bool		mod_stmt_set;	/* is mod_stmt valid yet? */
 	PLpgSQL_rec *rec;			/* INTO target, if record */
 	PLpgSQL_row *row;			/* INTO target, if row */
 } PLpgSQL_stmt_execsql;