Fix plpgsql's handling of "simple" expression evaluation.

In general, expression execution state trees aren't re-entrantly usable,
since functions can store private state information in them.
For efficiency reasons, plpgsql tries to cache and reuse state trees for
"simple" expressions.  It can get away with that most of the time, but it
can fail if the state tree is dirty from a previous failed execution (as
in an example from Alvaro) or is being used recursively (as noted by me).

Fix by tracking whether a state tree is in use, and falling back to the
"non-simple" code path if so.  This results in a pretty considerable speed
hit when the non-simple path is taken, but the available alternatives seem
even more unpleasant because they add overhead in the simple path.  Per
idea from Heikki.

Back-patch to all supported branches.

src/pl/plpgsql/src/pl_exec.c

@@ -3920,7 +3920,14 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 	 * directly
 	 */
 	if (expr->expr_simple_expr != NULL)
-		return exec_eval_simple_expr(estate, expr, isNull, rettype);
+	{
+		/*
+		 * If expression is in use in current xact, don't touch it.
+		 */
+		if (!(expr->expr_simple_in_use &&
+			  expr->expr_simple_id == estate->eval_estate_simple_id))
+			return exec_eval_simple_expr(estate, expr, isNull, rettype);
+	}
 
 	rc = exec_run_select(estate, expr, 2, NULL);
 	if (rc != SPI_OK_SELECT)
@@ -4071,6 +4078,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 	{
 		expr->expr_simple_state = ExecPrepareExpr(expr->expr_simple_expr,
 												  estate->eval_estate);
+		expr->expr_simple_in_use = false;
 		expr->expr_simple_id = estate->eval_estate_simple_id;
 	}
 
@@ -4130,6 +4138,11 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 			ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
 		}
 
+		/*
+		 * Mark expression as busy for the duration of the ExecEvalExpr call.
+		 */
+		expr->expr_simple_in_use = true;
+
 		/*
 		 * Finally we can call the executor to evaluate the expression
 		 */
@@ -4137,11 +4150,15 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 							  econtext,
 							  isNull,
 							  NULL);
+
+		/* Assorted cleanup */
+		expr->expr_simple_in_use = false;
 		MemoryContextSwitchTo(oldcontext);
 	}
 	PG_CATCH();
 	{
 		/* Restore global vars and propagate error */
+		/* note we intentionally don't reset expr_simple_in_use here */
 		ActiveSnapshot = saveActiveSnapshot;
 		PG_RE_THROW();
 	}
@@ -4741,6 +4758,7 @@ exec_simple_check_plan(PLpgSQL_expr *expr)
 	 */
 	expr->expr_simple_expr = tle->expr;
 	expr->expr_simple_state = NULL;
+	expr->expr_simple_in_use = false;
 	expr->expr_simple_id = -1;
 	/* Also stash away the expression result type */
 	expr->expr_simple_type = exprType((Node *) tle->expr);

src/pl/plpgsql/src/plpgsql.h

@@ -181,10 +181,11 @@ typedef struct PLpgSQL_expr
 
 	/*
 	 * if expr is simple AND prepared in current eval_estate,
-	 * expr_simple_state is valid.  Test validity by seeing if expr_simple_id
+	 * expr_simple_state and expr_simple_in_use are valid. Test validity by
-	 * matches eval_estate_simple_id.
+	 * seeing if expr_simple_id matches eval_estate_simple_id.
 	 */
-	ExprState  *expr_simple_state;
+	ExprState  *expr_simple_state;		/* eval tree for expr_simple_expr */
+	bool		expr_simple_in_use;		/* true if eval tree is active */
 	long int	expr_simple_id;
 
 	/* params to pass to expr */

src/test/regress/expected/plpgsql.out

@@ -2977,3 +2977,50 @@ SELECT nonsimple_expr_test();
 (1 row)
 
 DROP FUNCTION nonsimple_expr_test();
+--
+-- Test cases involving recursion and error recovery in simple expressions
+-- (bugs in all versions before October 2010).  The problems are most
+-- easily exposed by mutual recursion between plpgsql and sql functions.
+--
+create function recurse(float8) returns float8 as
+$$
+begin
+  if ($1 < 10) then
+    return sql_recurse($1 + 1);
+  else
+    return $1;
+  end if;
+end;
+$$ language plpgsql;
+-- "limit" is to prevent this from being inlined
+create function sql_recurse(float8) returns float8 as
+$$ select recurse($1) limit 1; $$ language sql;
+select recurse(0);
+ recurse 
+---------
+      10
+(1 row)
+
+create function error1(text) returns text language sql as
+$$ SELECT relname::text FROM pg_class c WHERE c.oid = $1::regclass $$;
+create function error2(p_name_table text) returns text language plpgsql as $$
+begin
+  return error1(p_name_table);
+end$$;
+BEGIN;
+create table public.stuffs (stuff text);
+SAVEPOINT a;
+select error2('nonexistent.stuffs');
+ERROR:  schema "nonexistent" does not exist
+CONTEXT:  SQL function "error1" statement 1
+PL/pgSQL function "error2" line 2 at return
+ROLLBACK TO a;
+select error2('public.stuffs');
+ error2 
+--------
+ stuffs
+(1 row)
+
+rollback;
+drop function error2(p_name_table text);
+drop function error1(text);

src/test/regress/sql/plpgsql.sql

@@ -2480,3 +2480,45 @@ $$ LANGUAGE plpgsql;
 SELECT nonsimple_expr_test();
 
 DROP FUNCTION nonsimple_expr_test();
+
+--
+-- Test cases involving recursion and error recovery in simple expressions
+-- (bugs in all versions before October 2010).  The problems are most
+-- easily exposed by mutual recursion between plpgsql and sql functions.
+--
+
+create function recurse(float8) returns float8 as
+$$
+begin
+  if ($1 < 10) then
+    return sql_recurse($1 + 1);
+  else
+    return $1;
+  end if;
+end;
+$$ language plpgsql;
+
+-- "limit" is to prevent this from being inlined
+create function sql_recurse(float8) returns float8 as
+$$ select recurse($1) limit 1; $$ language sql;
+
+select recurse(0);
+
+create function error1(text) returns text language sql as
+$$ SELECT relname::text FROM pg_class c WHERE c.oid = $1::regclass $$;
+
+create function error2(p_name_table text) returns text language plpgsql as $$
+begin
+  return error1(p_name_table);
+end$$;
+
+BEGIN;
+create table public.stuffs (stuff text);
+SAVEPOINT a;
+select error2('nonexistent.stuffs');
+ROLLBACK TO a;
+select error2('public.stuffs');
+rollback;
+
+drop function error2(p_name_table text);
+drop function error1(text);