Fix pg_plan_queries() to restore the previous setting of ActiveSnapshot

(probably NULL) before exiting.  Up to now it's just left the variable as it
set it, which means that after we're done processing the current client
message, ActiveSnapshot is probably pointing at garbage (because this function
is typically run in MessageContext which will get reset).  There doesn't seem
to have been any code path in which that mattered before 8.3, but now the
plancache module might try to use the stale value if the next client message
is a Bind for a prepared statement that is in need of replanning.  Per report
from Alex Hunsaker.

diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 5a70e952fc96fda41ca4e50798753cf865e49f22..a03d29a20e10cd0ec852d9c25e6b2ab402c45db7 100644 (file)
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -729,31 +729,49 @@ List *
 pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams,
                                bool needSnapshot)
 {
-       List       *stmt_list = NIL;
-       ListCell   *query_list;
+       List       * volatile stmt_list = NIL;
+       Snapshot        saveActiveSnapshot = ActiveSnapshot;
 
-       foreach(query_list, querytrees)
+       /* PG_TRY to ensure previous ActiveSnapshot is restored on error */
+       PG_TRY();
        {
-               Query      *query = (Query *) lfirst(query_list);
-               Node       *stmt;
+               Snapshot        mySnapshot = NULL;
+               ListCell   *query_list;
 
-               if (query->commandType == CMD_UTILITY)
+               foreach(query_list, querytrees)
                {
-                       /* Utility commands have no plans. */
-                       stmt = query->utilityStmt;
-               }
-               else
-               {
-                       if (needSnapshot)
+                       Query      *query = (Query *) lfirst(query_list);
+                       Node       *stmt;
+
+                       if (query->commandType == CMD_UTILITY)
+                       {
+                               /* Utility commands have no plans. */
+                               stmt = query->utilityStmt;
+                       }
+                       else
                        {
-                               ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
-                               needSnapshot = false;
+                               if (needSnapshot && mySnapshot == NULL)
+                               {
+                                       mySnapshot = CopySnapshot(GetTransactionSnapshot());
+                                       ActiveSnapshot = mySnapshot;
+                               }
+                               stmt = (Node *) pg_plan_query(query, cursorOptions,
+                                                                                         boundParams);
                        }
-                       stmt = (Node *) pg_plan_query(query, cursorOptions, boundParams);
+
+                       stmt_list = lappend(stmt_list, stmt);
                }
 
-               stmt_list = lappend(stmt_list, stmt);
+               if (mySnapshot)
+                       FreeSnapshot(mySnapshot);
+       }
+       PG_CATCH();
+       {
+               ActiveSnapshot = saveActiveSnapshot;
+               PG_RE_THROW();
        }
+       PG_END_TRY();
+       ActiveSnapshot = saveActiveSnapshot;
 
        return stmt_list;
 }