AlterSubscription_refresh: avoid stomping on global variable

This patch replaces use of the global "wrconn" variable in
AlterSubscription_refresh with a local variable of the same name, making
it consistent with other functions in subscriptioncmds.c (e.g.
DropSubscription).

The global wrconn is only meant to be used for logical apply/tablesync worker.
Abusing it this way is known to cause trouble if an apply worker
manages to do a subscription refresh, such as reported by Jeremy Finzel
and diagnosed by Andres Freund back in November 2020, at
https://www.postgresql.org/message-id/20201111215820.qihhrz7fayu6myfi@alap3.anarazel.de

Backpatch to 10.  In branch master, also move the connection establishment
to occur outside the PG_TRY block; this way we can remove a test for NULL in
PG_FINALLY, and it also makes the code more consistent with similar code in
the same file.

Author: Peter Smith <peter.b.smith@fujitsu.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/CAHut+Pu7Jv9L2BOEx_Z0UtJxfDevQSAUW2mJqWU+CtmDrEZVAg@mail.gmail.com

diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index 517c8edd3b2602a9bee0f684fddd891f2b7c550c..bbb2f5d029ea779b9ab11539de8feedcec74fabe 100644 (file)
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -556,18 +556,19 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
        char        state;
    } SubRemoveRels;
    SubRemoveRels *sub_remove_rels;
+   WalReceiverConn *wrconn;
 
    /* Load the library providing us libpq calls. */
    load_file("libpqwalreceiver", false);
 
+   /* Try to connect to the publisher. */
+   wrconn = walrcv_connect(sub->conninfo, true, sub->name, &err);
+   if (!wrconn)
+       ereport(ERROR,
+               (errmsg("could not connect to the publisher: %s", err)));
+
    PG_TRY();
    {
-       /* Try to connect to the publisher. */
-       wrconn = walrcv_connect(sub->conninfo, true, sub->name, &err);
-       if (!wrconn)
-           ereport(ERROR,
-                   (errmsg("could not connect to the publisher: %s", err)));
-
        /* Get the table list from publisher. */
        pubrel_names = fetch_table_list(wrconn, sub->publications);
 
@@ -737,8 +738,7 @@ AlterSubscription_refresh(Subscription *sub, bool copy_data)
    }
    PG_FINALLY();
    {
-       if (wrconn)
-           walrcv_disconnect(wrconn);
+       walrcv_disconnect(wrconn);
    }
    PG_END_TRY();
 
@@ -1062,7 +1062,7 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
    ListCell   *lc;
    char        originname[NAMEDATALEN];
    char       *err = NULL;
-   WalReceiverConn *wrconn = NULL;
+   WalReceiverConn *wrconn;
    Form_pg_subscription form;
    List       *rstates;