Raise a WARNING for max_slot_wal_keep_size in pg_createsubscriber.

During the pg_createsubscriber execution, it is possible that the required
WAL is removed from the primary/publisher node due to
'max_slot_wal_keep_size'.

This patch raises a WARNING during the '--dry-run' mode if the
'max_slot_wal_keep_size' is set to a non-default value on the
primary/publisher node.

Author: Shubham Khanna <khannashubham1197@gmail.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Discussion: https://postgr.es/m/CAHv8Rj+deqsQXOMa7Tck8CBQUbsua=+4AuMVQ2=MPM0f-ZHbjA@mail.gmail.com

diff --git a/doc/src/sgml/ref/pg_createsubscriber.sgml b/doc/src/sgml/ref/pg_createsubscriber.sgml
index 26b8e64a4e03b07eb67be526c003b9360147bc35..d56487fe2ca8158a6c3bf8bdb5e93b08dbe8da58 100644 (file)
--- a/doc/src/sgml/ref/pg_createsubscriber.sgml
+++ b/doc/src/sgml/ref/pg_createsubscriber.sgml
@@ -377,6 +377,13 @@ PostgreSQL documentation
     server.  If the target server has a standby, replication will break and a
     fresh standby should be created.
    </para>
+
+   <para>
+    Replication failures can occur if required WAL files are missing. To prevent
+    this, the source server must set
+    <xref linkend="guc-max-slot-wal-keep-size"/> to <literal>-1</literal> to
+    ensure that required WAL files are not prematurely removed.
+   </para>
   </refsect2>
 
   <refsect2>

diff --git a/src/bin/pg_basebackup/pg_createsubscriber.c b/src/bin/pg_basebackup/pg_createsubscriber.c
index 2d881d54f5b25c437f1976bce3d72704edc6b548..37fdf150b413cc9da79e57ad3d6200cc950b1c20 100644 (file)
--- a/src/bin/pg_basebackup/pg_createsubscriber.c
+++ b/src/bin/pg_basebackup/pg_createsubscriber.c
@@ -849,6 +849,7 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
    int         max_walsenders;
    int         cur_walsenders;
    int         max_prepared_transactions;
+   char       *max_slot_wal_keep_size;
 
    pg_log_info("checking settings on publisher");
 
@@ -872,6 +873,7 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
     * - wal_level = logical
     * - max_replication_slots >= current + number of dbs to be converted
     * - max_wal_senders >= current + number of dbs to be converted
+    * - max_slot_wal_keep_size = -1 (to prevent deletion of required WAL files)
     * -----------------------------------------------------------------------
     */
    res = PQexec(conn,
@@ -880,7 +882,8 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
                 " (SELECT count(*) FROM pg_catalog.pg_replication_slots),"
                 " pg_catalog.current_setting('max_wal_senders'),"
                 " (SELECT count(*) FROM pg_catalog.pg_stat_activity WHERE backend_type = 'walsender'),"
-                " pg_catalog.current_setting('max_prepared_transactions')");
+                " pg_catalog.current_setting('max_prepared_transactions'),"
+                " pg_catalog.current_setting('max_slot_wal_keep_size')");
 
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
@@ -895,6 +898,7 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
    max_walsenders = atoi(PQgetvalue(res, 0, 3));
    cur_walsenders = atoi(PQgetvalue(res, 0, 4));
    max_prepared_transactions = atoi(PQgetvalue(res, 0, 5));
+   max_slot_wal_keep_size = pg_strdup(PQgetvalue(res, 0, 6));
 
    PQclear(res);
 
@@ -905,6 +909,8 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
    pg_log_debug("publisher: current wal senders: %d", cur_walsenders);
    pg_log_debug("publisher: max_prepared_transactions: %d",
                 max_prepared_transactions);
+   pg_log_debug("publisher: max_slot_wal_keep_size: %s",
+                max_slot_wal_keep_size);
 
    disconnect_database(conn, false);
 
@@ -939,6 +945,18 @@ check_publisher(const struct LogicalRepInfo *dbinfo)
                              "Prepared transactions will be replicated at COMMIT PREPARED.");
    }
 
+   /*
+    * Validate 'max_slot_wal_keep_size'. If this parameter is set to a
+    * non-default value, it may cause replication failures due to required
+    * WAL files being prematurely removed.
+    */
+   if (dry_run && (strcmp(max_slot_wal_keep_size, "-1") != 0))
+   {
+       pg_log_warning("required WAL could be removed from the publisher");
+       pg_log_warning_hint("Set the configuration parameter \"%s\" to -1 to ensure that required WAL files are not prematurely removed.",
+                           "max_slot_wal_keep_size");
+   }
+
    pg_free(wal_level);
 
    if (failed)