Fix cross-checking of ReservedBackends/max_wal_senders/MaxConnections.

We were independently checking ReservedBackends < MaxConnections and
max_wal_senders < MaxConnections, but because walsenders aren't allowed
to use superuser-reserved connections, that's really the wrong thing.
Correct behavior is to insist on ReservedBackends + max_wal_senders being
less than MaxConnections.  Fix the code and associated documentation.

This has been wrong for a long time, but since the situation probably
hardly ever arises in the field (especially pre-v10, when the default
for max_wal_senders was zero), no back-patch.

Discussion: https://postgr.es/m/28271.1520195491@sss.pgh.pa.us

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 259a2d83b4a4671088ca083c4ef61ab5aaeb62a4..3a8fc7d803c5c3c3223911b6ff4083cf894d5bc0 100644 (file)
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -696,8 +696,9 @@ include_dir 'conf.d'
 
        <para>
         The default value is three connections. The value must be less
-        than the value of <varname>max_connections</varname>. This
-        parameter can only be set at server start.
+        than <varname>max_connections</varname> minus
+        <xref linkend="guc-max-wal-senders"/>.
+        This parameter can only be set at server start.
        </para>
       </listitem>
      </varlistentry>
@@ -2982,13 +2983,16 @@ include_dir 'conf.d'
         maximum number of simultaneously running WAL sender
         processes). The default is 10. The value 0 means replication is
         disabled. WAL sender processes count towards the total number
-        of connections, so the parameter cannot be set higher than
-        <xref linkend="guc-max-connections"/>.  Abrupt streaming client
-        disconnection might cause an orphaned connection slot until
+        of connections, so this parameter's value must be less than
+        <xref linkend="guc-max-connections"/> minus
+        <xref linkend="guc-superuser-reserved-connections"/>.
+        Abrupt streaming client disconnection might leave an orphaned
+        connection slot behind until
         a timeout is reached, so this parameter should be set slightly
         higher than the maximum number of expected clients so disconnected
         clients can immediately reconnect.  This parameter can only
-        be set at server start. <varname>wal_level</varname> must be set to
+        be set at server start.
+        Also, <varname>wal_level</varname> must be set to
         <literal>replica</literal> or higher to allow connections from standby
         servers.
        </para>
@@ -3007,10 +3011,11 @@ include_dir 'conf.d'
          (see <xref linkend="streaming-replication-slots"/>) that the server
          can support. The default is 10.  This parameter can only be set at
          server start.
-         <varname>wal_level</varname> must be set
-         to <literal>replica</literal> or higher to allow replication slots to
-         be used. Setting it to a lower value than the number of currently
+         Setting it to a lower value than the number of currently
          existing replication slots will prevent the server from starting.
+         Also, <varname>wal_level</varname> must be set
+         to <literal>replica</literal> or higher to allow replication slots to
+         be used.
         </para>
        </listitem>
       </varlistentry>

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index f3ddf828bbec6f963d266e503feb8d9e97c82ed0..660f3185e6bc8822de8bf350dfd6689cb087b7e5 100644 (file)
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -202,9 +202,9 @@ char           *ListenAddresses;
 
 /*
  * ReservedBackends is the number of backends reserved for superuser use.
- * This number is taken out of the pool size given by MaxBackends so
+ * This number is taken out of the pool size given by MaxConnections so
  * number of backend slots available to non-superusers is
- * (MaxBackends - ReservedBackends).  Note what this really means is
+ * (MaxConnections - ReservedBackends).  Note what this really means is
  * "if there are <= ReservedBackends connections available, only superusers
  * can make new connections" --- pre-existing superuser connections don't
  * count against the limit.
@@ -882,14 +882,11 @@ PostmasterMain(int argc, char *argv[])
        /*
         * Check for invalid combinations of GUC settings.
         */
-       if (ReservedBackends >= MaxConnections)
+       if (ReservedBackends + max_wal_senders >= MaxConnections)
        {
-               write_stderr("%s: superuser_reserved_connections must be less than max_connections\n", progname);
-               ExitPostmaster(1);
-       }
-       if (max_wal_senders >= MaxConnections)
-       {
-               write_stderr("%s: max_wal_senders must be less than max_connections\n", progname);
+               write_stderr("%s: superuser_reserved_connections (%d) plus max_wal_senders (%d) must be less than max_connections (%d)\n",
+                                        progname,
+                                        ReservedBackends, max_wal_senders, MaxConnections);
                ExitPostmaster(1);
        }
        if (XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level == WAL_LEVEL_MINIMAL)

diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 484628987f44ea0b306afe7d2d03fa6f9891e7fc..d8f45b3c43f51912bea19cc06c726eb92e580b37 100644 (file)
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -778,7 +778,7 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
        }
 
        /*
-        * The last few connections slots are reserved for superusers. Although
+        * The last few connection slots are reserved for superusers.  Although
         * replication connections currently require superuser privileges, we
         * don't allow them to consume the reserved slots, which are intended for
         * interactive use.

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 0c4cc9160d47d0b6555e16af726f47eb8dba747e..a4f9b3668e0d0958541b45bafa2270a816b90640 100644 (file)
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1873,6 +1873,7 @@ static struct config_int ConfigureNamesInt[] =
        },
 
        {
+               /* see max_connections and max_wal_senders */
                {"superuser_reserved_connections", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
                        gettext_noop("Sets the number of connection slots reserved for superusers."),
                        NULL
@@ -2375,7 +2376,7 @@ static struct config_int ConfigureNamesInt[] =
        },
 
        {
-               /* see max_connections */
+               /* see max_connections and superuser_reserved_connections */
                {"max_wal_senders", PGC_POSTMASTER, REPLICATION_SENDING,
                        gettext_noop("Sets the maximum number of simultaneously running WAL sender processes."),
                        NULL
@@ -2386,7 +2387,7 @@ static struct config_int ConfigureNamesInt[] =
        },
 
        {
-               /* see max_connections */
+               /* see max_wal_senders */
                {"max_replication_slots", PGC_POSTMASTER, REPLICATION_SENDING,
                        gettext_noop("Sets the maximum number of simultaneously defined replication slots."),
                        NULL