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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
This commit is contained in:
Tom Lane 2018-03-08 11:25:26 -05:00
parent f9f8784c8b
commit 4e0c743c18
4 changed files with 24 additions and 21 deletions
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23
doc/src/sgml/config.sgml
View File

@ -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>

15
src/backend/postmaster/postmaster.c
View File

@ -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)

2
src/backend/utils/init/postinit.c
View File

@ -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.

5
src/backend/utils/misc/guc.c
View File

@ -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