Add support for host failover with PostgreSQL

Since version 10, PostgreSQL allows to specify serveral hosts in the
connnection string, and can, if the connection option
target_session_atts=read-write is added, automatically choose the primary
(writable) node in a cluster. If the primary changes (due to a failover or
switchover), the client connection will be retried for the next host until a
writable primary is found again.

To leverage this, add the "dbfailover" option to dboptions. If it is set, add
"target_session_attr=read-write" to the connection string. This, in conjunction
with specifying several hosts in $CFG->dbhost config variable makes it possible
to point Moodle at a high-available PostgreSQL cluster and let it follow
failovers automatically.

lib/dml/pgsql_native_moodle_database.php

@@ -172,6 +172,18 @@ public function raw_connect(string $dbhost, string $dbuser, string $dbpass, stri
             $connection .= " connect_timeout=".$this->dboptions['connecttimeout'];
         }
 
+        if (!empty($this->dboptions['dbfailover'])) {
+	    // Failover handling has been requested. However, we only add the
+	    // "target_session_attrs=read-write" option to the connection if
+	    // there is more than one host specified in $CFG->dbhost, because
+	    // otherwise this option has no effect and would then be added to
+	    // possible read-only standbys as well.
+	    $myhosts = explode(',', $this->dbhost);
+            if (count($myhosts) > 1) {
+                $connection .= " target_session_attrs=read-write";
+            }
+        }
+
         if (empty($this->dboptions['dbhandlesoptions'])) {
             // ALTER USER and ALTER DATABASE are overridden by these settings.
             $options = array('--client_encoding=utf8', '--standard_conforming_strings=on');