MDL-78212 dml: Added MYSQLI_CLIENT_COMPRESS option for mysqli

Config dboptions key 'clientcompress'.
Decreases traffic between the application server and the database host.

config-dist.php

@@ -91,6 +91,10 @@
                                 // set to zero if you are using pg_bouncer in
                                 // 'transaction' mode (it is fine in 'session'
                                 // mode).
+    // 'clientcompress' => true // Use compression protocol to communicate with the database server.
+                                // Decreases traffic from the database server.
+                                // Not needed if the databse is on the same host.
+                                // Currently supported only with mysqli driver.
     /*
     'connecttimeout' => null, // Set connect timeout in seconds. Not all drivers support it.
     'readonly' => [          // Set to read-only slave details, to get safe reads

lib/dml/mysqli_native_moodle_database.php

@@ -573,11 +573,16 @@ public function raw_connect(string $dbhost, string $dbuser, string $dbpass, stri
             $this->mysqli->options(MYSQLI_OPT_CONNECT_TIMEOUT, $this->dboptions['connecttimeout']);
         }
 
+        $flags = 0;
+        if ($this->dboptions['clientcompress'] ?? false) {
+            $flags |= MYSQLI_CLIENT_COMPRESS;
+        }
+
         $conn = null;
         $dberr = null;
         try {
             // real_connect() is doing things we don't expext.
-            $conn = @$this->mysqli->real_connect($dbhost, $dbuser, $dbpass, $dbname, $dbport, $dbsocket);
+            $conn = @$this->mysqli->real_connect($dbhost, $dbuser, $dbpass, $dbname, $dbport, $dbsocket, $flags);
         } catch (\Exception $e) {
             $dberr = "$e";
         }

lib/dml/tests/mysqli_native_moodle_database_test.php

@@ -0,0 +1,109 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace core;
+
+use ReflectionClass;
+use mysqli;
+use moodle_database, mysqli_native_moodle_database;
+use moodle_exception;
+
+/**
+ * Test specific features of the MySql dml.
+ *
+ * @package core
+ * @category test
+ * @copyright 2023 Catalyst IT
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers  \mysqli_native_moodle_database
+ */
+class mysqli_native_moodle_database_test extends \advanced_testcase {
+
+    /**
+     * Set up.
+     */
+    public function setUp(): void {
+        global $DB;
+        parent::setUp();
+        // Skip tests if not using Postgres.
+        if (!($DB instanceof mysqli_native_moodle_database)) {
+            $this->markTestSkipped('MySql-only test');
+        }
+    }
+
+    /**
+     * SSL connection helper.
+     *
+     * @param bool|null $compress
+     * @return mysqli
+     * @throws moodle_exception
+     */
+    public function new_connection(?bool $compress = false): mysqli {
+        global $DB;
+
+        // Open new connection.
+        $cfg = $DB->export_dbconfig();
+        if (!isset($cfg->dboptions)) {
+            $cfg->dboptions = [];
+        }
+
+        $cfg->dboptions['clientcompress'] = $compress;
+
+        // Get a separate disposable db connection handle with guaranteed 'readonly' config.
+        $db2 = moodle_database::get_driver_instance($cfg->dbtype, $cfg->dblibrary);
+        $db2->raw_connect($cfg->dbhost, $cfg->dbuser, $cfg->dbpass, $cfg->dbname, $cfg->prefix, $cfg->dboptions);
+
+        $reflector = new ReflectionClass($db2);
+        $rp = $reflector->getProperty('mysqli');
+        $rp->setAccessible(true);
+        return $rp->getValue($db2);
+    }
+
+    /**
+     * Test client compression helper.
+     *
+     * @param mysqli $mysqli
+     * @return array
+     */
+    public function connection_status($mysqli): array {
+        $mysqli->query("SELECT * FROM INFORMATION_SCHEMA.TABLES");
+
+        $stats = [];
+        foreach ($mysqli->query('SHOW SESSION STATUS')->fetch_all(MYSQLI_ASSOC) as $r) {
+            $stats[$r['Variable_name']] = $r['Value'];
+        }
+        return $stats;
+    }
+
+    /**
+     * Test client compression.
+     *
+     * @return void
+     */
+    public function test_client_compression(): void {
+        $mysqli = $this->new_connection();
+        $stats = $this->connection_status($mysqli);
+        $this->assertEquals('OFF', $stats['Compression']);
+        $sent = $stats['Bytes_sent'];
+
+        $mysqlic = $this->new_connection(true);
+        $stats = $this->connection_status($mysqlic);
+        $this->assertEquals('ON', $stats['Compression']);
+        $sentc = $stats['Bytes_sent'];
+
+        $this->assertLessThan($sent, $sentc);
+    }
+}