MDL-59866 session: Add retry for redis sessions

There is an edge case whereby redis will fail
to accept connections on the first try but
retrying the connection seems to make it work

Included in this commit:
* Retry functionality in the session init

lib/classes/session/redis.php

@@ -157,39 +157,61 @@ class redis extends handler {
             throw new exception('redissessionhandlerproblem', 'error');
         }
 
-        try {
-            // One second timeout was chosen as it is long for connection, but short enough for a user to be patient.
-            if (!$this->connection->connect($this->host, $this->port, 1)) {
-                throw new RedisException('Unable to connect to host.');
-            }
+        // MDL-59866: Add retries for connections (up to 5 times) to make sure it goes through.
+        $counter = 1;
+        $maxnumberofretries = 5;
+
+        while ($counter <= $maxnumberofretries) {
+
+            try {
+
+                $delay = rand(100000, 500000);
 
-            if ($this->auth !== '') {
-                if (!$this->connection->auth($this->auth)) {
-                    throw new RedisException('Unable to authenticate.');
+                // One second timeout was chosen as it is long for connection, but short enough for a user to be patient.
+                if (!$this->connection->connect($this->host, $this->port, 1, null, $delay)) {
+                    throw new RedisException('Unable to connect to host.');
                 }
-            }
 
-            if (!$this->connection->setOption(\Redis::OPT_SERIALIZER, $this->serializer)) {
-                throw new RedisException('Unable to set Redis PHP Serializer option.');
-            }
+                if ($this->auth !== '') {
+                    if (!$this->connection->auth($this->auth)) {
+                        throw new RedisException('Unable to authenticate.');
+                    }
+                }
 
-            if ($this->prefix !== '') {
-                // Use custom prefix on sessions.
-                if (!$this->connection->setOption(\Redis::OPT_PREFIX, $this->prefix)) {
-                    throw new RedisException('Unable to set Redis Prefix option.');
+                if (!$this->connection->setOption(\Redis::OPT_SERIALIZER, $this->serializer)) {
+                    throw new RedisException('Unable to set Redis PHP Serializer option.');
                 }
-            }
-            if ($this->database !== 0) {
-                if (!$this->connection->select($this->database)) {
-                    throw new RedisException('Unable to select Redis database '.$this->database.'.');
+
+                if ($this->prefix !== '') {
+                    // Use custom prefix on sessions.
+                    if (!$this->connection->setOption(\Redis::OPT_PREFIX, $this->prefix)) {
+                        throw new RedisException('Unable to set Redis Prefix option.');
+                    }
+                }
+                if ($this->database !== 0) {
+                    if (!$this->connection->select($this->database)) {
+                        throw new RedisException('Unable to select Redis database '.$this->database.'.');
+                    }
                 }
+                $this->connection->ping();
+                return true;
+            } catch (RedisException $e) {
+                $logstring = "Failed to connect (try {$counter} out of {$maxnumberofretries}) to redis ";
+                $logstring .= "at {$this->host}:{$this->port}, error returned was: {$e->getMessage()}";
+
+                // @codingStandardsIgnoreStart
+                error_log($logstring);
+                // @codingStandardsIgnoreEnd
             }
-            $this->connection->ping();
-            return true;
-        } catch (RedisException $e) {
-            error_log('Failed to connect to redis at '.$this->host.':'.$this->port.', error returned was: '.$e->getMessage());
-            return false;
+
+            $counter++;
+
+            // Introduce a random sleep between 100ms and 500ms.
+            usleep(rand(100000, 500000));
         }
+
+        // We have exhausted our retries, time to give up.
+        return false;
     }
 
     /**