Send an email when not able to get last block header

lib/worker.js

@@ -521,6 +521,30 @@ function monitorNodes() {
     });
 }
 
+function monitorNodes() {
+    global.coinFuncs.getLastBlockHeader((err, block) => {
+        if (err !== null) {
+            return global.support.sendEmail(
+                global.config.general.adminEmail,
+                'Failed to query daemon for last block header',
+                `The worker failed to return last block header. Please verify if the daemon is running properly.`
+            );
+        }
+        const sql = 'SELECT blockID, hostname, ip FROM pools WHERE last_checkin > DATE_SUB(NOW(), INTERVAL 30 MINUTE)';
+        global.mysql.query(sql).then(pools => {
+            pools.forEach(({ blockID, hostname, ip }) => {
+                if (blockID < block.height - 3) {
+                    global.support.sendEmail(
+                        global.config.general.adminEmail,
+                        'Pool server is behind in blocks',
+                        `The pool server: ${hostname} with IP: ${ip} is ${(block.height - blockID)} blocks behind.`
+                    );
+                }
+            })
+        });
+    });
+}
+
 updateShareStats();
 updateBlockHeader();
 updatePoolStats();