Prevent Phabot from spinning out of control

Summary: When the remote closes the connection, phabot goes into a busy loop
because of PHP's "nothing should ever be an error" semantics. Instead, detect
connection termination.
Test Plan: Disabled the "PONG" response in the protocol handler and let freenode
disconnect phabot. It spun out of control before, now it detects the issue and
exits to await automatic restart.
Reviewed By: jungejason
Reviewers: codeblock, jungejason, aran, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 582

src/infrastructure/daemon/irc/bot/PhabricatorIRCBot.php

@@ -130,6 +130,14 @@ final class PhabricatorIRCBot extends PhabricatorDaemon {
       }
 
       if ($read) {
+        // Test for connection termination; in PHP, fread() off a nonblocking,
+        // closed socket is empty string.
+        if (feof($this->socket)) {
+          // This indicates the connection was terminated on the other side,
+          // just exit via exception and let the overseer restart us after a
+          // delay so we can reconnect.
+          throw new Exception("Remote host closed connection.");
+        }
         do {
           $data = fread($this->socket, 4096);
           if ($data === false) {