Read the last commands from a light process

poll can set both POLLIN and POLLHUP. We were checking for
POLLHUP first, which meant we often missed the "exit" command. This
meant the LightProcess would incorrectly report that it was exiting
because it lost its parent, rather than because it was told to exit.

Fixing this so its easier to diagnose what was going on.

hphp/util/light_process.cpp

@@ -479,15 +479,10 @@ void LightProcess::runShadow(int fdin, int fdout) {
     if (ret < 0 && errno == EINTR) {
       continue;
     }
-    if (pfd[0].revents & POLLHUP) {
-      // no more command can come in
-      Logger::Error("Lost parent, LightProcess exiting");
-      break;
-    }
-    else if (pfd[0].revents & POLLIN) {
+    if (pfd[0].revents & POLLIN) {
       if (!fgets(buf, BUFFER_SIZE, fin)) buf[0] = '\0';
       if (strncmp(buf, "exit", 4) == 0) {
-        Logger::Info("LightProces exiting upon request");
+        Logger::Info("LightProcess exiting upon request");
         break;
       } else if (strncmp(buf, "popen", 5) == 0) {
         do_popen(fin, fout, m_afdt_fd);
@@ -499,7 +494,13 @@ void LightProcess::runShadow(int fdin, int fdout) {
         do_waitpid(fin, fout);
       } else if (strncmp(buf, "change_user", 11) == 0) {
         do_change_user(fin, fout);
+      } else if (buf[0]) {
+        Logger::Info("LightProcess got invalid command: %.20s", buf);
       }
+    } else if (pfd[0].revents & POLLHUP) {
+      // no more command can come in
+      Logger::Error("Lost parent, LightProcess exiting");
+      break;
     }
   }