base: Fix race in PollQueue and remove SIGALRM workaround

There is a race between enabling asynchronous IO for a file descriptor
and IO events happening on that descriptor. A SIGIO won't normally be
delivered if an event is pending when asynchronous IO is
enabled. Instead, the signal will be raised the next time there is an
event on the FD. This changeset simulates a SIGIO by setting the
async_io flag when setting up asynchronous IO for an FD. This causes
the main event loop to poll all file descriptors to check for pending
IO. As a consequence of this, the old SIGALRM hack should no longer be
needed and is therefore removed.

src/base/pollevent.cc

@@ -214,4 +214,14 @@ PollQueue::setupAsyncIO(int fd, bool set)
 
     if (fcntl(fd, F_SETFL, flags) == -1)
         panic("Could not set up async IO");
+
+    // The file descriptor might already have events pending. We won't
+    // see them if they occurred before we set the FASYNC
+    // flag. Simulate a SIGIO to ensure that the FD will be polled in
+    // next iteration of the simulation loop. We could just poll it,
+    // but this is much simpler.
+    if (set) {
+        async_event = true;
+        async_io = true;
+    }
 }