poll-loop: Enable checking whether a FD caused a wakeup.

Each time we run through the poll loop, we check all file descriptors
that we were waiting on to see if there is data available.  However,
this requires a system call and poll already provides information on
which FDs caused the wakeup so it is inefficient as the number of
active FDs grows.  This provides a way to check whether a given FD
has data.

lib/poll-loop.c

@@ -62,6 +62,15 @@ static int timeout = -1;
 /* Location where waiter created. */
 static const char *timeout_where;
 
+/* Array of file descriptors from last run of poll_block(). */
+static struct pollfd *pollfds;
+
+/* Allocated size of pollfds. */
+static size_t max_pollfds;
+
+/* Current number of elements in pollfds. */
+static int n_pollfds;
+
 static struct poll_waiter *new_waiter(int fd, short int events,
                                       const char *where);
 
@@ -210,11 +219,8 @@ log_wakeup(const char *where, const struct pollfd *pollfd, int timeout)
 void
 poll_block(void)
 {
-    static struct pollfd *pollfds;
-    static size_t max_pollfds;
-
     struct poll_waiter *pw, *next;
-    int n_waiters, n_pollfds;
+    int n_waiters;
     int retval;
 
     /* Register fatal signal events before actually doing any real work for
@@ -275,6 +281,25 @@ poll_cancel(struct poll_waiter *pw)
         free(pw);
     }
 }
+
+/* Checks whether the given file descriptor caused the poll loop to wake up
+ * in the previous iteration.  If it did, returns a bitmask of the events
+ * that caused the wakeup.  Otherwise returns 0;
+ */
+short int
+poll_fd_woke(int fd)
+{
+    int i;
+    short int events = 0;
+
+    for (i = 0; i < n_pollfds; i++) {
+        if (pollfds[i].fd == fd) {
+            events |= pollfds[i].revents;
+        }
+    }
+
+    return events;
+}
 
 /* Creates and returns a new poll_waiter for 'fd' and 'events'. */
 static struct poll_waiter *