Isolate the "socketpair or a pipe" logic for alerting main thread

This way we can use the linux eventfd extension where available.
Using EVFILT_USER on the BSDs will be a teeny bit trickier, and will
require libevent hacking.

src/common/workqueue.c

@@ -9,11 +9,6 @@
 #include "tor_queue.h"
 #include "torlog.h"
 
-#ifdef HAVE_UNISTD_H
-// XXXX move wherever we move the write/send stuff
-#include <unistd.h>
-#endif
-
 /*
   design:
 
@@ -44,9 +39,7 @@ struct replyqueue_s {
   tor_mutex_t lock;
   TOR_TAILQ_HEAD(, workqueue_entry_s) answers;
 
-  void (*alert_fn)(struct replyqueue_s *); // lock not held on this, next 2. 
-  tor_socket_t write_sock;
-  tor_socket_t read_sock;
+  alert_sockets_t alert; // lock not held on this.
 };
 
 typedef struct workerthread_s {
@@ -169,22 +162,12 @@ queue_reply(replyqueue_t *queue, workqueue_entry_t *work)
   tor_mutex_release(&queue->lock);
 
   if (was_empty) {
-    queue->alert_fn(queue);
+    if (queue->alert.alert_fn(queue->alert.write_fd) < 0) {
+      /* XXXX complain! */
+    }
   }
 }
 
-
-static void
-alert_by_fd(replyqueue_t *queue)
-{
-  /* XXX extract this into new function */
-#ifndef _WIN32
-  (void) send(queue->write_sock, "x", 1, 0);
-#else
-  (void) write(queue->write_sock, "x", 1);
-#endif
-}
-
 static workerthread_t *
 workerthread_new(void *state, replyqueue_t *replyqueue)
 {
@@ -293,59 +276,32 @@ threadpool_get_replyqueue(threadpool_t *tp)
 replyqueue_t *
 replyqueue_new(void)
 {
-  tor_socket_t pair[2];
   replyqueue_t *rq;
-  int r;
-
-  /* XXX extract this into new function */
-#ifdef _WIN32
-  r = tor_socketpair(AF_UNIX, SOCK_STREAM, 0, pair);
-#else
-  r = pipe(pair);
-#endif
-  if (r < 0)
-    return NULL;
-
-  set_socket_nonblocking(pair[0]); /* the read-size should be nonblocking. */
-#if defined(FD_CLOEXEC)
-  fcntl(pair[0], F_SETFD, FD_CLOEXEC);
-  fcntl(pair[1], F_SETFD, FD_CLOEXEC);
-#endif
 
   rq = tor_malloc_zero(sizeof(replyqueue_t));
+  if (alert_sockets_create(&rq->alert) < 0) {
+    tor_free(rq);
+    return NULL;
+  }
 
   tor_mutex_init(&rq->lock);
   TOR_TAILQ_INIT(&rq->answers);
 
-  rq->read_sock = pair[0];
-  rq->write_sock = pair[1];
-  rq->alert_fn = alert_by_fd;
-
   return rq;
 }
 
 tor_socket_t
 replyqueue_get_socket(replyqueue_t *rq)
 {
-  return rq->read_sock;
+  return rq->alert.read_fd;
 }
 
 void
 replyqueue_process(replyqueue_t *queue)
 {
-  ssize_t r;
-
-  /* XXX extract this into new function */
-  do {
-    char buf[64];
-#ifdef _WIN32
-    r = recv(queue->read_sock, buf, sizeof(buf), 0);
-#else
-    r = read(queue->read_sock, buf, sizeof(buf));
-#endif
-  } while (r > 0);
-
-  /* XXXX freak out on r == 0, or r == "error, not retryable". */
+  if (queue->alert.drain_fn(queue->alert.read_fd) < 0) {
+    /* XXXX complain! */
+  }
 
   tor_mutex_acquire(&queue->lock);
   while (!TOR_TAILQ_EMPTY(&queue->answers)) {