Disable IOCP and retry event_base_new_with_config once on failure

This is a fancier bug4457 workaround for 0.2.3.  In 0.2.2, we could
just tell Libevent "Don't enable locking!" so it wouldn't try to make
the event_base notifiable.  But for IOCP, we need a notifiable base.
(Eventually, we'll want a notifiable base for other stuff, like
multithreaded crypto.)  So the solution is to try a full-featured
initialization, and then retry with all the options turned off if that
fails.

changes/bug4457_master

@@ -0,0 +1,6 @@
+  o Minor features:
+    - When we fail to initialize Libevent, retry with IOCP disabled so we
+      don't need to turn on multi-threading support in Libevent, which in
+      turn requires a working socketpair().  This is a workaround for bug
+      4457, which affects Libevent versions from 2.0.1-alpha through
+      2.0.15-stable.

src/common/compat_libevent.c

@@ -195,7 +195,14 @@ tor_libevent_initialize(tor_libevent_cfg *torcfg)
 
 #ifdef HAVE_EVENT2_EVENT_H
   {
-    struct event_config *cfg = event_config_new();
+    int attempts = 0;
+    int using_threads;
+    struct event_config *cfg;
+
+  retry:
+    ++attempts;
+    using_threads = 0;
+    cfg = event_config_new();
     tor_assert(cfg);
 
 #if defined(MS_WINDOWS) && defined(USE_BUFFEREVENTS)
@@ -203,9 +210,18 @@ tor_libevent_initialize(tor_libevent_cfg *torcfg)
       evthread_use_windows_threads();
       event_config_set_flag(cfg, EVENT_BASE_FLAG_STARTUP_IOCP);
       using_iocp_bufferevents = 1;
+      using_threads = 1;
+    } else {
+      using_iocp_bufferevents = 0;
     }
 #endif
 
+    if (!using_threads) {
+      /* Telling Libevent not to try to turn locking on can avoid a needless
+       * socketpair() attempt. */
+      event_config_set_flag(cfg, EVENT_BASE_FLAG_NOLOCK);
+    }
+
 #if defined(LIBEVENT_VERSION_NUMBER) && LIBEVENT_VERSION_NUMBER >= V(2,0,7)
     if (torcfg->num_cpus > 0)
       event_config_set_num_cpus_hint(cfg, torcfg->num_cpus);
@@ -220,6 +236,25 @@ tor_libevent_initialize(tor_libevent_cfg *torcfg)
     the_event_base = event_base_new_with_config(cfg);
 
     event_config_free(cfg);
+
+    if (using_threads && the_event_base == NULL && attempts < 2) {
+      /* This could be a socketpair() failure, which can happen sometimes on
+       * windows boxes with obnoxious firewall rules.  Downgrade and try
+       * again. */
+#if defined(MS_WINDOWS) && defined(USE_BUFFEREVENTS)
+      if (torcfg->disable_iocp == 0) {
+        log_warn(LD_GENERAL, "Unable to initialize Libevent. Trying again with "
+                 "IOCP disabled.");
+      } else
+#endif
+      {
+          log_warn(LD_GENERAL, "Unable to initialize Libevent. Trying again.");
+      }
+
+      torcfg->disable_iocp = 1;
+      goto retry;
+    }
+
   }
 #else
   the_event_base = event_init();