unix: fix EINPROGRESS busy loop

Don't make the event loop spin when connect() returns EINPROGRESS.

Test case:

  #include "uv.h"

  static void connect_cb(uv_connect_t* req, int status) {
    // ...
  }

  int main() {
    uv_tcp_t handle;
    uv_connect_t req;
    struct sockaddr_in addr;
    addr = uv_ip4_addr("8.8.8.8", 1234); // unreachable
    uv_tcp_init(uv_default_loop(), &handle);
    uv_tcp_connect(&req, (uv_stream_t*)&handle, addr, connect_cb);
    uv_run(uv_default_loop()); // busy loops until connection times out
    return 0;
  }

After EINPROGRESS, there are zero active handles and one active request. That
in turn makes uv__poll_timeout() believe that it should merely poll, not block,
in epoll() / kqueue() / port_getn().

Sidestep that by artificially starting the handle on connect() and stopping it
again once the TCP handshake completes / is rejected / times out.

It's a slightly hacky approach because I don't want to change the ABI of the
stable branch. I'll address it properly in the master branch.

src/unix/internal.h

@@ -93,7 +93,8 @@ enum {
   UV_STREAM_WRITABLE  = 0x40,   /* The stream is writable */
   UV_STREAM_BLOCKING  = 0x80,   /* Synchronous writes. */
   UV_TCP_NODELAY      = 0x100,  /* Disable Nagle. */
-  UV_TCP_KEEPALIVE    = 0x200   /* Turn on keep-alive. */
+  UV_TCP_KEEPALIVE    = 0x200,  /* Turn on keep-alive. */
+  UV_TCP_CONNECTING   = 0x400   /* Not alway set. See uv_connect() in tcp.c */
 };
 
 inline static void uv__req_init(uv_loop_t* loop,

src/unix/stream.c

@@ -790,6 +790,12 @@ static void uv__stream_connect(uv_stream_t* stream) {
   stream->connect_req = NULL;
   uv__req_unregister(stream->loop, req);
 
+  /* Hack. See uv__connect() in tcp.c */
+  if (stream->flags & UV_TCP_CONNECTING) {
+    assert(stream->type == UV_TCP);
+    uv__handle_stop(stream);
+  }
+
   if (req->cb) {
     uv__set_sys_error(stream->loop, error);
     req->cb(req, error ? -1 : 0);

src/unix/tcp.c

@@ -109,8 +109,17 @@ static int uv__connect(uv_connect_t* req,
   while (r == -1 && errno == EINTR);
 
   if (r == -1) {
-    if (errno == EINPROGRESS)
+    if (errno == EINPROGRESS) {
-      ; /* not an error */
+      /* Not an error. However, we need to keep the event loop from spinning
+       * while the connection is in progress. Artificially start the handle
+       * and stop it again in uv__stream_connect() in stream.c. Yes, it's a
+       * hack but there's no good alternative, the v0.8 ABI is frozen.
+       */
+      if (!uv__is_active(handle)) {
+        handle->flags |= UV_TCP_CONNECTING;
+        uv__handle_start(handle);
+      }
+    }
     else if (errno == ECONNREFUSED)
     /* If we get a ECONNREFUSED wait until the next tick to report the
      * error. Solaris wants to report immediately--other unixes want to