• Version: 1.13
• Platform: Darwin zanarpro 16.6.0 Darwin Kernel Version 16.6.0: Fri Apr 14 16:21:16 PDT 2017; root:xnu-3789.60.24~6/RELEASE_X86_64 x86_64 i386 MacBookPro11,3 Darwin

I saw a hint in the document:

Warning libuv will coalesce calls to uv_async_send(), that is, not every call to it will yield an execution of the callback. For example: if uv_async_send() is called 5 times in a row before the callback is called, the callback will only be called once. If uv_async_send() is called again after the callback was called, it will be called again.

How about uv_async_send() two different handles? Will this function swallow one of the callback function?

eg:

uv_async_send(async_1);
uv_async_send(async_2);

One of the problem is Node.js cares_wrap.cc (https://github.com/nodejs/node/blob/v8.3.0/src/cares_wrap.cc#L681):

dns.resolve4(..., function() {});
dns.resolve4(..., function() {});

Refs: nodejs/node#13050