官方资料参考:
Module:terminate(Reason, State)
Types:
This function is called by a gen_server when it is about to terminate. It should be the opposite of Module:init/1 and do any necessary cleaning up. When it returns, the gen_server terminates with Reason. The return value is ignored.
Reason is a term denoting the stop reason and State is the internal state of the gen_server.
Reason depends on why the gen_server is terminating.
If it is because another callback function has returned a stop tuple {stop,..}, Reason will have the value specified in that tuple.
If it is due to a failure, Reason is the error reason.
If the gen_server is part of a supervision tree and is ordered by its supervisor to terminate, this function will be called with Reason=shutdown if the following conditions apply:
- the gen_server has been set to trap exit signals, and
- the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill.
Even if the gen_server is not part of a supervision tree, this function will be called if it receives an 'EXIT' message from its parent. Reason will be the same as in the'EXIT' message.
Otherwise, the gen_server will be immediately terminated.
实际场景：
gen_server的三个进程,A、B、C，其中A是B、C的父进程，即A进程调用的 B、C的 gen_server:start_link.
A、B、C三个进程全都未捕获 trap_exit，在C进程退出的时候 A、B进程的terminate()/2函数未执行。
原因分析：
参考官方资料可以知，如果一个gen_server进程不是监控树的一部分，gen_sever进程在收到父进程的 'EXIT'信息时将会调用terminate()/2函数。
(注：不是只有收到父进程的 'EXIT'信息时才会调用，别理解歧义)
需要设置 process_flag(trap_exit, true),link的进程死掉时,当前gen_server进程的handle_info()/2将会收到 :{'EXIT',Pid,Reason}
代码修改：
在A、B进程对应的模块中添加 handle_info()/2 对 {'EXIT',Pid,Reason} 的匹配，并 返回 {stop,Reason,NewState}
参考：
http://blog.csdn.net/zcc_0015/article/details/18054889
http://blog.sina.com.cn/s/blog_96b8a1540101314t.html
