系统技术非业余研究

今天晚上mingchaoyan同学在线上问以下这个问题：

152489 =ERROR REPORT==== 2013-06-28 19:57:53 ===
152490 Discarding message {send,<<19 bytes>>} from <0.86.1> to <0.6743.0> in an old incarnation (1 ) of this node (2)
152491
152492
152493 =ERROR REPORT==== 2013-06-28 19:57:55 ===
152494 Discarding message {send,<<22 bytes>>} from <0.1623.1> to <0.6743.0> in an old incarnation (1) of this node (2

我们中午服务器更新后，日志上满屏的这些错误，请问您有遇到过类似的错误吗？或者提过些定位问题，解决问题的思路，谢谢

这个问题有点意思，从日志提示来再结合源码来看，马上我们就可以找到打出这个提示的地方：

/*bif.c*/
Sint
do_send(Process *p, Eterm to, Eterm msg, int suspend) {
    Eterm portid;
...
} else if (is_external_pid(to)) {
        dep = external_pid_dist_entry(to);
        if(dep == erts_this_dist_entry) {
            erts_dsprintf_buf_t *dsbufp = erts_create_logger_dsbuf();
            erts_dsprintf(dsbufp,
                          "Discarding message %T from %T to %T in an old "
                          "incarnation (%d) of this node (%d)\n",
                          msg,
                          p->id,
                          to,
                          external_pid_creation(to),
                          erts_this_node->creation);
            erts_send_error_to_logger(p->group_leader, dsbufp);
            return 0;
        }
..
}

触发这句警告提示必须满足以下条件：
1. 目标Pid必须是external_pid。
2. 该pid归宿的外部节点所对应的dist_entry和当前节点的dist_entry相同。

通过google引擎，我找到了和这个描述很相近的问题：参见 这里 ，该作者很好的描述和重现了这个现象，但是他没有解释出具体的原因。

好，那我们顺着他的路子来重新下这个问题.
但演示之前，我们先巩固下基础，首先需要明白pid的格式：
可以参见这篇文章：

pid的核心内容摘抄如下：

Printed process ids < A.B.C > are composed of [6]:
A, the node number (0 is the local node, an arbitrary number for a remote node)
B, the first 15 bits of the process number (an index into the process table) [7]
C, bits 16-18 of the process number (the same process number as B) [7]

再参见Erlang External Term Format 文档的章节9.10
描述了PID_EXT的组成：

1 N 4 4 1
103 Node ID Serial Creation
Table 9.16:
Encode a process identifier object (obtained from spawn/3 or friends). The ID and Creation fields works just like in REFERENCE_EXT, while the Serial field is used to improve safety. In ID, only 15 bits are significant; the rest should be 0.

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