上节介绍了PostgreSQL的后台进程walsender中的函数WalSndLoop->WaitLatchOrSocket->WaitEventSetWait->WaitEventSetWaitBlock,在跟踪分析的时候进程退出,现就此问题进行分析.在调用函数ProcessRepliesIfAny后进程退出,因此重点分析ProcessRepliesIfAny函数.

调用栈如下:

(gdb) bt
#0  0x00007fb6e6390903 in __epoll_wait_nocancel () from /lib64/libc.so.6
#1  0x000000000088e668 in WaitEventSetWaitBlock (set=0x10ac808, cur_timeout=29999, occurred_events=0x7ffd634441b0, nevents=1) at latch.c:1048
#2  0x000000000088e543 in WaitEventSetWait (set=0x10ac808, timeout=29999, occurred_events=0x7ffd634441b0, nevents=1, wait_event_info=83886092) at latch.c:1000
#3  0x000000000088dcec in WaitLatchOrSocket (latch=0x7fb6dcbfc4d4, wakeEvents=27, sock=10, timeout=29999, wait_event_info=83886092) at latch.c:385
#4  0x000000000085405b in WalSndLoop (send_data=0x8547fe <XLogSendPhysical>) at walsender.c:2229
#5  0x0000000000851c93 in StartReplication (cmd=0x10ab750) at walsender.c:684
#6  0x00000000008532f0 in exec_replication_command (cmd_string=0x101dd78 "START_REPLICATION 0/5D000000 TIMELINE 16")at walsender.c:1539
#7  0x00000000008c0170 in PostgresMain (argc=1, argv=0x1049cb8, dbname=0x1049ba8 "", username=0x1049b80 "replicator")at postgres.c:4178
#8  0x000000000081e06c in BackendRun (port=0x103fb50) at postmaster.c:4361
#9  0x000000000081d7df in BackendStartup (port=0x103fb50) at postmaster.c:4033
#10 0x0000000000819bd9 in ServerLoop () at postmaster.c:1706
#11 0x000000000081948f in PostmasterMain (argc=1, argv=0x1018a50) at postmaster.c:1379
#12 0x0000000000742931 in main (argc=1, argv=0x1018a50) at main.c:228

一、数据结构

N/A

二、源码解读

ProcessRepliesIfAny
在streaming期间,处理接收到的消息,同时检查远程终端是否关闭了连接,执行相关处理.
代码不多也不复杂,可自行阅读.

/** Process any incoming messages while streaming. Also checks if the remote* end has closed the connection.* 在streaming期间,处理接收到的消息.* 同时检查远程终端是否关闭了连接,执行相关处理.*/
static void
ProcessRepliesIfAny(void)
{unsigned char firstchar;int         r;bool        received = false;//当前时间last_processing = GetCurrentTimestamp();for (;;){//---------- 循环接收相关消息pq_startmsgread();r = pq_getbyte_if_available(&firstchar);if (r < 0){/* unexpected error or EOF *///未知异常或者EOFereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("unexpected EOF on standby connection")));//进程退出proc_exit(0);}if (r == 0){/* no data available without blocking *///已无阻塞的消息数据,退出pq_endmsgread();break;}/* Read the message contents *///读取消息内容resetStringInfo(&reply_message);if (pq_getmessage(&reply_message, 0)){ereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("unexpected EOF on standby connection")));proc_exit(0);}/** If we already received a CopyDone from the frontend, the frontend* should not send us anything until we've closed our end of the COPY.* XXX: In theory, the frontend could already send the next command* before receiving the CopyDone, but libpq doesn't currently allow* that.* 如果已在前台接收到CopyDone消息,前台不应该再发送消息,直至关闭COPY.* XXX:理论上来说,在接收到CopyDone前,前台可能已经发送了下一个命令,但libpq不允许这种情况发生*/if (streamingDoneReceiving && firstchar != 'X')ereport(FATAL,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("unexpected standby message type \"%c\", after receiving CopyDone",firstchar)));/* Handle the very limited subset of commands expected in this phase *///处理有限几个命令switch (firstchar){/** 'd' means a standby reply wrapped in a CopyData packet.* 'd'意味着standby节点的应答封装了CopyData包*/case 'd':ProcessStandbyMessage();received = true;break;/** CopyDone means the standby requested to finish streaming.* Reply with CopyDone, if we had not sent that already.* CopyDone意味着standby节点请求结束streaming.* 如尚未发送,则使用CopyDone应答.*/case 'c':if (!streamingDoneSending){pq_putmessage_noblock('c', NULL, 0);streamingDoneSending = true;}streamingDoneReceiving = true;received = true;break;/** 'X' means that the standby is closing down the socket.* 'X'意味着standby节点正在关闭socket*/case 'X':proc_exit(0);default:ereport(FATAL,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("invalid standby message type \"%c\"",firstchar)));}}/** Save the last reply timestamp if we've received at least one reply.* 如接收到至少一条应答信息,则保存最后的应答时间戳.*/if (received){last_reply_timestamp = last_processing;waiting_for_ping_response = false;}
}

二、跟踪分析

在主节点上用gdb跟踪postmaster,在PostgresMain上设置断点后启动standby节点,进入断点

(gdb) set follow-fork-mode child
(gdb) b ProcessRepliesIfAny
Breakpoint 2 at 0x85343b: file walsender.c, line 1597.
(gdb) c
Continuing.
Breakpoint 2, ProcessRepliesIfAny () at walsender.c:1597
1597        bool        received = false;
(gdb)

查看进程信息

[xdb@localhost ~]$ ps -ef|grep postgres
xdb       1376     1  0 14:16 ?        00:00:00 /appdb/xdb/pg11.2/bin/postgres
xdb       1377  1376  0 14:16 ?        00:00:00 postgres: logger
xdb       1550  1376  0 16:53 ?        00:00:00 postgres: checkpointer
xdb       1551  1376  0 16:53 ?        00:00:00 postgres: background writer
xdb       1552  1376  0 16:53 ?        00:00:00 postgres: walwriter
xdb       1553  1376  0 16:53 ?        00:00:00 postgres: autovacuum launcher
xdb       1554  1376  0 16:53 ?        00:00:00 postgres: archiver
xdb       1555  1376  0 16:53 ?        00:00:00 postgres: stats collector
xdb       1556  1376  0 16:53 ?        00:00:00 postgres: logical replication launcher
xdb       1633  1376  0 17:26 ?        00:00:00 postgres: walsender replicator 192.168.26.26(40528) idle

循环接收相关消息

(gdb) n
1599        last_processing = GetCurrentTimestamp();
(gdb)
1603            pq_startmsgread();
(gdb)
1604            r = pq_getbyte_if_available(&firstchar);
(gdb)
1605            if (r < 0)
(gdb) p r
$1 = 1
(gdb) p firstchar
$2 = 100 'd'
(gdb)

命令是’d’,执行相关处理

(gdb) n
1613            if (r == 0)
(gdb)
1621            resetStringInfo(&reply_message);
(gdb)
1622            if (pq_getmessage(&reply_message, 0))
(gdb)
1637            if (streamingDoneReceiving && firstchar != 'X')
(gdb)
1644            switch (firstchar)
(gdb)
1650                    ProcessStandbyMessage();
(gdb)
1651                    received = true;
(gdb)
1652                    break;
(gdb)
1681        }
(gdb)

设置断点

(gdb) b walsender.c:1643
Breakpoint 3 at 0x8535b6: file walsender.c, line 1643.
(gdb) b walsender.c:1672
Breakpoint 4 at 0x85361a: file walsender.c, line 1672.
(gdb) c
Continuing.
Breakpoint 3, ProcessRepliesIfAny () at walsender.c:1644
1644            switch (firstchar)
(gdb)
Continuing.
...
Breakpoint 4, ProcessRepliesIfAny () at walsender.c:1673
1673                    proc_exit(0);
(gdb)

进程即将退出,查看进程信息

[xdb@localhost ~]$ ps -ef|grep postgres
xdb       1376     1  0 14:16 ?        00:00:00 /appdb/xdb/pg11.2/bin/postgres
xdb       1377  1376  0 14:16 ?        00:00:00 postgres: logger
xdb       1550  1376  0 16:53 ?        00:00:00 postgres: checkpointer
xdb       1551  1376  0 16:53 ?        00:00:00 postgres: background writer
xdb       1552  1376  0 16:53 ?        00:00:00 postgres: walwriter
xdb       1553  1376  0 16:53 ?        00:00:00 postgres: autovacuum launcher
xdb       1554  1376  0 16:53 ?        00:00:00 postgres: archiver
xdb       1555  1376  0 16:53 ?        00:00:00 postgres: stats collector
xdb       1556  1376  0 16:53 ?        00:00:00 postgres: logical replication launcher
xdb       1633  1376  0 17:26 ?        00:00:00 postgres: walsender replicator 192.168.26.26(40528) idle
xdb       1637  1376  0 17:27 ?        00:00:00 postgres: walsender replicator 192.168.26.26(40530) streaming 0/5D075248
[xdb@localhost ~]$

进程退出(PID=1633),启动了新的进程(PID=1637)

(gdb) n
[Inferior 2 (process 1633) exited normally]
(gdb)

DONE!

四、参考资料

PG Source Code