备库读涉及快照的流程
rbreak procarray.c:.
比较简单，直接拿快照就好了。
堆栈：
备库更新procarray流程
主库的事务状态会拼成xlog发送给备库：
- 主库上有三个事务在运行
- 备库收到
info == XLOG_RUNNING_XACTS类型xlog，解析后还原为RunningTransactionsData running - 使用running刷新备库snapshot状态
备库 【1】
代码语言：javascript复制(gdb) bt
#0 RecordKnownAssignedTransactionIds (xid=612) at procarray.c:3199
#1 0x0000000000549019 in StartupXLOG () at xlog.c:7279
#2 0x00000000007e181f in StartupProcessMain () at startup.c:230
#3 0x000000000055d5c9 in AuxiliaryProcessMain (argc=2, argv=0x7ffe04557c20) at bootstrap.c:426
#4 0x00000000007e0710 in StartChildProcess (type=StartupProcess) at postmaster.c:5463
#5 0x00000000007db5bb in PostmasterMain (argc=3, argv=0x1a79c80) at postmaster.c:1377
#6 0x00000000007195f6 in main (argc=3, argv=0x1a79c80) at main.c:228【2】
代码语言：javascript复制#0 KnownAssignedXidsAdd (from_xid=612, to_xid=612, exclusive_lock=0 ' 00') at procarray.c:3478
#1 0x0000000000851a23 in RecordKnownAssignedTransactionIds (xid=612) at procarray.c:3247
#2 0x0000000000549019 in StartupXLOG () at xlog.c:7279
#3 0x00000000007e181f in StartupProcessMain () at startup.c:230
#4 0x000000000055d5c9 in AuxiliaryProcessMain (argc=2, argv=0x7ffe04557c20) at bootstrap.c:426
#5 0x00000000007e0710 in StartChildProcess (type=StartupProcess) at postmaster.c:5463
#6 0x00000000007db5bb in PostmasterMain (argc=3, argv=0x1a79c80) at postmaster.c:1377
#7 0x00000000007195f6 in main (argc=3, argv=0x1a79c80) at main.c:228【3】
代码语言：javascript复制#0 RecordKnownAssignedTransactionIds (xid=612) at procarray.c:3199
#1 0x000000000053d080 in xact_redo_commit (parsed=0x7ffe04557010, xid=612, lsn=4169788496, origin_id=0) at xact.c:5483
#2 0x000000000053d3ed in xact_redo (record=0x1aa3d50) at xact.c:5671
#3 0x0000000000549048 in StartupXLOG () at xlog.c:7282
#4 0x00000000007e181f in StartupProcessMain () at startup.c:230
#5 0x000000000055d5c9 in AuxiliaryProcessMain (argc=2, argv=0x7ffe04557c20) at bootstrap.c:426
#6 0x00000000007e0710 in StartChildProcess (type=StartupProcess) at postmaster.c:5463
#7 0x00000000007db5bb in PostmasterMain (argc=3, argv=0x1a79c80) at postmaster.c:1377
#8 0x00000000007195f6 in main (argc=3, argv=0x1a79c80) at main.c:228主库在哪发快照信息
在BackgroundWriterMain进程中拼RunningTransactions，并写xlog，后面就交给wal sender了。
代码语言：javascript复制(gdb) bt
#0 LogStandbySnapshot () at standby.c:1040
#1 0x00000000007ceffc in BackgroundWriterMain () at bgwriter.c:336
#2 0x000000000055d5ce in AuxiliaryProcessMain (argc=2, argv=0x7ffd48dd6cc0) at bootstrap.c:431
#3 0x00000000007e0710 in StartChildProcess (type=BgWriterProcess) at postmaster.c:5463
#4 0x00000000007ddadd in reaper (postgres_signal_arg=17) at postmaster.c:2936
#5 <signal handler called>
#6 0x00007f66847f4793 in __select_nocancel () from /lib64/libc.so.6
#7 0x00000000007dbe9f in ServerLoop () at postmaster.c:1741
#8 0x00000000007db602 in PostmasterMain (argc=3, argv=0xf8dc70) at postmaster.c:1385
#9 0x00000000007195f6 in main (argc=3, argv=0xf8dc70) at main.c:228
