PostgreSQL同步复制主库挂起分析

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在Streaming Replication环境中PostgreSQL主节点设置为同步复制,如standby节点没有启动或者网络出现问题没法连接到主节点时,主节点如执行DML则进程会挂起,下面分析这个挂起的问题.

一、数据结构

Latch
Latch结构体应被视为opaque"不透明的",并且只能通过公共的函数访问.在这里定义是运行把Latchs作为更大的结构体的一部分.

//通常情况下，int类型的变量通常是原子访问的，也可以认为 sig_atomic_t就是int类型的数据，//因为对这些变量要求一条指令完成，所以sig_atomic_t不可能是结构体，只会是数字类型。typedef int __sig_atomic_t;/* * Latch structure should be treated as opaque and only accessed through * the public functions. It is defined here to allow embedding Latches as * part of bigger structs. * Latch结构体应被视为"不透明的"opaque,并且只能通过公共的函数访问. * 在这里定义是运行把Latchs作为更大的结构体的一部分. */typedef struct Latch{    sig_atomic_t is_set;    bool        is_shared;    int         owner_pid;#ifdef WIN32    HANDLE      event;#endif} Latch;

二、源码解读

N/A

二、跟踪分析

启动master节点,不启动standby节点,使用psql连接数据库,执行SQL,Session挂起:

testdb=# drop table t1;

使用gdb跟踪挂起的进程

[xdb@localhost ~]$ ps -ef|grep postgresxdb       1318     1  0 12:14 pts/0    00:00:00 /appdb/xdb/pg11.2/bin/postgresxdb       1319  1318  0 12:14 ?        00:00:00 postgres: logger   xdb       1321  1318  0 12:14 ?        00:00:00 postgres: checkpointer   xdb       1322  1318  0 12:14 ?        00:00:00 postgres: background writer   xdb       1323  1318  0 12:14 ?        00:00:00 postgres: walwriter   xdb       1324  1318  0 12:14 ?        00:00:00 postgres: autovacuum launcher  xdb       1325  1318  0 12:14 ?        00:00:00 postgres: archiver   xdb       1326  1318  0 12:14 ?        00:00:00 postgres: stats collector   xdb       1327  1318  0 12:14 ?        00:00:00 postgres: logical replication launcher  xdb       1331  1318  0 12:15 ?        00:00:00 postgres: xdb testdb [local] DROP TABLE waiting for 0/5D07B668[xdb@localhost ~]$ gdb -p 1331GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-100.el7...

查看调用栈

(gdb) bt#0  0x00007f4636d48903 in __epoll_wait_nocancel () from /lib64/libc.so.6#1  0x000000000088e668 in WaitEventSetWaitBlock (set=0x21640e8, cur_timeout=-1, occurred_events=0x7ffc96572f40, nevents=1)    at latch.c:1048#2  0x000000000088e543 in WaitEventSetWait (set=0x21640e8, timeout=-1, occurred_events=0x7ffc96572f40, nevents=1,     wait_event_info=134217761) at latch.c:1000#3  0x000000000088dcec in WaitLatchOrSocket (latch=0x7f462d5b44d4, wakeEvents=17, sock=-1, timeout=-1,     wait_event_info=134217761) at latch.c:385#4  0x000000000088dbcd in WaitLatch (latch=0x7f462d5b44d4, wakeEvents=17, timeout=-1, wait_event_info=134217761)    at latch.c:339#5  0x0000000000863e2d in SyncRepWaitForLSN (lsn=1560786536, commit=true) at syncrep.c:286#6  0x0000000000546279 in RecordTransactionCommit () at xact.c:1359#7  0x0000000000546da3 in CommitTransaction () at xact.c:2074#8  0x0000000000547a3f in CommitTransactionCommand () at xact.c:2817#9  0x00000000008be250 in finish_xact_command () at postgres.c:2523#10 0x00000000008bbf45 in exec_simple_query (query_string=0x20a1d78 "drop table t1;") at postgres.c:1170#11 0x00000000008c0191 in PostgresMain (argc=1, argv=0x20cdcd8, dbname=0x20cdb40 "testdb", username=0x209ea98 "xdb")    at postgres.c:4182#12 0x000000000081e06c in BackendRun (port=0x20c3b10) at postmaster.c:4361#13 0x000000000081d7df in BackendStartup (port=0x20c3b10) at postmaster.c:4033#14 0x0000000000819bd9 in ServerLoop () at postmaster.c:1706#15 0x000000000081948f in PostmasterMain (argc=1, argv=0x209ca50) at postmaster.c:1379#16 0x0000000000742931 in main (argc=1, argv=0x209ca50) at main.c:228(gdb)

kill进程,重新进入在WaitLatch上设置断点进行跟踪

#########[xdb@localhost ~]$ kill -9 1331#########testdb=# select pg_backend_pid(); pg_backend_pid ----------------           1377(1 row)#########[xdb@localhost ~]$ gdb -p 1377...(gdb) b WaitLatchBreakpoint 1 at 0x88dbac: file latch.c, line 339.(gdb) #########testdb=# drop table t1;ERROR:  table "t1" does not existtestdb=# create table t1(id int);

进入断点

(gdb) b WaitLatchBreakpoint 1 at 0x88dbac: file latch.c, line 339.(gdb) cContinuing.Breakpoint 1, WaitLatch (latch=0x7f462d5b44d4, wakeEvents=17, timeout=-1, wait_event_info=134217761) at latch.c:339339     return WaitLatchOrSocket(latch, wakeEvents, PGINVALID_SOCKET, timeout,(gdb)

进入WaitLatchOrSocket

(gdb) stepWaitLatchOrSocket (latch=0x7f462d5b44d4, wakeEvents=17, sock=-1, timeout=-1, wait_event_info=134217761) at latch.c:359359     int         ret = 0;(gdb) (gdb) p *latch$1 = {is_set = 0, is_shared = true, owner_pid = 1377}

构建等待事件集

(gdb) n362     WaitEventSet *set = CreateWaitEventSet(CurrentMemoryContext, 3);(gdb) n364     if (wakeEvents & WL_TIMEOUT)(gdb) 367         timeout = -1;(gdb) 369     if (wakeEvents & WL_LATCH_SET)(gdb) p *set$2 = {nevents = 0, nevents_space = 3, events = 0x2181eb8, latch = 0x0, latch_pos = 0, epoll_fd = 37,   epoll_ret_events = 0x2181f00}(gdb) p *set->events$3 = {pos = 0, events = 0, fd = 0, user_data = 0x0}(gdb) p *set->epoll_ret_events$4 = {events = 0, data = {ptr = 0x0, fd = 0, u32 = 0, u64 = 0}}(gdb) $5 = {events = 0, data = {ptr = 0x0, fd = 0, u32 = 0, u64 = 0}}(gdb) n370         AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,(gdb) 373     if (wakeEvents & WL_POSTMASTER_DEATH && IsUnderPostmaster)(gdb) 374         AddWaitEventToSet(set, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,(gdb) 377     if (wakeEvents & WL_SOCKET_MASK)(gdb) 385     rc = WaitEventSetWait(set, timeout, &event, 1, wait_event_info);(gdb) p *set$6 = {nevents = 2, nevents_space = 3, events = 0x2181eb8, latch = 0x7f462d5b44d4, latch_pos = 0, epoll_fd = 37,   epoll_ret_events = 0x2181f00}(gdb) p *set->events$7 = {pos = 0, events = 1, fd = 11, user_data = 0x0}(gdb) p *set->epoll_ret_events$8 = {events = 0, data = {ptr = 0x0, fd = 0, u32 = 0, u64 = 0}}(gdb)

进入WaitEventSetWait

(gdb) stepWaitEventSetWait (set=0x2181e90, timeout=-1, occurred_events=0x7ffc96572f40, nevents=1, wait_event_info=134217761)    at latch.c:925925     int         returned_events = 0;(gdb)

输入参数

(gdb) n928     long        cur_timeout = -1;(gdb) p *set$9 = {nevents = 2, nevents_space = 3, events = 0x2181eb8, latch = 0x7f462d5b44d4, latch_pos = 0, epoll_fd = 37,   epoll_ret_events = 0x2181f00}(gdb) p *occurred_events$10 = {pos = 35135068, events = 0, fd = -1772664741, user_data = 0x7ffc96572fa0}(gdb)

执行相关判断和设置参数

(gdb) n930     Assert(nevents > 0);(gdb) 936     if (timeout >= 0)(gdb) 943     pgstat_report_wait_start(wait_event_info);(gdb) 946     waiting = true;(gdb)

未有事件出现,则循环

951     while (returned_events == 0)(gdb)

不符合set->latch->is_set为T的条件,继续循环

982         if (set->latch && set->latch->is_set)(gdb) p *set->latch$11 = {is_set = 0, is_shared = true, owner_pid = 1377}(gdb)

进入WaitEventSetWaitBlock

(gdb) n1000            rc = WaitEventSetWaitBlock(set, cur_timeout,(gdb) stepWaitEventSetWaitBlock (set=0x2181e90, cur_timeout=-1, occurred_events=0x7ffc96572f40, nevents=1) at latch.c:10421042        int         returned_events = 0;(gdb)

调用epoll_wait,挂起

(gdb) n1048        rc = epoll_wait(set->epoll_fd, set->epoll_ret_events,(gdb) p *set$12 = {nevents = 2, nevents_space = 3, events = 0x2181eb8, latch = 0x7f462d5b44d4, latch_pos = 0, epoll_fd = 37,   epoll_ret_events = 0x2181f00}(gdb) (gdb) n

启动standby节点

####[xdb@localhost ~]$ pg_ctl startpg_ctl: another server might be running; trying to start server anyway...

接收到信号

Program received signal SIGUSR1, User defined signal 1.0x00007f4636d48903 in __epoll_wait_nocancel () from /lib64/libc.so.6(gdb) (gdb) nSingle stepping until exit from function __epoll_wait_nocancel,which has no line number information.procsignal_sigusr1_handler (postgres_signal_arg=-1) at procsignal.c:262262 {(gdb)

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