From 0f5ca02f53ac2b211d8518f0882c49284c0c9610 Mon Sep 17 00:00:00 2001 From: Alexander Korotkov Date: Tue, 7 Apr 2020 23:51:10 +0300 Subject: Implement waiting for given lsn at transaction start This commit adds following optional clause to BEGIN and START TRANSACTION commands. WAIT FOR LSN lsn [ TIMEOUT timeout ] New clause pospones transaction start till given lsn is applied on standby. This clause allows user be sure, that changes previously made on primary would be visible on standby. New shared memory struct is used to track awaited lsn per backend. Recovery process wakes up backend once required lsn is applied. Author: Ivan Kartyshov, Anna Akenteva Reviewed-by: Craig Ringer, Thomas Munro, Robert Haas, Kyotaro Horiguchi Reviewed-by: Masahiko Sawada, Ants Aasma, Dmitry Ivanov, Simon Riggs Reviewed-by: Amit Kapila, Alexander Korotkov Discussion: https://postgr.es/m/0240c26c-9f84-30ea-fca9-93ab2df5f305%40postgrespro.ru --- src/backend/commands/wait.c | 295 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 295 insertions(+) create mode 100644 src/backend/commands/wait.c (limited to 'src/backend/commands/wait.c') diff --git a/src/backend/commands/wait.c b/src/backend/commands/wait.c new file mode 100644 index 00000000000..b7aee5b794b --- /dev/null +++ b/src/backend/commands/wait.c @@ -0,0 +1,295 @@ +/*------------------------------------------------------------------------- + * + * wait.c + * Implements WAIT FOR clause for BEGIN and START TRANSACTION commands. + * This clause allows waiting for given LSN to be replayed on standby. + * + * Copyright (c) 2020, PostgreSQL Global Development Group + * + * IDENTIFICATION + * src/backend/commands/wait.c + * + *------------------------------------------------------------------------- + */ +#include "postgres.h" + +#include + +#include "access/xlog.h" +#include "access/xlogdefs.h" +#include "commands/wait.h" +#include "funcapi.h" +#include "miscadmin.h" +#include "pgstat.h" +#include "storage/backendid.h" +#include "storage/pmsignal.h" +#include "storage/proc.h" +#include "storage/shmem.h" +#include "storage/sinvaladt.h" +#include "storage/spin.h" +#include "utils/builtins.h" +#include "utils/pg_lsn.h" +#include "utils/timestamp.h" + +/* + * Shared memory structure representing information about LSNs, which backends + * are waiting for replay. + */ +typedef struct +{ + slock_t mutex; /* mutex protecting the fields below */ + int max_backend_id; /* max backend_id present in lsns[] */ + pg_atomic_uint64 min_lsn; /* minimal waited LSN */ + /* per-backend array of waited LSNs */ + XLogRecPtr lsns[FLEXIBLE_ARRAY_MEMBER]; +} WaitLSNState; + +static WaitLSNState * state; + +/* + * Add the wait event of the current backend to shared memory array + */ +static void +WaitLSNAdd(XLogRecPtr lsn_to_wait) +{ + SpinLockAcquire(&state->mutex); + if (state->max_backend_id < MyBackendId) + state->max_backend_id = MyBackendId; + + state->lsns[MyBackendId] = lsn_to_wait; + + if (lsn_to_wait < state->min_lsn.value) + state->min_lsn.value = lsn_to_wait; + SpinLockRelease(&state->mutex); +} + +/* + * Delete wait event of the current backend from the shared memory array. + */ +void +WaitLSNDelete(void) +{ + int i; + XLogRecPtr deleted_lsn; + + SpinLockAcquire(&state->mutex); + + deleted_lsn = state->lsns[MyBackendId]; + state->lsns[MyBackendId] = InvalidXLogRecPtr; + + /* If we are deleting the minimal LSN, then choose the next min_lsn */ + if (!XLogRecPtrIsInvalid(deleted_lsn) && + deleted_lsn == state->min_lsn.value) + { + state->min_lsn.value = InvalidXLogRecPtr; + for (i = 2; i <= state->max_backend_id; i++) + { + if (!XLogRecPtrIsInvalid(state->lsns[i]) && + (state->lsns[i] < state->min_lsn.value || + XLogRecPtrIsInvalid(state->min_lsn.value))) + { + state->min_lsn.value = state->lsns[i]; + } + } + } + + /* If deleting from the end of the array, shorten the array's used part */ + if (state->max_backend_id == MyBackendId) + { + for (i = (MyBackendId); i >= 2; i--) + if (!XLogRecPtrIsInvalid(state->lsns[i])) + { + state->max_backend_id = i; + break; + } + } + + SpinLockRelease(&state->mutex); +} + +/* + * Report amount of shared memory space needed for WaitLSNState + */ +Size +WaitLSNShmemSize(void) +{ + Size size; + + size = offsetof(WaitLSNState, lsns); + size = add_size(size, mul_size(MaxBackends + 1, sizeof(XLogRecPtr))); + return size; +} + +/* + * Initialize an shared memory structure for waiting for LSN + */ +void +WaitLSNShmemInit(void) +{ + bool found; + uint32 i; + + state = (WaitLSNState *) ShmemInitStruct("pg_wait_lsn", + WaitLSNShmemSize(), + &found); + if (!found) + { + SpinLockInit(&state->mutex); + + for (i = 0; i < (MaxBackends + 1); i++) + state->lsns[i] = InvalidXLogRecPtr; + + state->max_backend_id = 0; + pg_atomic_init_u64(&state->min_lsn, InvalidXLogRecPtr); + } +} + +/* + * Set latches in shared memory to signal that new LSN has been replayed + */ +void +WaitLSNSetLatch(XLogRecPtr cur_lsn) +{ + uint32 i; + int max_backend_id; + PGPROC *backend; + + SpinLockAcquire(&state->mutex); + max_backend_id = state->max_backend_id; + + for (i = 2; i <= max_backend_id; i++) + { + backend = BackendIdGetProc(i); + + if (backend && state->lsns[i] != 0 && + state->lsns[i] <= cur_lsn) + { + SetLatch(&backend->procLatch); + } + } + SpinLockRelease(&state->mutex); +} + +/* + * Get minimal LSN that some backend is waiting for + */ +XLogRecPtr +WaitLSNGetMin(void) +{ + return state->min_lsn.value; +} + +/* + * On WAIT use a latch to wait till LSN is replayed, postmaster dies or timeout + * happens. Timeout is specified in milliseconds. Returns true if LSN was + * reached and false otherwise. + */ +bool +WaitLSNUtility(XLogRecPtr target_lsn, const int timeout_ms) +{ + XLogRecPtr cur_lsn; + int latch_events; + float8 endtime; + bool res = false; + bool wait_forever = (timeout_ms <= 0); + + endtime = GetNowFloat() + timeout_ms / 1000.0; + + latch_events = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH; + + /* Check if we already reached the needed LSN */ + cur_lsn = GetXLogReplayRecPtr(NULL); + if (cur_lsn >= target_lsn) + return true; + + WaitLSNAdd(target_lsn); + ResetLatch(MyLatch); + + /* Recheck if LSN was reached while WaitLSNAdd() and ResetLatch() */ + cur_lsn = GetXLogReplayRecPtr(NULL); + if (cur_lsn >= target_lsn) + return true; + + for (;;) + { + int rc; + float8 time_left = 0; + long time_left_ms = 0; + + time_left = endtime - GetNowFloat(); + + /* Use 1 second as the default timeout to check for interrupts */ + if (wait_forever || time_left < 0 || time_left > 1.0) + time_left_ms = 1000; + else + time_left_ms = (long) ceil(time_left * 1000.0); + + /* If interrupt, LockErrorCleanup() will do WaitLSNDelete() for us */ + CHECK_FOR_INTERRUPTS(); + + /* If postmaster dies, finish immediately */ + if (!PostmasterIsAlive()) + break; + + rc = WaitLatch(MyLatch, latch_events, time_left_ms, + WAIT_EVENT_CLIENT_READ); + + ResetLatch(MyLatch); + + if (rc & WL_LATCH_SET) + cur_lsn = GetXLogReplayRecPtr(NULL); + + if (rc & WL_TIMEOUT) + { + time_left = endtime - GetNowFloat(); + /* If the time specified by user has passed, stop waiting */ + if (!wait_forever && time_left <= 0.0) + break; + cur_lsn = GetXLogReplayRecPtr(NULL); + } + + /* If LSN has been replayed */ + if (target_lsn <= cur_lsn) + break; + } + + WaitLSNDelete(); + + if (cur_lsn < target_lsn) + ereport(WARNING, + (errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION), + errmsg("didn't start transaction because LSN was not reached"), + errhint("Try to increase wait timeout."))); + else + res = true; + + return res; +} + +/* + * Implementation of WAIT FOR clause for BEGIN and START TRANSACTION commands + */ +int +WaitLSNMain(WaitClause *stmt, DestReceiver *dest) +{ + TupleDesc tupdesc; + TupOutputState *tstate; + XLogRecPtr target_lsn; + bool res = false; + + target_lsn = DatumGetLSN(DirectFunctionCall1(pg_lsn_in, + CStringGetDatum(stmt->lsn))); + res = WaitLSNUtility(target_lsn, stmt->timeout); + + /* Need a tuple descriptor representing a single TEXT column */ + tupdesc = CreateTemplateTupleDesc(1); + TupleDescInitEntry(tupdesc, (AttrNumber) 1, "LSN reached", TEXTOID, -1, 0); + + /* Prepare for projection of tuples */ + tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsMinimalTuple); + + /* Send the result */ + do_text_output_oneline(tstate, res ? "t" : "f"); + end_tup_output(tstate); + return res; +} -- cgit v1.2.3