PostgreSQL中函數(shù)AssignTransactionId的實(shí)現(xiàn)邏輯是什么
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一、數(shù)據(jù)結(jié)構(gòu)
靜態(tài)變量
當(dāng)前事務(wù)狀態(tài)CurrentTransactionState
/*
* CurrentTransactionState always points to the current transaction state
* block. It will point to TopTransactionStateData when not in a
* transaction at all, or when in a top-level transaction.
* CurrentTransactionState通常指向當(dāng)前事務(wù)塊.
* 如不處于事務(wù)中或者處于頂層事務(wù)中,則指向TopTransactionStateData
*/
static TransactionStateData TopTransactionStateData = {
.state = TRANS_DEFAULT,
.blockState = TBLOCK_DEFAULT,
};
/*
* unreportedXids holds XIDs of all subtransactions that have not yet been
* reported in an XLOG_XACT_ASSIGNMENT record.
* unreportedXids保存所有尚未在XLOG_XACT_ASSIGNMENT記錄的子事務(wù).
*/
static int nUnreportedXids;
static TransactionId unreportedXids[PGPROC_MAX_CACHED_SUBXIDS];
static TransactionState CurrentTransactionState = &TopTransactionStateData;
/*
* The subtransaction ID and command ID assignment counters are global
* to a whole transaction, so we do not keep them in the state stack.
* subtransaction ID和command ID全局計(jì)數(shù)器,對(duì)事務(wù)可見,在state棧中不記錄這些信息.
*/
static SubTransactionId currentSubTransactionId;
static CommandId currentCommandId;
static bool currentCommandIdUsed;TransactionState
事務(wù)狀態(tài)結(jié)構(gòu)體
/* * transaction states - transaction state from server perspective * 事務(wù)狀態(tài)枚舉 - 服務(wù)器視角的事務(wù)狀態(tài) */ typedef enum TransState { TRANS_DEFAULT, /* idle 空閑 */ TRANS_START, /* transaction starting 事務(wù)啟動(dòng) */ TRANS_INPROGRESS, /* inside a valid transaction 進(jìn)行中 */ TRANS_COMMIT, /* commit in progress 提交中 */ TRANS_ABORT, /* abort in progress 回滾中 */ TRANS_PREPARE /* prepare in progress 準(zhǔn)備中 */ } TransState; /* * transaction block states - transaction state of client queries * 事務(wù)塊狀態(tài) - 客戶端查詢的事務(wù)狀態(tài) * * Note: the subtransaction states are used only for non-topmost * transactions; the others appear only in the topmost transaction. * 注意:subtransaction只用于非頂層事務(wù);其他字段用于頂層事務(wù). */ typedef enum TBlockState { /* not-in-transaction-block states 未進(jìn)入事務(wù)塊狀態(tài) */ TBLOCK_DEFAULT, /* idle 空閑 */ TBLOCK_STARTED, /* running single-query transaction 單個(gè)查詢事務(wù) */ /* transaction block states 事務(wù)塊狀態(tài) */ TBLOCK_BEGIN, /* starting transaction block 開始事務(wù)塊 */ TBLOCK_INPROGRESS, /* live transaction 進(jìn)行中 */ TBLOCK_IMPLICIT_INPROGRESS, /* live transaction after implicit BEGIN 隱式事務(wù),進(jìn)行中 */ TBLOCK_PARALLEL_INPROGRESS, /* live transaction inside parallel worker 并行worker中的事務(wù),進(jìn)行中 */ TBLOCK_END, /* COMMIT received 接收到COMMIT */ TBLOCK_ABORT, /* failed xact, awaiting ROLLBACK 失敗,等待ROLLBACK */ TBLOCK_ABORT_END, /* failed xact, ROLLBACK received 失敗,已接收ROLLBACK */ TBLOCK_ABORT_PENDING, /* live xact, ROLLBACK received 進(jìn)行中,接收到ROLLBACK */ TBLOCK_PREPARE, /* live xact, PREPARE received 進(jìn)行中,接收到PREPARE */ /* subtransaction states 子事務(wù)狀態(tài) */ TBLOCK_SUBBEGIN, /* starting a subtransaction 開啟 */ TBLOCK_SUBINPROGRESS, /* live subtransaction 進(jìn)行中 */ TBLOCK_SUBRELEASE, /* RELEASE received 接收到RELEASE */ TBLOCK_SUBCOMMIT, /* COMMIT received while TBLOCK_SUBINPROGRESS 進(jìn)行中,接收到COMMIT */ TBLOCK_SUBABORT, /* failed subxact, awaiting ROLLBACK 失敗,等待ROLLBACK */ TBLOCK_SUBABORT_END, /* failed subxact, ROLLBACK received 失敗,已接收ROLLBACK */ TBLOCK_SUBABORT_PENDING, /* live subxact, ROLLBACK received 進(jìn)行中,接收到ROLLBACK */ TBLOCK_SUBRESTART, /* live subxact, ROLLBACK TO received 進(jìn)行中,接收到ROLLBACK TO */ TBLOCK_SUBABORT_RESTART /* failed subxact, ROLLBACK TO received 失敗,已接收ROLLBACK TO */ } TBlockState; /* * transaction state structure * 事務(wù)狀態(tài)結(jié)構(gòu)體 */ typedef struct TransactionStateData { //事務(wù)ID TransactionId transactionId; /* my XID, or Invalid if none */ //子事務(wù)ID SubTransactionId subTransactionId; /* my subxact ID */ //保存點(diǎn)名稱 char *name; /* savepoint name, if any */ //保存點(diǎn)級(jí)別 int savepointLevel; /* savepoint level */ //低級(jí)別的事務(wù)狀態(tài) TransState state; /* low-level state */ //高級(jí)別的事務(wù)狀態(tài) TBlockState blockState; /* high-level state */ //事務(wù)嵌套深度 int nestingLevel; /* transaction nesting depth */ //GUC上下文嵌套深度 int gucNestLevel; /* GUC context nesting depth */ //事務(wù)生命周期上下文 MemoryContext curTransactionContext; /* my xact-lifetime context */ //查詢資源 ResourceOwner curTransactionOwner; /* my query resources */ //按XID順序保存的已提交的子事務(wù)ID TransactionId *childXids; /* subcommitted child XIDs, in XID order */ //childXids數(shù)組大小 int nChildXids; /* # of subcommitted child XIDs */ //分配的childXids數(shù)組空間 int maxChildXids; /* allocated size of childXids[] */ //上一個(gè)CurrentUserId Oid prevUser; /* previous CurrentUserId setting */ //上一個(gè)SecurityRestrictionContext int prevSecContext; /* previous SecurityRestrictionContext */ //上一事務(wù)是否只讀? bool prevXactReadOnly; /* entry-time xact r/o state */ //是否處于Recovery? bool startedInRecovery; /* did we start in recovery? */ //XID是否已保存在WAL Record中? bool didLogXid; /* has xid been included in WAL record? */ //Enter/ExitParallelMode計(jì)數(shù)器 int parallelModeLevel; /* Enter/ExitParallelMode counter */ //父事務(wù)狀態(tài) struct TransactionStateData *parent; /* back link to parent */ } TransactionStateData; //結(jié)構(gòu)體指針 typedef TransactionStateData *TransactionState;
二、源碼解讀
AssignTransactionId函數(shù),給定的TransactionState分配一個(gè)新的持久化事務(wù)號(hào)XID,在此函數(shù)調(diào)用前,不會(huì)為事務(wù)分配XIDs.
/*
* AssignTransactionId
*
* Assigns a new permanent XID to the given TransactionState.
* We do not assign XIDs to transactions until/unless this is called.
* Also, any parent TransactionStates that don't yet have XIDs are assigned
* one; this maintains the invariant that a child transaction has an XID
* following its parent's.
* 為給定的TransactionState分配一個(gè)新的持久化事務(wù)號(hào)XID.
* 在此函數(shù)調(diào)用前,我們不會(huì)為事務(wù)分配XIDs.
* 同時(shí),所有尚未獲得XIDs的父TransactionStates也會(huì)分配事務(wù)號(hào),
* 這可以確保子事務(wù)的事務(wù)號(hào)在父事務(wù)之后.
*/
static void
AssignTransactionId(TransactionState s)
{
bool isSubXact = (s->parent != NULL);
ResourceOwner currentOwner;
bool log_unknown_top = false;
/* Assert that caller didn't screw up */
//確保調(diào)用者沒有搞砸
Assert(!TransactionIdIsValid(s->transactionId));
Assert(s->state == TRANS_INPROGRESS);
/*
* Workers synchronize transaction state at the beginning of each parallel
* operation, so we can't account for new XIDs at this point.
* 在每個(gè)并行操作前,Parallel Workers同步事務(wù)狀態(tài),
* 因此我們不能在這時(shí)候請求XIDs
*/
if (IsInParallelMode() || IsParallelWorker())
elog(ERROR, "cannot assign XIDs during a parallel operation");
/*
* Ensure parent(s) have XIDs, so that a child always has an XID later
* than its parent. Mustn't recurse here, or we might get a stack overflow
* if we're at the bottom of a huge stack of subtransactions none of which
* have XIDs yet.
* 確保父事務(wù)已分配XIDs,這樣可以確保子事務(wù)的XID后于父事務(wù).
* 不能在這里遞歸執(zhí)行,否則如果我們在一個(gè)未分配XID子事務(wù)大棧的底部,可能會(huì)遇到棧溢出
*/
if (isSubXact && !TransactionIdIsValid(s->parent->transactionId))
{
TransactionState p = s->parent;
TransactionState *parents;
size_t parentOffset = 0;
parents = palloc(sizeof(TransactionState) * s->nestingLevel);
while (p != NULL && !TransactionIdIsValid(p->transactionId))
{
parents[parentOffset++] = p;
p = p->parent;
}
/*
* This is technically a recursive call, but the recursion will never
* be more than one layer deep.
* 遞歸調(diào)用,但遞歸不會(huì)超過一層
*/
while (parentOffset != 0)
AssignTransactionId(parents[--parentOffset]);
pfree(parents);
}
/*
* When wal_level=logical, guarantee that a subtransaction's xid can only
* be seen in the WAL stream if its toplevel xid has been logged before.
* If necessary we log an xact_assignment record with fewer than
* PGPROC_MAX_CACHED_SUBXIDS. Note that it is fine if didLogXid isn't set
* for a transaction even though it appears in a WAL record, we just might
* superfluously log something. That can happen when an xid is included
* somewhere inside a wal record, but not in XLogRecord->xl_xid, like in
* xl_standby_locks.
* 如wal_level=logical,確保子事務(wù)XID在頂層XID已被"日志"的情況只可以被WAL stream看到.
* 如果有必要，我們將使用小于PGPROC_MAX_CACHED_SUBXIDS的值來記錄xact_assignment記錄.
* 請注意，即使didLogXid出現(xiàn)在WAL記錄中，如果它沒有為事務(wù)設(shè)置，也沒有問題，
* 我們可能只是多余地記錄了一些東西。
* 當(dāng)一個(gè)xid出現(xiàn)在WAL Record中的某個(gè)地方，但不在XLogRecord->xl_xid中
* (比如在xl_standby_locks中)時(shí)，就會(huì)發(fā)生這種情況。
*/
if (isSubXact && XLogLogicalInfoActive() &&
!TopTransactionStateData.didLogXid)
log_unknown_top = true;
/*
* Generate a new Xid and record it in PG_PROC and pg_subtrans.
* 生成一個(gè)新的XID,并記錄在PG_PROC和pg_subtrans中
*
* NB: we must make the subtrans entry BEFORE the Xid appears anywhere in
* shared storage other than PG_PROC; because if there's no room for it in
* PG_PROC, the subtrans entry is needed to ensure that other backends see
* the Xid as "running". See GetNewTransactionId.
* 注意:我們必須在Xid出現(xiàn)在除PG_PROC之外的共享存儲(chǔ)之前構(gòu)造subtrans條目.
* 因?yàn)槿绻赑G_PROC沒有空閑空間,子事務(wù)條目需要確保其他進(jìn)程看到該XID正在運(yùn)行.
* 參考函數(shù)GetNewTransactionId說明.
*
*/
s->transactionId = GetNewTransactionId(isSubXact);
if (!isSubXact)
XactTopTransactionId = s->transactionId;
if (isSubXact)
SubTransSetParent(s->transactionId, s->parent->transactionId);
/*
* If it's a top-level transaction, the predicate locking system needs to
* be told about it too.
* 如為頂層事務(wù),謂詞鎖系統(tǒng)也需要了解此事務(wù).
*/
if (!isSubXact)
RegisterPredicateLockingXid(s->transactionId);
/*
* Acquire lock on the transaction XID. (We assume this cannot block.) We
* have to ensure that the lock is assigned to the transaction's own
* ResourceOwner.
* 請求鎖(我們假定這樣做不好引起阻塞).我們必須確保鎖已被分配給事務(wù)自己的ResourceOwner.
*/
currentOwner = CurrentResourceOwner;
CurrentResourceOwner = s->curTransactionOwner;
XactLockTableInsert(s->transactionId);
CurrentResourceOwner = currentOwner;
/*
* Every PGPROC_MAX_CACHED_SUBXIDS assigned transaction ids within each
* top-level transaction we issue a WAL record for the assignment. We
* include the top-level xid and all the subxids that have not yet been
* reported using XLOG_XACT_ASSIGNMENT records.
* 在每個(gè)頂級(jí)事務(wù)中分配的每個(gè)PGPROC_MAX_CACHED_SUBXIDS事務(wù)id，
* 我們都會(huì)為分配記錄一條WAL記錄。
* 該記錄包括頂級(jí)的xid和所有尚未使用XLOG_XACT_ASSIGNMENT記錄報(bào)告的子xid。
*
* This is required to limit the amount of shared memory required in a hot
* standby server to keep track of in-progress XIDs. See notes for
* RecordKnownAssignedTransactionIds().
* 在跟蹤進(jìn)行中的XIDs的備機(jī)上,需要控制共享內(nèi)存的大小.
* 參見RecordKnownAssignedTransactionIds()函數(shù)說明.
*
* We don't keep track of the immediate parent of each subxid, only the
* top-level transaction that each subxact belongs to. This is correct in
* recovery only because aborted subtransactions are separately WAL
* logged.
* 我們不需要跟蹤父事務(wù)的每個(gè)子事務(wù),只需要跟蹤子事務(wù)歸屬的頂層事務(wù)即可.
* 這樣可行是因?yàn)樵诨謴?fù)中,已回滾的子事務(wù)是通過WAL單獨(dú)記錄的.
*
* This is correct even for the case where several levels above us didn't
* have an xid assigned as we recursed up to them beforehand.
* 即使在我們之前遞歸到上面的幾個(gè)級(jí)別時(shí)沒有分配xid的情況下，這也是正確的。
*/
if (isSubXact && XLogStandbyInfoActive())
{
unreportedXids[nUnreportedXids] = s->transactionId;
nUnreportedXids++;
/*
* ensure this test matches similar one in
* RecoverPreparedTransactions()
* 確保在RecoverPreparedTransactions()中可以匹配到相似的.
*/
if (nUnreportedXids >= PGPROC_MAX_CACHED_SUBXIDS ||
log_unknown_top)
{
xl_xact_assignment xlrec;
/*
* xtop is always set by now because we recurse up transaction
* stack to the highest unassigned xid and then come back down
* xtop現(xiàn)在已經(jīng)設(shè)置好了，因?yàn)槲覀儗⑹聞?wù)堆棧遞歸到最高的未分配的xid，然后再返回
*/
xlrec.xtop = GetTopTransactionId();
Assert(TransactionIdIsValid(xlrec.xtop));
xlrec.nsubxacts = nUnreportedXids;
XLogBeginInsert();
XLogRegisterData((char *) &xlrec, MinSizeOfXactAssignment);
XLogRegisterData((char *) unreportedXids,
nUnreportedXids * sizeof(TransactionId));
(void) XLogInsert(RM_XACT_ID, XLOG_XACT_ASSIGNMENT);
nUnreportedXids = 0;
/* mark top, not current xact as having been logged */
//標(biāo)記為最頂層,而不是當(dāng)前已記錄日志的xact
TopTransactionStateData.didLogXid = true;
}
}
}三、跟蹤分析
執(zhí)行txid_current,觸發(fā)函數(shù)調(diào)用
11:10:36 (xdb@[local]:5432)testdb=# begin; BEGIN 11:40:20 (xdb@[local]:5432)testdb=#* select txid_current_if_assigned(); txid_current_if_assigned -------------------------- (1 row) 11:40:43 (xdb@[local]:5432)testdb=#* select txid_current();
啟動(dòng)gdb,設(shè)置斷點(diǎn)
(gdb) b AssignTransactionId Breakpoint 5 at 0x546a4c: file xact.c, line 491. (gdb) c Continuing. Breakpoint 5, AssignTransactionId (s=0xf9c720 <TopTransactionStateData>) at xact.c:491 491 bool isSubXact = (s->parent != NULL); (gdb)
查看調(diào)用棧
(gdb) bt #0 AssignTransactionId (s=0xf9c720 <TopTransactionStateData>) at xact.c:491 #1 0x000000000054693d in GetTopTransactionId () at xact.c:392 #2 0x00000000009fe1f3 in txid_current (fcinfo=0x25835a0) at txid.c:443 #3 0x00000000006cfebd in ExecInterpExpr (state=0x25834b8, econtext=0x25831a8, isnull=0x7ffe3d4a31f7) at execExprInterp.c:654 #4 0x00000000006d1ac6 in ExecInterpExprStillValid (state=0x25834b8, econtext=0x25831a8, isNull=0x7ffe3d4a31f7) at execExprInterp.c:1786 #5 0x00000000007140dd in ExecEvalExprSwitchContext (state=0x25834b8, econtext=0x25831a8, isNull=0x7ffe3d4a31f7) at ../../../src/include/executor/executor.h:303 #6 0x000000000071414b in ExecProject (projInfo=0x25834b0) at ../../../src/include/executor/executor.h:337 #7 0x0000000000714323 in ExecResult (pstate=0x2583090) at nodeResult.c:136 #8 0x00000000006e4c30 in ExecProcNodeFirst (node=0x2583090) at execProcnode.c:445 #9 0x00000000006d9974 in ExecProcNode (node=0x2583090) at ../../../src/include/executor/executor.h:237 #10 0x00000000006dc22d in ExecutePlan (estate=0x2582e78, planstate=0x2583090, use_parallel_mode=false, operation=CMD_SELECT, sendTuples=true, numberTuples=0, direction=ForwardScanDirection, dest=0x24cd0a0, execute_once=true) at execMain.c:1723 #11 0x00000000006d9f5c in standard_ExecutorRun (queryDesc=0x256b0c8, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:364 #12 0x00000000006d9d7f in ExecutorRun (queryDesc=0x256b0c8, direction=ForwardScanDirection, count=0, execute_once=true) at execMain.c:307 #13 0x00000000008ccf5a in PortalRunSelect (portal=0x250c748, forward=true, count=0, dest=0x24cd0a0) at pquery.c:932 #14 0x00000000008ccbf3 in PortalRun (portal=0x250c748, count=9223372036854775807, isTopLevel=true, run_once=true, dest=0x24cd0a0, altdest=0x24cd0a0, completionTag=0x7ffe3d4a3570 "") at pquery.c:773 #15 0x00000000008c6b1e in exec_simple_query (query_string=0x24a6ec8 "select txid_current();") at postgres.c:1145 #16 0x00000000008cae70 in PostgresMain (argc=1, argv=0x24d2dc8, dbname=0x24d2c30 "testdb", username=0x24a3ba8 "xdb") at postgres.c:4182 #17 0x000000000082642b in BackendRun (port=0x24c8c00) at postmaster.c:4361 ---Type <return> to continue, or q <return> to quit--- #18 0x0000000000825b8f in BackendStartup (port=0x24c8c00) at postmaster.c:4033 #19 0x0000000000821f1c in ServerLoop () at postmaster.c:1706 #20 0x00000000008217b4 in PostmasterMain (argc=1, argv=0x24a1b60) at postmaster.c:1379 #21 0x00000000007488ef in main (argc=1, argv=0x24a1b60) at main.c:228
輸入?yún)?shù)TransactionState(全局變量,指向TopTransactionStateData)
(gdb) p s
$13 = (TransactionState) 0xf9c720 <TopTransactionStateData>
(gdb) p *s
$14 = {transactionId = 0, subTransactionId = 1, name = 0x0, savepointLevel = 0, state = TRANS_INPROGRESS,
blockState = TBLOCK_INPROGRESS, nestingLevel = 1, gucNestLevel = 1, curTransactionContext = 0x2523850,
curTransactionOwner = 0x24d4868, childXids = 0x0, nChildXids = 0, maxChildXids = 0, prevUser = 10, prevSecContext = 0,
prevXactReadOnly = false, startedInRecovery = false, didLogXid = false, parallelModeLevel = 0, parent = 0x0}
(gdb)初始化部分變量并驗(yàn)證
(gdb) n 493 bool log_unknown_top = false; (gdb) 496 Assert(!TransactionIdIsValid(s->transactionId)); (gdb) 497 Assert(s->state == TRANS_INPROGRESS); (gdb)
獲取事務(wù)號(hào)
(gdb) 503 if (IsInParallelMode() || IsParallelWorker()) (gdb) 512 if (isSubXact && !TransactionIdIsValid(s->parent->transactionId)) (gdb) 545 if (isSubXact && XLogLogicalInfoActive() && (gdb) 557 s->transactionId = GetNewTransactionId(isSubXact); (gdb) 558 if (!isSubXact) (gdb) p s->transactionId $15 = 2407 (gdb)
注冊,并設(shè)置其他信息
(gdb) n 559 XactTopTransactionId = s->transactionId; (gdb) 561 if (isSubXact) (gdb) 568 if (!isSubXact) (gdb) 569 RegisterPredicateLockingXid(s->transactionId); (gdb) 576 currentOwner = CurrentResourceOwner; (gdb) 577 CurrentResourceOwner = s->curTransactionOwner; (gdb) 579 XactLockTableInsert(s->transactionId); (gdb) 581 CurrentResourceOwner = currentOwner; (gdb) 601 if (isSubXact && XLogStandbyInfoActive()) (gdb) 635 }
完成調(diào)用
(gdb) GetTopTransactionId () at xact.c:393 393 return XactTopTransactionId; (gdb)
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