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Improve documentation about reasoning behind the order of operations

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in GetSnapshotData, GetNewTransactionId, CommitTransaction, AbortTransaction,
etc.  Correct race condition in transaction status testing in
HeapTupleSatisfiesVacuum --- this wasn't important for old VACUUM with
exclusive lock on its table, but it sure is important now.  All per
pghackers discussion 7/11/01 and 7/12/01.
This commit is contained in:
Tom Lane
2001-07-16 22:43:34 +00:00
parent ffbd97c8ac
commit ed5c4e4a14
4 changed files with 110 additions and 49 deletions
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60
src/backend/storage/ipc/sinval.c
View File

@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.36 2001/07/12 04:11:13 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinval.c,v 1.37 2001/07/16 22:43:34 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -193,8 +193,10 @@ TransactionIdIsInProgress(TransactionId xid)
if (pOffset != INVALID_OFFSET)
{
PROC *proc = (PROC *) MAKE_PTR(pOffset);
/* Fetch xid just once - see GetNewTransactionId */
TransactionId pxid = proc->xid;
if (TransactionIdEquals(proc->xid, xid))
if (TransactionIdEquals(pxid, xid))
{
result = true;
break;
@@ -236,9 +238,9 @@ GetXmaxRecent(TransactionId *XmaxRecent)
if (pOffset != INVALID_OFFSET)
{
PROC *proc = (PROC *) MAKE_PTR(pOffset);
TransactionId xid;
/* Fetch xid just once - see GetNewTransactionId */
TransactionId xid = proc->xid;
xid = proc->xid;
if (! TransactionIdIsSpecial(xid))
{
if (TransactionIdPrecedes(xid, result))
@@ -256,8 +258,19 @@ GetXmaxRecent(TransactionId *XmaxRecent)
*XmaxRecent = result;
}
/*
/*----------
* GetSnapshotData -- returns information about running transactions.
*
* The returned snapshot includes xmin (lowest still-running xact ID),
* xmax (next xact ID to be assigned), and a list of running xact IDs
* in the range xmin <= xid < xmax. It is used as follows:
* All xact IDs < xmin are considered finished.
* All xact IDs >= xmax are considered still running.
* For an xact ID xmin <= xid < xmax, consult list to see whether
* it is considered running or not.
* This ensures that the set of transactions seen as "running" by the
* current xact will not change after it takes the snapshot.
*----------
*/
Snapshot
GetSnapshotData(bool serializable)
@@ -287,12 +300,33 @@ GetSnapshotData(bool serializable)
elog(ERROR, "Memory exhausted in GetSnapshotData");
}
/*
* Unfortunately, we have to call ReadNewTransactionId() after
* acquiring SInvalLock above. It's not good because
* ReadNewTransactionId() does SpinAcquire(XidGenLockId) but
* _necessary_.
/*--------------------
* Unfortunately, we have to call ReadNewTransactionId() after acquiring
* SInvalLock above. It's not good because ReadNewTransactionId() does
* SpinAcquire(XidGenLockId), but *necessary*. We need to be sure that
* no transactions exit the set of currently-running transactions
* between the time we fetch xmax and the time we finish building our
* snapshot. Otherwise we could have a situation like this:
*
* 1. Tx Old is running (in Read Committed mode).
* 2. Tx S reads new transaction ID into xmax, then
* is swapped out before acquiring SInvalLock.
* 3. Tx New gets new transaction ID (>= S' xmax),
* makes changes and commits.
* 4. Tx Old changes some row R changed by Tx New and commits.
* 5. Tx S finishes getting its snapshot data. It sees Tx Old as
* done, but sees Tx New as still running (since New >= xmax).
*
* Now S will see R changed by both Tx Old and Tx New, *but* does not
* see other changes made by Tx New. If S is supposed to be in
* Serializable mode, this is wrong.
*
* By locking SInvalLock before we read xmax, we ensure that TX Old
* cannot exit the set of running transactions seen by Tx S. Therefore
* both Old and New will be seen as still running => no inconsistency.
*--------------------
*/
ReadNewTransactionId(&(snapshot->xmax));
for (index = 0; index < segP->lastBackend; index++)
@@ -302,6 +336,7 @@ GetSnapshotData(bool serializable)
if (pOffset != INVALID_OFFSET)
{
PROC *proc = (PROC *) MAKE_PTR(pOffset);
/* Fetch xid just once - see GetNewTransactionId */
TransactionId xid = proc->xid;
/*
@@ -325,11 +360,12 @@ GetSnapshotData(bool serializable)
if (serializable)
MyProc->xmin = snapshot->xmin;
/* Serializable snapshot must be computed before any other... */
Assert(MyProc->xmin != InvalidTransactionId);
SpinRelease(SInvalLock);
/* Serializable snapshot must be computed before any other... */
Assert(MyProc->xmin != InvalidTransactionId);
snapshot->xcnt = count;
return snapshot;
}