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Do not commit already-committed prepared transactions in recovery

pull/1696/head
Marco Slot 2017-11-20 13:18:31 +01:00
parent fe798cf0f9
commit 9793218122
1 changed files with 40 additions and 39 deletions
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75
src/backend/distributed/transaction/transaction_recovery.c
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@ -246,7 +246,8 @@ RecoverWorkerTransactions(WorkerNode *workerNode)
{
bool isNull = false;
bool isTransactionInProgress = false;
bool isTransactionPending = false;
bool foundPreparedTransactionBeforeCommit = false;
bool foundPreparedTransactionAfterCommit = false;
Datum transactionNameDatum = heap_getattr(heapTuple,
Anum_pg_dist_transaction_gid,
@ -270,13 +271,20 @@ RecoverWorkerTransactions(WorkerNode *workerNode)
* that need to be aborted remain at the end.
*/
hash_search(pendingTransactionSet, transactionName, HASH_REMOVE,
&isTransactionPending);
&foundPreparedTransactionBeforeCommit);
if (isTransactionPending)
hash_search(recheckTransactionSet, transactionName, HASH_FIND,
&foundPreparedTransactionAfterCommit);
if (foundPreparedTransactionBeforeCommit && foundPreparedTransactionAfterCommit)
{
/*
* The transaction was committed, but the prepared transaction still exists
* on the worker. Try committing it.
*
* We double check that the recovery record exists both before and after
* checking ActiveDistributedTransactionNumbers(), since we may have
* observed a prepared transaction that was committed immediately after.
*/
bool shouldCommit = true;
bool commitSucceeded = RecoverPreparedTransactionOnWorker(connection,
@ -300,53 +308,46 @@ RecoverWorkerTransactions(WorkerNode *workerNode)
* the recovery record.
*/
}
else
else if (foundPreparedTransactionAfterCommit)
{
bool foundNewPreparedTransaction = false;
/*
* We found a committed pg_dist_transaction record without a prepared
* transaction. In the typical case, this means that the prepared
* transaction was successfully committed from the post-commit callback
* and we can therefore delete the recovery record. However, there is a
* race condition.
* We found a committed pg_dist_transaction record that initially did
* not have a prepared transaction, but did when we checked again.
*
* If a transaction started and committed just after we observed the
* set of prepared transactions, and just before we took our
* pg_dist_transaction snapshot, then we would see its records, but it may
* have prepared transactions that failed to commit and that we did not
* observe. We should not delete the records, since we would otherwise
* roll back the prepared transaction on the next call to
* set of prepared transactions, and just before we called
* ActiveDistributedTransactionNumbers, then we would see a recovery
* record without a prepared transaction in pendingTransactionSet,
* but there may be prepared transactions that failed to commit.
* We should not delete the records for those prepared transactions,
* since we would otherwise roll back them on the next call to
* recover_prepared_transactions.
*
* In addition, if the transaction started after the call it
* ActiveDistributedTransactionNumbers, then it may still be in the process
* of comitting the prepared transactions in the post-commit callback.
* In addition, if the transaction started after the call to
* ActiveDistributedTransactionNumbers and finished just before our
* pg_dist_transaction snapshot, then it may still be in the process
* of comitting the prepared transactions in the post-commit callback
* and we should not touch the prepared transactions.
*
* We check the prepared transactions again after taking the
* pg_dist_transaction snapshot. Any prepared transactions that appear
* in this set, but not in pendingTransactionSet, may have been created
* before the pg_dist_transaction snapshot. We skip deleting the records
* for those transactions since there are prepared transactions that
* need to be commit, but we cannot commit them now, since they might
* still be getting committed and committing them twice would result in
* failures.
*
* In this case, we just leave the records and prepared transactions
* for the next call to recover_prepared_transactions.
* To handle these cases, we just leave the records and prepared
* transactions for the next call to recover_prepared_transactions
* and skip them here.
*/
hash_search(recheckTransactionSet, transactionName, HASH_FIND,
&foundNewPreparedTransaction);
if (foundNewPreparedTransaction)
{
continue;
}
else
{
/*
* There is a recovery record and definitely no prepared transaction, it
* must have been committed immediately, so it's safe to delete the
* We found a recovery record without any prepared transaction. It
* must have already been committed, so it's safe to delete the
* recovery record.
*
* Transactions that started after we observed pendingTransactionSet,
* but successfully committed their prepared transactions before
* ActiveDistributedTransactionNumbers are indistinguishable from
* transactions that committed at an earlier time, in which case it's
* safe delete the recovery record as well.
*/
}