database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-11-28 11:44:57 +03:00
Code Activity

Make release of 2PC identifier and locks consistent in COMMIT PREPARED

Browse Source 
When preparing a transaction in two-phase commit, a dummy PGPROC entry
holding the GID used for the transaction is registered, which gets
released once COMMIT PREPARED is run.  Prior releasing its shared memory
state, all the locks taken in the prepared transaction are released
using a dedicated set of callbacks (pgstat and multixact having similar
callbacks), which may cause the locks to be released before the GID is
set free.

Hence, there is a small window where lock conflicts could happen, for
example:
- Transaction A releases its locks, still holding its GID in shared
memory.
- Transaction B held a lock which conflicted with locks of transaction
A.
- Transaction B continues its processing, reusing the same GID as
transaction A.
- Transaction B fails because of a conflicting GID, already in use by
transaction A.

This commit changes the shared memory state release so as post-commit
callbacks and predicate lock cleanup happen consistently with the shared
memory state cleanup for the dummy PGPROC entry.  The race window is
small and 2PC had this issue from the start, so no backpatch is done.
On top if that fixes discussed involved ABI breakages, which are not
welcome in stable branches.

Reported-by: Oleksii Kliukin, Ildar Musin
Diagnosed-by: Oleksii Kliukin, Ildar Musin
Author: Michael Paquier
Reviewed-by: Masahiko Sawada, Oleksii Kliukin
Discussion: https://postgr.es/m/BF9B38A4-2BFF-46E8-BA87-A2D00A8047A6@hintbits.com
This commit is contained in:
Michael Paquier
2019-02-25 14:19:34 +09:00
parent 29ddb548f6
commit effe7d9552
4 changed files with 44 additions and 19 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
src/backend/access/transam/multixact.c
View File

@@ -1713,7 +1713,7 @@ PostPrepare_MultiXact(TransactionId xid)
myOldestMember = OldestMemberMXactId[MyBackendId];
if (MultiXactIdIsValid(myOldestMember))
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, false);
/*
* Even though storing MultiXactId is atomic, acquire lock to make
@@ -1755,7 +1755,7 @@ void
multixact_twophase_recover(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, false);
MultiXactId oldestMember;
/*
@@ -1776,7 +1776,7 @@ void
multixact_twophase_postcommit(TransactionId xid, uint16 info,
void *recdata, uint32 len)
{
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid);
BackendId dummyBackendId = TwoPhaseGetDummyBackendId(xid, true);
Assert(len == sizeof(MultiXactId));