The race condition goes like this:
1. GetNewTransactionId advances nextXid e.g. from 100 to 101 2. GetOldestActiveTransactionId reads the new nextXid, 101 3. GetOldestActiveTransactionId loops through the proc array. There are no active XIDs there, so it returns 101 as the oldest active XID. 4. GetNewTransactionid stores XID 100 to MyPgXact->xid
So, GetOldestActiveTransactionId returned XID 101, even though 100 only just started and is surely still running.
This would be hard to hit in practice, and even harder to spot any ill effect if it happens. GetOldestActiveTransactionId is only used when creating a checkpoint in a master server, and the race condition can only happen on an online checkpoint, as there are no backends running during a shutdown checkpoint. The oldestActiveXid value of an online checkpoint is only used when starting up a hot standby server, to determine the starting point where pg_subtrans is initialized from. For the race condition to happen, there must be no other XIDs in the proc array that would hold back the oldest-active XID value, which means that the missed XID must be a top transaction's XID. However, pg_subtrans is not used for top XIDs, so I believe an off-by-one error is in fact inconsequential. Nevertheless, let's fix it, as it's clearly wrong and the fix is simple.
This has been wrong ever since hot standby was introduced, so backport to all supported versions.
74fc838 Fix race between GetNewTransactionId and GetOldestActiveTransactionId.
src/backend/storage/ipc/procarray.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)