GetSnapshotData(), is not quite right, because at the time this is
executed, we already hold ProcArrayLock, which is the only lock that
gets grabbed here:
* If we're in recovery then snapshot data comes from a different place,
* so decide which route we take before grab the lock. It is
* recovery to end before we finish taking snapshot, and for newly
* assigned transaction ids to be added to the procarray. Xmax cannot
* change while we hold ProcArrayLock, so those newly added transaction
* ids would be filtered away, so we need not be concerned about them.
snapshot->takenDuringRecovery = RecoveryInProgress();
Having thought it over for a bit, I believe that the code is correct
and it's only the comment that needs to be updated. If we were to set
snapshot->takenDuringRecovery before acquiring ProcArrayLock, then the
following sequence of events might occur:
T1: Enter GetSnapshotData(). Set snapshot->takenDuringRecovery = true.
T2: Begin, get an XID.
T3: Begin, get an XID.
T3: Commit, advancing latestCompletedXid.
T1: Acquire ProcArrayLock and use the "in recovery" path, missing T2's
XID (because it's not in the subxip array).
T1: Do some stuff with the snapshot... not seeing T2's XID...
T1: Do some stuff with the snapshot... now seeing T2's XID...
I think if we just delete the first sentence of the comment, the rest
is all correct.
A little further down, there is this comment:
* Spin over procArray checking xid, xmin, and
subxids. The goal is
* to gather all active xids, find the lowest xmin,
and try to record
* subxids. During recovery no xids will be assigned,
so all normal
* backends can be ignored, nor are there any VACUUMs
* prepared transaction xids are held in
KnownAssignedXids, so these
* will be seen without needing to loop through procs here.
...but this code is only executed when recovery is not in progress.
So I feel like everything after "try to record subxids" should either
be removed, or relocated to the following "else" block, where the
recovery path is discussed in detail.
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