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由 Heikki Linnakangas 提交于
If the caller specifies DF_WITH_SNAPSHOT, so that the command is dispatched to the segments with a snapshot, but it currently has no active snapshot in the QD itself, that seems like a mistake. In qdSerializeDtxContextInfo(), the comment talked about which snapshot to use when the transaction has already been aborted. I didn't quite understand that. I don't think the function is used to dispatch the "ABORT" statement itself, and we shouldn't be dispatching anything else in an already-aborted transaction. This makes it more clear which snapshot is dispatched along with the command. In theory, the latest or serializable snapshot can be different from the one being used when the command is dispatched, although I'm not sure if there are any such cases in practice. In the upcoming 8.4 merge, there are more changes coming up to snapshot management, which make it more difficult to get hold of the latest acquired snapshot in the transaction, so changing this now will ease the pain of merging that. I don't know why, but after making the change in qdSerializeDtxContextInfo, I started to get a lot of "Too many distributed transactions for snapshot (maxCount %d, count %d)" errors. Looking at the code, I don't understand how it ever worked. I don't see any no guarantee that the array in TempQDDtxContextInfo or TempDtxContextInfo was pre-allocated correctly. Or maybe it got allocated big enough to hold max_prepared_xacts, which was always large enough, but it seemed rather haphazard to me. So in the spirit of "if you don't understand it, rewrite it until you do", I changed the way the allocation of the inProgressXidArray array works. In statically allocated snapshots, i.e. SerializableSnapshot and LatestSnapshot, the array is malloc'd. In a snapshot copied with CopySnapshot(), it is points to a part of the palloc'd space for the snapshot. Nothing new so far, but I changed CopySnapshot() to set "maxCount" to -1 to indicate that it's not malloc'd. Then I modified DistributedSnapshot_Copy and DistributedSnapshot_Deserialize to not give up if the target array is not large enough, but enlarge it as needed. Finally, I made a little optimization in GetSnapshotData() when running in a QE, to move the copying of the distributed snapshot data to outside the section guarded by ProcArrayLock. ProcArrayLock can be heavily contended, so that's a nice little optimization anyway, but especially now that DistributedSnapshot_Copy() might need to realloc the array.
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