Thread

Re: A few patches to clarify snapshot management, part 2

Kirill Reshke <reshkekirill@gmail.com> — 2025-12-19T21:32:20Z

On Fri, 19 Dec 2025 at 16:51, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 19/12/2025 07:15, Chao Li wrote:
> >> On Dec 19, 2025, at 07:30, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> >>
> >> Patch 0002: Split SnapshotData into separate structs for each kind of snapshot
> >> >> This implements the long-standing TODO and splits SnapshotData up
> >> into multiple structs. This makes it more clear which fields are
> >> used with which kind of snapshot. For example, we now have
> >> properly named fields for the XID arrays in historic snapshots.
> >> Previously, they abused the 'xip' and 'subxip' arrays to mean
> >> something different than what they mean in regular MVCC snapshots.
> >>
> >> This introduces some new casts between Snapshots and the new
> >> MVCCSnapshots. I struggled to decide which functions should use
> >> the new MVCCSnapshot type and which should continue to use
> >> Snapshot. It's a balancing act between code churn and where we
> >> want to have casts. For example, PushActiveSnapshot() takes a
> >> Snapshot argument, but assumes that it's an MVCC snapshot, so it
> >> really should take MVCCSnapshot. But most of its callers have a
> >> Snapshot rather than MVCCSnapshot. Another example: the executor
> >> assumes that it's dealing with MVCC snapshots, so it would be
> >> appropriate to use MVCCSnapshot for EState->es_snapshot. But it'd
> >> cause a lot code churn and casts elsewhere. I think the patch is a
> >> reasonable middle ground.
> > > The core of 0002 is the new union Snapshot:
> > ```
> > +/*
> > + * Generic union representing all kind of possible snapshots.  Some have
> > + * type-specific structs.
> > + */
> > +typedef union SnapshotData
> > +{
> > +     SnapshotType snapshot_type; /* type of snapshot */
> > +
> > +     MVCCSnapshotData mvcc;
> > +     DirtySnapshotData dirty;
> > +     HistoricMVCCSnapshotData historic_mvcc;
> > +     NonVacuumableSnapshotData nonvacuumable;
> >   } SnapshotData;
> > ```
> > > And my big concern is here. This union definition looks unusual,
> > snapshot_type shares the same space with real snapshot bodies, so
> > that once snapshot is assigned to the union, that type info is lost,
> > there would be no way to decide what exact snapshot is stored in
> > SnapshotData.
>
> Each of the structs, MVCCSnapshotData, DirtySnapshotData, etc., contain
> 'snapshot_type' as the first field, so it's always available.
>
> > I think SnapshotData should be a structure and utilize anonymous union technique:
> > ```
> > typedef struct SnapshotData
> > {
> >      SnapshotType snapshot_type; /* type of snapshot */
> >
> >      union {
> >         MVCCSnapshotData mvcc;
> >         DirtySnapshotData dirty;
> >         HistoricMVCCSnapshotData historic_mvcc;
> >         NonVacuumableSnapshotData nonvacuumable;
> >      }
> > ```
>
> Hmm, yeah, maybe that would be more clear. But then you cannot cast an
> "MVCCSnapshotData *" to "SnapshotData *", or vice versa.
>
> > If you agree with this, then 0002 will need some rework. A set of helper micros/functions would need to be added, e.g. InitDirtySnapshot(), DirtySnapshotGetSnapshot()...
>
> Right, I feel those helper macros / functions would be excessive.
>
> - Heikki
>
>
>


Hi! I have been looking through this series of patches, when I noticed
that UnregisterSnapshotFromOwner/UnregisterSnapshot use this coding:

```
if (snapshot == InvalidSnapshot)
return;
```

First of all, I am not sure this is the type of check we usually do in
core. If this coding practice is OK, should we remove this check from
UnregisterSnapshot* caller?
Like in PFA.

Sorry for bikeshedding

-- 
Best regards,
Kirill Reshke