Re: Introduce XID age and inactive timeout based replication slot invalidation

On Wed, Feb 12, 2025 at 12:36 AM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
>
> On Tue, Feb 11, 2025 at 8:49 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> >
> > Hi Nisha.
> >
> > Some review comments about v74-0001
> >
> > ======
> > src/backend/replication/slot.c
> >
> > 1.
> > /* Maximum number of invalidation causes */
> > -#define RS_INVAL_MAX_CAUSES RS_INVAL_WAL_LEVEL
> > -
> > -StaticAssertDecl(lengthof(SlotInvalidationCauses) == (RS_INVAL_MAX_CAUSES + 1),
> > - "array length mismatch");
> > +#define RS_INVAL_MAX_CAUSES (lengthof(InvalidationCauses)-1)
> >
> > The static assert was here to protect against dev mistakes in keeping
> > the lookup table up-to-date with the enum of slot.h. So it's not a
> > good idea to remove it...
> >
> > IMO the RS_INVAL_MAX_CAUSES should be relocated to slot.h where the
> > enum is defined and where the devs know exactly how many invalidation
> > types there are. Then this static assert can be put back in to do its
> > job of ensuring the integrity properly again for this lookup table.
> >
>
> How about keeping RS_INVAL_MAX_CAUSES dynamic in slot.c (as it was)
> and updating the static assert to ensure the lookup table stays
> up-to-date with the enums?
> The change has been implemented in v75.
>

Latest v75-001 patch code looks like:

+static const SlotInvalidationCauseMap InvalidationCauses[] = {
+ {RS_INVAL_NONE, "none"},
+ {RS_INVAL_WAL_REMOVED, "wal_removed"},
+ {RS_INVAL_HORIZON, "rows_removed"},
+ {RS_INVAL_WAL_LEVEL, "wal_level_insufficient"},
+ {RS_INVAL_IDLE_TIMEOUT, "idle_timeout"},
};

/* Maximum number of invalidation causes */
-#define RS_INVAL_MAX_CAUSES RS_INVAL_WAL_LEVEL
+#define RS_INVAL_MAX_CAUSES (lengthof(InvalidationCauses)-1)

-StaticAssertDecl(lengthof(SlotInvalidationCauses) == (RS_INVAL_MAX_CAUSES + 1),
+/*
+ * Ensure that the lookup table is up-to-date with the enums defined in
+ * ReplicationSlotInvalidationCause. Shifting 1 left by
+ * (RS_INVAL_MAX_CAUSES - 1) should give the highest defined value in
+ * the enum.
+ */
+StaticAssertDecl(RS_INVAL_IDLE_TIMEOUT == (1 << (RS_INVAL_MAX_CAUSES - 1)),
"array length mismatch");

Where:
1. RS_INVAL_MAX_CAUSES is based on the length of lookup table so it is 4
2. the StaticAssert then confirms that the enum RS_INVAL_IDLE_TIMEOUT
is the 4th enum entry

AFAICT that is not useful. The purpose of the static assert is (like
your comment says) to "Ensure that the lookup table is up-to-date with
the enums". Imagine if I added another (5th cause) enum called
RS_INVAL_BANANA but accidentally overlook updating the lookup table.
The code above isn't going to detect that -- the lookup table length
is still 4 (instead of 5) but RS_INVAL_IDLE_TIMEOUT is still the 4th
enum so the assert is happy. Hence my original suggestion to define
RS_INVAL_MAX_CAUSES adjacent to the enum in slot.h.

======
Kind Regards,
Peter Smith.
Fujitsu Australia