Re: Fix bug in multixact Oldest*MXactId initialization and access

Hi,

> But proc structs reserved for two phase commit are placed after auxiliary
> procs, therefore writes to OldestMemberMXactId[dummy] starts to overwrites
> slots of OldestVisibleMXactId.
>
> Then PostgreSQL 18 increased NUM_AUXILIARY_PROCS due to reserve of workers
> for AIO. And it is possible to make such test postgresql.conf with so
> extremely low MaxBackend so writes to OldestMemberMXactId[dummy] overwrites
> first entry of BufferDescriptors, which are allocated next in shared memory.

From what I can tell, PreparedXactProcs proc numbers start at
MaxBackends + NUM_AUXILIARY_PROCS, but MaxOldestSlot is calculated as
MaxBackends + max_prepared_xacts, missing the NUM_AUXILIARY_PROCS offset.

MyProcNumber (for regular backends) is always < MaxBackends, so it's safe.
But dummyProcNumber (for prepared transactions) starts at
MaxBackends + NUM_AUXILIARY_PROCS, which exceeds MaxOldestSlot.

Using this workload:

```
psql -c "DROP TABLE IF EXISTS test;"
psql -c "CREATE TABLE IF NOT EXISTS test (id int PRIMARY KEY, data text);"
psql -c "INSERT INTO test VALUES (1, 'test data');"

for i in {1..25}; do
    psql -q <<EOF
BEGIN;
SELECT * FROM test WHERE id = 1 FOR SHARE;
PREPARE TRANSACTION 'prep_$i';
EOF
    echo "  Created prep_$i"
done
```
I can see the assert in your patch fail here:

```
@@ -1628,8 +1631,8 @@ PostPrepare_MultiXact(FullTransactionId fxid)
          */
         LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);

-        OldestMemberMXactId[dummyProcNumber] = myOldestMember;
-        OldestMemberMXactId[MyProcNumber] = InvalidMultiXactId;
+        setOldest(OldestMemberMXactId, dummyProcNumber, myOldestMember);
```

> Patch in attach replaces direct accesses to this arrays with inline
> functions which include asserts. And changes calculation of MaxOldestSlot
> to include NUM_AUXILIARY_PROCS.

This is the important part of the patch to correctly size MaxOldestSlot

```
+#define MaxOldestSlot (MaxBackends + NUM_AUXILIARY_PROCS + max_prepared_xacts)
```

but, I am not sure if we need all the extra asserts, as this can only
be an issue for the
dummyProcNumber, right?

Maybe we can also define a new macro:

```
#define TOTAL_PROCS MaxBackends + NUM_AUXILIARY_PROCS + max_prepared_xacts
```

and use it in InitProcGlobal instead of TotalProcs, as well as MaxOldestSlot?

--
Sami Imseih
Amazon Web Services