On 27/11/2025 05:39, Chao Li wrote:
<v11-0001-Avoid-multixact-edge-case-2-by-writing-the-next-.patch>
1
```
+ if (*offptr != offset)
+ {
+ /* should already be set to the correct value, or not at all */
+ Assert(*offptr == 0);
+ *offptr = offset;
+ MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
+ }
```
This is a more like a question. Since pre-write should always happen, in theory
*offptr != offset should never be true, why do we still need to handle the case
instead of just assert(false)?
No, *offptr == 0 can happen, if multiple backends are generating
multixids concurrently. It's possible that we get here before the
RecordNewMultiXact() call for the previous multixid.
Similarly, the *next_offptr != 0 case can happen if another backend
calls RecordNewMultiXact() concurrently for the next multixid.
2
```
+ next = multi + 1;
+ if (next < FirstMultiXactId)
+ next = FirstMultiXactId;
```
next < FirstMultiXactId will only be true when next wraps around to 0, maybe
deserve one-line comment to explain that.
This is a common pattern used in many places in the file.
3
```
+ if (*next_offptr != offset + nmembers)
+ {
+ /* should already be set to the correct value, or not at all */
+ Assert(*next_offptr == 0);
+ *next_offptr = offset + nmembers;
+ MultiXactMemberCtl->shared->page_dirty[slotno] = true;
+ }
```
Should MultiXactMemberCtl be MultiXactOffsetCtl? As we are writing to the
offset SLRU.
Good catch! Yes, it should be MultiXactOffsetCtl.
- Heikki
