Обсуждение: heap_update() VM retry could break HOT?

Hi,

heap_update() retries pinning the vm pinning, as explained in the
following comment:
/* * Before locking the buffer, pin the visibility map page if it appears to * be necessary.  Since we haven't got the
lockyet, someone else might be * in the middle of changing this, so we'll need to recheck after we have * the lock.
*/if(PageIsAllVisible(page))    visibilitymap_pin(relation, block, &vmbuffer);
 

...
/* * If we didn't pin the visibility map page and the page has become all * visible while we were busy locking the
buffer,or during some * subsequent window during which we had it unlocked, we'll have to unlock * and re-lock, to avoid
holdingthe buffer lock across an I/O.  That's a * bit unfortunate, especially since we'll now have to recheck whether
the* tuple has been locked or updated under us, but hopefully it won't * happen very often. */if (vmbuffer ==
InvalidBuffer&& PageIsAllVisible(page)){    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);    visibilitymap_pin(relation,
block,&vmbuffer);    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);    goto l2;}
 

unfortunately the l2 target is after the following:/* * If we're not updating any "key" column, we can grab a weaker
locktype. * This allows for more concurrency when we are running simultaneously * with foreign key checks. * * Note
thatif a column gets detoasted while executing the update, but * the value ends up being the same, this test will fail
andwe will use * the stronger lock.  This is acceptable; the important case to optimize * is updates that don't
manipulatekey columns, not those that * serendipitiously arrive at the same key values.
*/HeapSatisfiesHOTandKeyUpdate(relation,hot_attrs, key_attrs, id_attrs,                             &satisfies_hot,
&satisfies_key,                            &satisfies_id, &oldtup, newtup);if (satisfies_key){    *lockmode =
LockTupleNoKeyExclusive;   mxact_status = MultiXactStatusNoKeyUpdate;    key_intact = true;
 
    /*     * If this is the first possibly-multixact-able operation in the     * current transaction, set my
per-backendOldestMemberMXactId     * setting. We can be certain that the transaction will never become a     * member
ofany older MultiXactIds than that.  (We have to do this     * even if we end up just using our own TransactionId
below,since     * some other backend could incorporate our XID into a MultiXact     * immediately afterwards.)     */
MultiXactIdSetOldestMember();}else{    *lockmode = LockTupleExclusive;    mxact_status = MultiXactStatusUpdate;
key_intact= false;}
 

as far as I can see that could mean that we perform hot updates when not
permitted, because the tuple has been replaced since, including the
pkey. Similarly, the wrong tuple lock mode could end up being used.

Am I missing something?

- Andres