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amcheck: Fix FullTransactionIdFromXidAndCtx() for xids before epoch 0

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64bit xids can't represent xids before epoch 0 (see also be504a3e97). When
FullTransactionIdFromXidAndCtx() was passed such an xid, it'd create a 64bit
xid far into the future. Noticed while adding assertions in the course of
investigating be504a3e97, as amcheck's test create such xids.

To fix the issue, just return FirstNormalFullTransactionId in this case. A
freshly initdb'd cluster already has a newer horizon. The most minimal version
of this would make the messages for some detected corruptions differently
inaccurate. To make those cases accurate, switch
FullTransactionIdFromXidAndCtx() to use the 32bit modulo difference between
xid and nextxid to compute the 64bit xid, yielding sensible "in the future" /
"in the past" answers.

Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/20230108002923.cyoser3ttmt63bfn@awork3.anarazel.de
Backpatch: 14-, where heapam verification was introduced
This commit is contained in:
Andres Freund
2023-03-11 14:12:51 -08:00
parent 6d9588108a
commit e8a9750d03
2 changed files with 48 additions and 13 deletions
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33
contrib/amcheck/verify_heapam.c
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@ -1576,17 +1576,40 @@ check_tuple(HeapCheckContext *ctx)
static FullTransactionId
FullTransactionIdFromXidAndCtx(TransactionId xid, const HeapCheckContext *ctx)
{
uint32 epoch;
uint64 nextfxid_i;
int32 diff;
FullTransactionId fxid;
Assert(TransactionIdIsNormal(ctx->next_xid));
Assert(FullTransactionIdIsNormal(ctx->next_fxid));
Assert(XidFromFullTransactionId(ctx->next_fxid) == ctx->next_xid);
if (!TransactionIdIsNormal(xid))
return FullTransactionIdFromEpochAndXid(0, xid);
epoch = EpochFromFullTransactionId(ctx->next_fxid);
if (xid > ctx->next_xid)
epoch--;
return FullTransactionIdFromEpochAndXid(epoch, xid);
nextfxid_i = U64FromFullTransactionId(ctx->next_fxid);
/* compute the 32bit modulo difference */
diff = (int32) (ctx->next_xid - xid);
/*
* In cases of corruption we might see a 32bit xid that is before epoch
* 0. We can't represent that as a 64bit xid, due to 64bit xids being
* unsigned integers, without the modulo arithmetic of 32bit xid. There's
* no really nice way to deal with that, but it works ok enough to use
* FirstNormalFullTransactionId in that case, as a freshly initdb'd
* cluster already has a newer horizon.
*/
if (diff > 0 && (nextfxid_i - FirstNormalTransactionId) < (int64) diff)
{
Assert(EpochFromFullTransactionId(ctx->next_fxid) == 0);
fxid = FirstNormalFullTransactionId;
}
else
fxid = FullTransactionIdFromU64(nextfxid_i - diff);
Assert(FullTransactionIdIsNormal(fxid));
return fxid;
}
/*