amcheck: Fix FullTransactionIdFromXidAndCtx() for xids before epoch 0

64bit xids can't represent xids before epoch 0 (see also be504a3e974). 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 be504a3e974, 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

1 files changed, 28 insertions, 5 deletions

diff --git a/contrib/amcheck/verify_heapam.c b/contrib/amcheck/verify_heapam.c
index 33c5b33895..94ddccd23a 100644
--- a/contrib/amcheck/verify_heapam.c
+++ b/contrib/amcheck/verify_heapam.c
@@ -1574,17 +1574,40 @@ 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;
 }
 
 /*