Do XID sequences need to be contiguous?

Hackers,

While working on the problem of XID wraparound within the LISTEN/NOTIFY
system, I tried to increment XIDs by more than one per transaction.
This leads to a number of test failures, many which look like:

+ERROR: could not access status of transaction 7485
+DETAIL: Could not read from file "pg_subtrans/0000" at offset 24576:
read too few bytes.

I might not have read the right documentation, but....

I do not see anything in src/backend/access/transam/README nor elsewhere
documenting a design decision or assumption that transaction IDs must
be assigned contiguously. I suppose this is such a fundamental
assumption that it is completely implicit and nobody thought to document
it, but I'd like to check for two reasons:

First, I'd like a good method of burning through transaction ids in
tests designed to check for problems in XID wrap-around.

Second, I'd like to add Asserts where appropriate regarding this
assumption. It seems strange to me that I should have gotten as far
as a failing read() without having tripped an Assert somewhere along the
way.

To duplicate the errors I hit, you can either apply this simple change:

diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 33fd052156..360b7335bb 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -83,7 +83,7 @@ FullTransactionIdFromEpochAndXid(uint32 epoch,
TransactionId xid)
static inline void
FullTransactionIdAdvance(FullTransactionId *dest)
{
- dest->value++;
+ dest->value += 2;
while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
dest->value++;
}

or apply the much larger WIP patch, attached, and then be sure to
provide the --enable-xidcheck flag to configure before building.

--
Mark Dilger