A assert failure when initdb with track_commit_timestamp=on

Hi,

When working with the commit_ts module, I find the following issue:

After configure with --enable-cassert option, then initdb with:

initdb -D x2 -c track_commit_timestamp=on

Then we can get the following core dump:

 0 in raise of /lib/x86_64-linux-gnu/libc.so.6
 1 in abort of /lib/x86_64-linux-gnu/libc.so.6
 2 in ExceptionalCondition of assert.c:66
 3 in TransactionIdSetCommitTs of commit_ts.c:257
 4 in SetXidCommitTsInPage of commit_ts.c:236
 5 in TransactionTreeSetCommitTsData of commit_ts.c:192
 6 in RecordTransactionCommit of xact.c:1468
 7 in CommitTransaction of xact.c:2365
 8 in CommitTransactionCommandInternal of xact.c:3202
 9 in CommitTransactionCommand of xact.c:3163
10 in BootstrapModeMain of bootstrap.c:390
11 in main of main.c:210

The reason are TransactionIdSetCommitTs think the given xid must be
normal

static void
TransactionIdSetCommitTs(TransactionId xid, TimestampTz ts,
						 RepOriginId nodeid, int slotno)
{
    ...
	Assert(TransactionIdIsNormal(xid));
}

However this is not true in BootstrapMode, this failure is masked by
default because TransactionTreeSetCommitTsData returns fast when
track_commit_timestamp is off.

void
TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
							   TransactionId *subxids, TimestampTz timestamp,
							   RepOriginId nodeid)
{

	/*
	 * No-op if the module is not active.
	 *
	 */
	if (!commitTsShared->commitTsActive)
		return;

    ..        
}

I can't think out a meaningful reason to record the commit timestamp for a
BootstrapTransactionId or FrozenTransactionId, so I think bypass it in
TransactionTreeSetCommitTsData could be a solution. Another solution is
just removing the Assert in TransactionIdSetCommitTs, it works during
initdb test at least.

I include both fixes in the attachment, I think just one of them could
be adopted however.

-- 
Best Regards
Andy Fan