Re: Be strict when request to flush past end of WAL in WaitXLogInsertionsToFinish

Here is v3, with the changes discussed upthread:

- s/clamp/adjust/ in the comments and commit messages.

- XLogBackgroundFlush() now has a comment explaining why the adjusted
position is applied conditionally instead of being assigned the way
XLogFlush() does.

- The C injector module is gone. The TAP test now reproduces the
incident naturally: it pads WAL with pg_logical_emit_message() until
the insert position is exactly SizeOfXLogRecord bytes before a
segment boundary and then calls pg_switch_wal(), as described in my
previous mail. Since no dedicated module is needed anymore, the
test moved to src/test/modules/test_misc. If a concurrent record
(e.g. a bgwriter snapshot) spoils the alignment, the test retries on
a later segment boundary; pg_switch_wal()'s return value tells
whether the window was hit.

On an unpatched assert build the test brings the walwriter down with
Assert("Insert >= Write") and fails; with the fix it passes. The fix
itself is unchanged from v2 apart from comments. Rebased onto current
master.

Regards,
Paul Kim