Do away with zero-padding assumption before WALRead()

Hi,

I noticed an assumption [1] at WALRead() call sites expecting the
flushed WAL page to be zero-padded after the flush LSN. I think this
can't always be true as the WAL can get flushed after determining the
flush LSN before reading it from the WAL file using WALRead(). I've
hacked the code up a bit to check if that's true -
https://github.com/BRupireddy2/postgres/tree/ensure_extra_read_WAL_page_is_zero_padded_at_the_end_WIP,
the tests hit the Assert(false); added. Which means, the zero-padding
comment around WALRead() call sites isn't quite right.

I'm wondering why the WALRead() callers are always reading XLOG_BLCKSZ
despite knowing exactly how much to read. Is it to tell the OS to
explicitly fetch the whole page from the disk? If yes, the OS will do
that anyway because the page transfers from disk to OS page cache are
always in terms of disk block sizes, no?

Although, there's no immediate problem with it right now, the
assumption is going to be incorrect when reading WAL from WAL buffers
using WALReadFromBuffers -
https://www.postgresql.org/message-id/CALj2ACV=C1GZT9XQRm4iN1NV1T=hLA_hsGWNx2Y5-G+mSwdhNg@mail.gmail.com.

If we have no reason, can the WALRead() callers just read how much
they want like walsender for physical replication? Attached a patch
for the change.

Thoughts?

[1]
    /*
     * Even though we just determined how much of the page can be validly read
     * as 'count', read the whole page anyway. It's guaranteed to be
     * zero-padded up to the page boundary if it's incomplete.
     */
    if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ, tli,
                 &errinfo))

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com