Add LSN along with offset to error messages reported for WAL file read/write/validate header failures

Hi,

I was recently asked about converting an offset reported in WAL read
error messages[1] to an LSN with which pg_waldump can be used to
verify the records or WAL file around that LSN (basically one can
filter out the output based on LSN). AFAICS, there's no function that
takes offset as an input and produces an LSN and I ended up figuring
out LSN manually. And, for some of my work too, I was hitting errors
in XLogReaderValidatePageHeader() and adding recptr to those error
messages helped me debug issues faster.

We have a bunch of messages [1] that have an offset, but not LSN in
the error message. Firstly, is there an easiest way to figure out LSN
from offset reported in the error messages? If not, is adding LSN to
these messages along with offset a good idea? Of course, we can't just
convert offset to LSN using XLogSegNoOffsetToRecPtr() and report, but
something meaningful like reporting the LSN of the page that we are
reading-in or writing-out etc.

Thoughts?

[1]
errmsg("could not read from WAL segment %s, offset %u: %m",
errmsg("could not read from WAL segment %s, offset %u: %m",
errmsg("could not write to log file %s "
       "at offset %u, length %zu: %m",
errmsg("unexpected timeline ID %u in WAL segment %s, offset %u",
errmsg("could not read from WAL segment %s, offset %u: read %d of %zu",
pg_log_error("received write-ahead log record for offset %u with no file open",
"invalid magic number %04X in WAL segment %s, offset %u",
"invalid info bits %04X in WAL segment %s, offset %u",
"invalid info bits %04X in WAL segment %s, offset %u",
"unexpected pageaddr %X/%X in WAL segment %s, offset %u",
"out-of-sequence timeline ID %u (after %u) in WAL segment %s, offset %u",

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