pgsql: Refactor WAL file-reading code into WALRead()

Refactor WAL file-reading code into WALRead()

XLogReader, walsender and pg_waldump all had their own routines to read
data from WAL files to memory, with slightly different approaches
according to the particular conditions of each environment. There's a
lot of commonality, so we can refactor that into a single routine
WALRead in XLogReader, and move the differences to a separate (simpler)
callback that just opens the next WAL-segment. This results in a
clearer (ahem) code flow.

The error reporting needs are covered by filling in a new error-info
struct, WALReadError, and it's the caller's responsibility to act on it.
The backend has WALReadRaiseError() to do so.

We no longer ever need to seek in this interface; switch to using
pg_pread().

Author: Antonin Houska, with contributions from Álvaro Herrera
Reviewed-by: Michaël Paquier, Kyotaro Horiguchi
Discussion: https://postgr.es/m/14984.1554998742@spoje.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0dc8ead46363fec6f621a12c7e1f889ba73b55a9

Modified Files
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src/backend/access/transam/xlogreader.c | 106 ++++++++++-
src/backend/access/transam/xlogutils.c | 205 ++++++++--------------
src/backend/replication/walsender.c | 300 +++++++++++++-------------------
src/bin/pg_waldump/pg_waldump.c | 168 ++++++------------
src/include/access/xlogreader.h | 39 ++++-
src/include/access/xlogutils.h | 2 +
6 files changed, 387 insertions(+), 433 deletions(-)