Use the right timeline when beginning to stream from master.

The xlogreader refactoring broke the logic to decide which timeline to start
streaming from. XLogPageRead() uses the timeline history to check which
timeline the requested WAL position falls into. However, after the
refactoring, XLogPageRead() is always first called with the first page in
the segment, to verify the segment header, and only then with the actual WAL
position we're interested in. That first read of the segment's header made
XLogPageRead() to always start streaming from the old timeline containing
the segment header, not the timeline containing the actual record, if there
was a timeline switch within the segment.

I thought I fixed this yesterday, but that fix was too narrow and only fixed
this for the corner-case that the timeline switch happened in the first page
of the segment. To fix this more robustly, pass explicitly the position of
the record we're actually interested in to XLogPageRead, and use that to
decide which timeline to read from, rather than deduce it from the page and
offset.

Per report from Fujii Masao.
Now that START_REPLICATION returns the next timeline's ID after reaching end
of timeline, take advantage of that in walreceiver.

Startup process is still in control of choosign the target timeline, by
scanning the timeline history files present in pg_xlog, but walreceiver now
uses the next timeline's ID to fetch its history file immediately after it
has finished streaming the old timeline. Before, the standby would first try
to restart streaming on the old timeline, which fetches the missing timeline
history file as a side-effect, and only then restart from the new timeline.
This patch eliminates the extra iteration, which speeds up the timeline
switch and reduces the noise in the log caused by the extra restart on the
old timeline.
psql latex fixes

Remove extra line at bottom of table for new 'latex' mode border=3.
Also update 'latex'-longtable 'tableattr' docs to say
'whitespace-separated' instead of 'space'.
Fix off-by-one bug in xlog reading logic

Bug reported by Michael Paquier

Author: Andres Freund
Improve pg_upgrade error report

If the cluster alignments don't match, output this suggestion:

	Likely one cluster is a 32-bit install, the other 64-bit
Unbreak lock conflict detection for Hot Standby.

This got broken in the original fast-path locking patch, because
I failed to account for the fact that Hot Standby startup process
might take a strong relation lock on a relation in a database to
which it is not bound, and confused MyDatabaseId with the database
ID of the relation being locked.

Report and diagnosis by Andres Freund.  Final form of patch by me.
Rename new latex longtable function name, for consistency
Protect against SnapshotNow race conditions in pg_tablespace scans.

Use of SnapshotNow is known to expose us to race conditions if the tuple(s)
being sought could be updated by concurrently-committing transactions.
CREATE DATABASE and DROP DATABASE are particularly exposed because they do
heavyweight filesystem operations during their scans of pg_tablespace,
so that the scans run for a very long time compared to most.  Furthermore,
the potential consequences of a missed or twice-visited row are nastier
than average:

* createdb() could fail with a bogus "file already exists" error, or
  silently fail to copy one or more tablespace's worth of files into the
  new database.

* remove_dbtablespaces() could miss one or more tablespaces, thus failing
  to free filesystem space for the dropped database.

* check_db_file_conflict() could likewise miss a tablespace, leading to an
  OID conflict that could result in data loss either immediately or in
  future operations.  (This seems of very low probability, though, since a
  duplicate database OID would be unlikely to start with.)

Hence, it seems worth fixing these three places to use MVCC snapshots, even
though this will someday be superseded by a generic solution to SnapshotNow
race conditions.

Back-patch to all active branches.

Stephen Frost and Tom Lane
libpq doc: Clarify what commands return PGRES_TUPLES_OK

The old text claimed that INSERT and UPDATE always return
PGRES_COMMAND_OK, but INSERT/UPDATE with RETURNING return
PGRES_TUPLES_OK.

Josh Kupershmidt