collect_corrupt_items() failed to initialize tuple.t_self.  While
HeapTupleSatisfiesVacuum() doesn't actually use that value, it does
Assert that it's valid, so that the code would dump core if ip_posid
chanced to be zero.  (That's somewhat unlikely, which probably explains
how this got missed.  In any case it wouldn't matter for field use.)

Also, collect_corrupt_items was returning the wrong TIDs, that is the
contents of t_ctid rather than the tuple's own location.  This would
be the same thing in simple cases, but it could be wrong if, for
example, a past update attempt had been rolled back, leaving a live
tuple whose t_ctid doesn't point at itself.

Also, in pg_visibility(), guard against trying to read a page past
the end of the rel.  The VM code handles inquiries beyond the end
of the map by silently returning zeroes, and it seems like we should
do the same thing here.

I ran into the assertion failure while using pg_visibility to check
pg_upgrade's behavior, and then noted the other problems while
reading the code.

Report: <29043.1475288648@sss.pgh.pa.us>

This requires some core changes as well so that we can properly
WAL-log the truncation.  Specifically, it changes the format of the
XLOG_SMGR_TRUNCATE WAL record, so bump XLOG_PAGE_MAGIC.

Patch by me, reviewed but not fully endorsed by Andres Freund.

The new pg_check_visible() and pg_check_frozen() functions can be used to
verify that the visibility map bits for a relation's data pages match the
actual state of the tuples on those pages.

Amit Kapila and Robert Haas, reviewed (in earlier versions) by Andres
Freund.  Additional testing help by Thomas Munro.

The reverted changes were intended to force a choice of whether any
newly-added BufferGetPage() calls needed to be accompanied by a
test of the snapshot age, to support the "snapshot too old"
feature.  Such an accompanying test is needed in about 7% of the
cases, where the page is being used as part of a scan rather than
positioning for other purposes (such as DML or vacuuming).  The
additional effort required for back-patching, and the doubt whether
the intended benefit would really be there, have indicated it is
best just to rely on developers to do the right thing based on
comments and existing usage, as we do with many other conventions.

This change should have little or no effect on generated executable
code.

Motivated by the back-patching pain of Tom Lane and Robert Haas

This patch is a no-op patch which is intended to reduce the chances
of failures of omission once the functional part of the "snapshot
too old" patch goes in.  It adds parameters for snapshot, relation,
and an enum to specify whether the snapshot age check needs to be
done for the page at this point.  This initial patch passes NULL
for the first two new parameters and BGP_NO_SNAPSHOT_TEST for the
third.  The follow-on patch will change the places where the test
needs to be made.

This lets you examine the visibility map as well as page-level
visibility information.  I initially wrote it as a debugging aid,
but was encouraged to polish it for commit.

Patch by me, reviewed by Masahiko Sawada.

Discussion: 56D77803.6080503@BlueTreble.com