Fix assorted bugs in CREATE/DROP INDEX CONCURRENTLY.

Commit 8cb53654db, which introduced DROP
INDEX CONCURRENTLY, managed to break CREATE INDEX CONCURRENTLY via a poor
choice of catalog state representation.  The pg_index state for an index
that's reached the final pre-drop stage was the same as the state for an
index just created by CREATE INDEX CONCURRENTLY.  This meant that the
(necessary) change to make RelationGetIndexList ignore about-to-die indexes
also made it ignore freshly-created indexes; which is catastrophic because
the latter do need to be considered in HOT-safety decisions.  Failure to
do so leads to incorrect index entries and subsequently wrong results from
queries depending on the concurrently-created index.

To fix, add an additional boolean column "indislive" to pg_index, so that
the freshly-created and about-to-die states can be distinguished.  (This
change obviously is only possible in HEAD.  This patch will need to be
back-patched, but in 9.2 we'll use a kluge consisting of overloading the
formerly-impossible state of indisvalid = true and indisready = false.)

In addition, change CREATE/DROP INDEX CONCURRENTLY so that the pg_index
flag changes they make without exclusive lock on the index are made via
heap_inplace_update() rather than a normal transactional update.  The
latter is not very safe because moving the pg_index tuple could result in
concurrent SnapshotNow scans finding it twice or not at all, thus possibly
resulting in index corruption.  This is a pre-existing bug in CREATE INDEX
CONCURRENTLY, which was copied into the DROP code.

In addition, fix various places in the code that ought to check to make
sure that the indexes they are manipulating are valid and/or ready as
appropriate.  These represent bugs that have existed since 8.2, since
a failed CREATE INDEX CONCURRENTLY could leave a corrupt or invalid
index behind, and we ought not try to do anything that might fail with
such an index.

Also fix RelationReloadIndexInfo to ensure it copies all the pg_index
columns that are allowed to change after initial creation.  Previously we
could have been left with stale values of some fields in an index relcache
entry.  It's not clear whether this actually had any user-visible
consequences, but it's at least a bug waiting to happen.

In addition, do some code and docs review for DROP INDEX CONCURRENTLY;
some cosmetic code cleanup but mostly addition and revision of comments.

This will need to be back-patched, but in a noticeably different form,
so I'm committing it to HEAD before working on the back-patch.

Problem reported by Amit Kapila, diagnosis by Pavan Deolassee,
fix by Tom Lane and Andres Freund.

src/backend/access/heap/README.HOT

@@ -386,6 +386,34 @@ from the index, as well as ensuring that no one can see any inconsistent
 rows in a broken HOT chain (the first condition is stronger than the
 second).  Finally, we can mark the index valid for searches.
 
+Note that we do not need to set pg_index.indcheckxmin in this code path,
+because we have outwaited any transactions that would need to avoid using
+the index.  (indcheckxmin is only needed because non-concurrent CREATE
+INDEX doesn't want to wait; its stronger lock would create too much risk of
+deadlock if it did.)
+
+
+DROP INDEX CONCURRENTLY
+-----------------------
+
+DROP INDEX CONCURRENTLY is sort of the reverse sequence of CREATE INDEX
+CONCURRENTLY.  We first mark the index as not indisvalid, and then wait for
+any transactions that could be using it in queries to end.  (During this
+time, index updates must still be performed as normal, since such
+transactions might expect freshly inserted tuples to be findable.)
+Then, we clear indisready and indislive, and again wait for transactions
+that could be updating the index to end.  Finally we can drop the index
+normally (though taking only ShareUpdateExclusiveLock on its parent table).
+
+The reason we need the pg_index.indislive flag is that after the second
+wait step begins, we don't want transactions to be touching the index at
+all; otherwise they might suffer errors if the DROP finally commits while
+they are reading catalog entries for the index.  If we had only indisvalid
+and indisready, this state would be indistinguishable from the first stage
+of CREATE INDEX CONCURRENTLY --- but in that state, we *do* want
+transactions to examine the index, since they must consider it in
+HOT-safety checks.
+
 
 Limitations and Restrictions
 ----------------------------