Nasty bug in heap_page_prune

While working on the previously discussed refactoring of
heap_page_prune, I came to the realization that its use of
CacheInvalidateHeapTuple is not just a PANIC risk but simply wrong :-(
The semantics aren't right: inval.c assumes that it's dealing with
transactional invalidations, but what we are dealing with in a redirect
collapse is non-transactional.  Once heap_page_prune completes, the
CTID update is a done deal even if the calling transaction rolls back.
However, inval.c will think it doesn't need to broadcast any
invalidations after a failed transaction.

This is fairly difficult to show a self-contained example of, because
it can only occur if VACUUM FULL errors out after doing a page prune,
and there's no very easy way to guarantee that will happen.  I resorted
to putting an elog(ERROR) call into vacuum.c right after the scan_heap()
call.  With that, it was possible to demonstrate the problem:

regression=# create table foo(f1 int);
CREATE TABLE
regression=# select ctid,relname from pg_class where relname = 'foo';  ctid  | relname 
--------+---------(9,32) | foo
(1 row)

-- need a HOT-candidate update to the pg_class row, eg
regression=# alter table foo owner to joe;
ALTER TABLE
-- check that update is on same page, else it's not HOT
regression=# select ctid,relname from pg_class where relname = 'foo'; ctid  | relname 
--------+---------(9,33) | foo
(1 row)

-- make sure the updated tuple is in local syscache
regression=# select 'foo'::regclass;regclass 
----------foo
(1 row)

-- now, in another backend, execute intentionally broken VACUUM FULL pg_class

-- and try to alter the updated tuple again using a syscache-based operation
regression=# alter table foo owner to postgres;
server closed the connection unexpectedly       This probably means the server terminated abnormally       before or
whileprocessing the request.
 
The connection to the server was lost. Attempting reset: Failed.

The crash is here:

TRAP: FailedAssertion("!(((lp)->lp_flags == 1))", File: "heapam.c", Line: 2330)
LOG:  server process (PID 8967) was terminated by signal 6
LOG:  terminating any other active server processes

because it's trying to find the tuple at a CTID that's no longer valid.

Not sure about a clean solution to this.  I don't really want to
bastardize inval.c by making it deal with nontransactional semantics,
but there may be no other way.

Or we could forget about letting VACUUM FULL collapse out LP_REDIRECT
pointers, at least in system catalogs.  That might be the best answer
for 8.3 in any case; I am not seeing a real fix that I'd care to risk
back-patching.  (Note that this is not exactly trivial in itself, since
then vacuum.c would need at least some minimal ability to deal with
LP_REDIRECT entries.)
        regards, tom lane