Horton

Re: [HACKERS] [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: "Wood, Dan" <hexpert@amazon.com>, Peter Geoghegan <pg@bowt.ie>, pgsql-committers@postgresql.org, "Wong, Yi Wen" <yiwong@amazon.com>, Michael Paquier <michael.paquier@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
Date: 2017-11-14T08:23:18Z
Lists: pgsql-hackers 
On 2017-11-13 19:03:41 -0800, Andres Freund wrote:
> > Staring at the vacuumlazy hunk I think I might have found a related bug:
> > heap_update_tuple() just copies the old xmax to the new tuple's xmax if
> > a multixact and still running.  It does so without verifying liveliness
> > of members.  Isn't that buggy? Consider what happens if we have three
> > blocks: 1 has free space, two is being vacuumed and is locked, three is
> > full and has a tuple that's key share locked by a live tuple and is
> > updated by a dead xmax from before the xmin horizon. In that case afaict
> > the multi will be copied from the third page to the first one.  Which is
> > quite bad, because vacuum already processed it, and we'll set
> > relfrozenxid accordingly.  I hope I'm missing something here?
> 
> Trying to write a testcase for that now.

This indeed happens, but I can't quite figure out a way to write an
isolationtester test for this. The problem is that to have something
reproducible one has to make vacuum block on a cleanup lock, but that
currently doesn't register as waiting for the purpose of
isolationtester's logic.

Greetings,

Andres Freund

Commits

  1. Backport addition of rs_old_rel to rewriteheap's state.

  2. Perform a lot more sanity checks when freezing tuples.

  3. Revert bogus fixes of HOT-freezing bug

  4. Fix traversal of half-frozen update chains

  5. Fix freezing of a dead HOT-updated tuple

  6. During index build, check and elog (not just Assert) for broken HOT chain.

  7. Fix WAL replay of locking an updated tuple

  8. Change the way we mark tuples as frozen.