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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: svn commit: r1164350 - in /couchdb/trunk: share/www/script/test/recreate_doc.js src/couchdb/couch_doc.erl src/couchdb/couch_key_tree.erl
Date Fri, 02 Sep 2011 15:47:34 GMT
I think we got this straightened out on IRC, but the key here is that
the #full_doc_info{} gets the udpate_seq+1 value, but the #doc_info{}
record that's generated in couch_db_updater:new_index_entries ends up
only consulting the leaves of the revision tree to figure out which
update_seq to remove. This was the crux of the bug. The solution was
to just fold over the entire tree looking for the largest update_seq
and use that for for the #full_doc_info{}.

On Fri, Sep 2, 2011 at 9:12 AM, Adam Kocoloski <adam.kocoloski@gmail.com> wrote:
> The part that I didn't quite get was the following
>
>> But what happens is at the point we calculate the seq to remove from docinfo_by_seq_tree,
we calculate the wrong value. Thus when the update continues we remove an update_seq that
doesn't exist in the tree and insert our new seq.
>
> Do we end up selecting the sequence from the non-leaf revision? Best,
>
> Adam
>
>
> On Friday, September 2, 2011 at 6:29 AM, Dave Cottlehuber wrote:
>
>> Paul,
>>
>> Love your work mate! Outstanding commit msg, I'm working through it (slowly).
>>
>> A+
>> D
>>
>> On 2 September 2011 16:31, <davisp@apache.org (mailto:davisp@apache.org)> wrote:
>> > Author: davisp
>> > Date: Fri Sep 2 04:31:10 2011
>> > New Revision: 1164350
>> >
>> > URL: http://svn.apache.org/viewvc?rev=1164350&view=rev
>> > Log:
>> > Fix introduction of duplicates into _changes feed
>> >
>> > When a document is updated the new update_seq is assigned as part of the
>> > rev_tree merging in couch_db_updater:merge_rev_trees/7 based on the
>> > condition of whether the new rev_tree is equal to the old tree. This
>> > equality is done as a simple Erlang term comparison. If the trees are
>> > not equal a new update_seq is assigned to the #full_doc_info{} record
>> > that is stored in fulldocinfo_by_id_btree.
>> >
>> > During replication it is possible that a document update merges into the
>> > rev_tree internally without creating a leaf. If the terminal node of the
>> > replicated path happens to land on a node with a value of ?REV_MISSING
>> > the new document information will be preferred and replace the
>> > ?REV_MISSING value.
>> >
>> > This preference ends up causing the rev_tree comparison to evaluate to
>> > false which ends up giving this document a new update_seq. Up until this
>> > point everything is fine. We write a bit of extra data (that will be
>> > cleared during compaction) because of a previous bug where we decided to
>> > be cautious and avoid losing data due to a broken rev_tree merging
>> > aglorithm. It is also important to note that this is the place were we
>> > calculate the update_seq to remove from the docinfo_by_seq_tree.
>> >
>> > After this point we get back to couch_db_udpater:update_docs_int/5 where
>> > we eventually call couch_db_updater:new_index_entries/3 which creates
>> > the new entries for the fulldocinfo_by_id_tree and docinfo_by_seq_btree.
>> > At this point we end up creating a #doc_info{} record based on the
>> > leaves in the rev_tree. As you recall, the update that caused the new
>> > update_seq was not a leaf, at this point we create a #doc_info{} record
>> > with an incorrect high_seq member pointing to the update_seq we are
>> > about to remove from the docinfo_by_seq_tree (remember we calculated the
>> > seq to remove before we consulted the leaves).
>> >
>> > The end result is that we remove the same update_seq we insert. This
>> > sets us up for the real bug. The next time we go to update this document
>> > the same procedure is applied. But what happens is at the point we
>> > calculate the seq to remove from docinfo_by_seq_tree, we calculate the
>> > wrong value. Thus when the update continues we remove an update_seq that
>> > doesn't exist in the tree and insert our new seq. But, the seq from the
>> > previous update is still in the tree. Thus, our docinfo_by_seq_tree now
>> > contains two entries pointing at the same document.
>> >
>> > At this point, we run into the observed behavior of this bug that ends
>> > up causing duplicate entries in views which then ends up throwing errors
>> > when the view is compaction. These errors are also particularly nasty
>> > because they bubble up the the couch_view gen_server which crashes and
>> > spiders out crashing every couch_view_group process. That part probably
>> > isn't important though.
>> >
>> > There's a simple test include with the patch to illustrate the behavior
>> > and maintain an assertion that it stays fixed.
>> >
>> > Fixes COUCHDB-1265
>> >
>> >
>> > Modified:
>> > couchdb/trunk/share/www/script/test/recreate_doc.js
>> > couchdb/trunk/src/couchdb/couch_doc.erl
>> > couchdb/trunk/src/couchdb/couch_key_tree.erl
>> >
>> > Modified: couchdb/trunk/share/www/script/test/recreate_doc.js
>> > URL: http://svn.apache.org/viewvc/couchdb/trunk/share/www/script/test/recreate_doc.js?rev=1164350&r1=1164349&r2=1164350&view=diff
>> > ==============================================================================
>> > --- couchdb/trunk/share/www/script/test/recreate_doc.js (original)
>> > +++ couchdb/trunk/share/www/script/test/recreate_doc.js Fri Sep 2 04:31:10 2011
>> > @@ -77,4 +77,45 @@ couchTests.recreate_doc = function(debug
>> >  } catch (e) {
>> >  T(e.error == "conflict");
>> >  }
>> > +
>> > + db.deleteDb();
>> > + db.createDb();
>> > +
>> > + // COUCHDB-1265
>> > + // Resuscitate an unavailable old revision and make sure that it
>> > + // doesn't introduce duplicates into the _changes feed.
>> > +
>> > + var doc = {_id: "bar", count: 0};
>> > + T(db.save(doc).ok);
>> > + var ghost = {_id: "bar", _rev: doc._rev, count: doc.count};
>> > + for(var i = 0; i < 2; i++) {
>> > + doc.count += 1;
>> > + T(db.save(doc).ok);
>> > + }
>> > +
>> > + // Compact so that the old revision to be resuscitated will be
>> > + // in the rev_tree as ?REV_MISSING
>> > + db.compact();
>> > + while(db.info (http://db.info)().compact_running) {}
>> > +
>> > + // Saving the ghost here puts it back in the rev_tree in such
>> > + // a way as to create a new update_seq but without changing a
>> > + // leaf revision. This would cause the #full_doc_info{} and
>> > + // #doc_info{} records to diverge in their idea of what the
>> > + // doc's update_seq is and end up introducing a duplicate in
>> > + // the _changes feed the next time this doc is updated.
>> > + T(db.save(ghost, {new_edits: false}).ok);
>> > +
>> > + // The duplicate would have been introduce here becuase the #doc_info{}
>> > + // would not have been removed correctly.
>> > + T(db.save(doc).ok);
>> > +
>> > + // And finally assert that there are no duplicates in _changes.
>> > + var req = CouchDB.request("GET", "/test_suite_db/_changes");
>> > + var resp = JSON.parse(req.responseText);
>> > + var docids = {};
>> > + for(var i = 0; i < resp.results.length; i++) {
>> > + T(docids[resp.results[i].id] === undefined, "Duplicates in _changes feed.");
>> > + docids[resp.results[i].id] = true;
>> > + }
>> > };
>> >
>> > Modified: couchdb/trunk/src/couchdb/couch_doc.erl
>> > URL: http://svn.apache.org/viewvc/couchdb/trunk/src/couchdb/couch_doc.erl?rev=1164350&r1=1164349&r2=1164350&view=diff
>> > ==============================================================================
>> > --- couchdb/trunk/src/couchdb/couch_doc.erl (original)
>> > +++ couchdb/trunk/src/couchdb/couch_doc.erl Fri Sep 2 04:31:10 2011
>> > @@ -319,10 +319,19 @@ to_doc_info(FullDocInfo) ->
>> >  {DocInfo, _Path} = to_doc_info_path(FullDocInfo),
>> >  DocInfo.
>> >
>> > -max_seq([], Max) ->
>> > - Max;
>> > -max_seq([#rev_info{seq=Seq}|Rest], Max) ->
>> > - max_seq(Rest, if Max > Seq -> Max; true -> Seq end).
>> > +max_seq(Tree) ->
>> > + FoldFun = fun({_Pos, _Key}, Value, _Type, MaxOldSeq) ->
>> > + case Value of
>> > + {_Deleted, _DiskPos, OldTreeSeq} ->
>> > + % Older versions didn't track data sizes.
>> > + erlang:max(MaxOldSeq, OldTreeSeq);
>> > + {_Deleted, _DiskPos, OldTreeSeq, _Size} ->
>> > + erlang:max(MaxOldSeq, OldTreeSeq);
>> > + _ ->
>> > + MaxOldSeq
>> > + end
>> > + end,
>> > + couch_key_tree:fold(FoldFun, 0, Tree).
>> >
>> > to_doc_info_path(#full_doc_info{id=Id,rev_tree=Tree}) ->
>> >  RevInfosAndPath = [
>> > @@ -342,7 +351,7 @@ to_doc_info_path(#full_doc_info{id=Id,re
>> >  end, RevInfosAndPath),
>> >  [{_RevInfo, WinPath}|_] = SortedRevInfosAndPath,
>> >  RevInfos = [RevInfo || {RevInfo, _Path} <- SortedRevInfosAndPath],
>> > - {#doc_info{id=Id, high_seq=max_seq(RevInfos, 0), revs=RevInfos}, WinPath}.
>> > + {#doc_info{id=Id, high_seq=max_seq(Tree), revs=RevInfos}, WinPath}.
>> >
>> >
>> >
>> >
>> > Modified: couchdb/trunk/src/couchdb/couch_key_tree.erl
>> > URL: http://svn.apache.org/viewvc/couchdb/trunk/src/couchdb/couch_key_tree.erl?rev=1164350&r1=1164349&r2=1164350&view=diff
>> > ==============================================================================
>> > --- couchdb/trunk/src/couchdb/couch_key_tree.erl (original)
>> > +++ couchdb/trunk/src/couchdb/couch_key_tree.erl Fri Sep 2 04:31:10 2011
>> > @@ -49,7 +49,7 @@
>> >
>> > -export([merge/3, find_missing/2, get_key_leafs/2, get_full_key_paths/2, get/2]).
>> > -export([get_all_leafs/1, count_leafs/1, remove_leafs/2, get_all_leafs_full/1,
stem/2]).
>> > --export([map/2, mapfold/3, map_leafs/2]).
>> > +-export([map/2, mapfold/3, map_leafs/2, fold/3]).
>> >
>> > -include("couch_db.hrl").
>> >
>> > @@ -320,6 +320,21 @@ count_leafs_simple([{_Key, _Value, SubTr
>> >  count_leafs_simple(SubTree) + count_leafs_simple(RestTree).
>> >
>> >
>> > +fold(_Fun, Acc, []) ->
>> > + Acc;
>> > +fold(Fun, Acc0, [{Pos, Tree}|Rest]) ->
>> > + Acc1 = fold_simple(Fun, Acc0, Pos, [Tree]),
>> > + fold(Fun, Acc1, Rest).
>> > +
>> > +fold_simple(_Fun, Acc, _Pos, []) ->
>> > + Acc;
>> > +fold_simple(Fun, Acc0, Pos, [{Key, Value, SubTree} | RestTree]) ->
>> > + Type = if SubTree == [] -> leaf; true -> branch end,
>> > + Acc1 = Fun({Pos, Key}, Value, Type, Acc0),
>> > + Acc2 = fold_simple(Fun, Acc1, Pos+1, SubTree),
>> > + fold_simple(Fun, Acc2, Pos, RestTree).
>> > +
>> > +
>> > map(_Fun, []) ->
>> >  [];
>> > map(Fun, [{Pos, Tree}|Rest]) ->
>
>
>

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