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From Dave Cottlehuber <d...@muse.net.nz>
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 10:29:18 GMT
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> 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().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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