Provide a more accurate size check for max_document_size limit

max_document_size currently checks document sizes based on Erlang's external
term size of the jiffy-decoded document body. This makes sense because that's
what used to store the data on disk and it's what manipulated by the CouchDB
internals.

However erlang term size is not always a good approximation of the size of json
encoded data. Sometimes it can be way off (I've seen 30% off) and It's hard for
users to estimate or check the external term size beforehand. So for example if
max_document_size is 1MB, CouchDB might reject user's 600KB json document
because Erlang's external term size of that document greater than 1MB.

To fix the issue provide a module which calculates the encoded size of a json
document. The size calculation approximates as well, since there is no
canonical json size as it depends on the encoder used.

Issue apache#659

src/chttpd/test/chttpd_db_doc_size_tests.erl

@@ -93,7 +93,7 @@ put_single_doc(Url) ->
 
 bulk_doc(Url) ->
     NewDoc = "{\"docs\": [{\"doc1\": 1}, {\"errordoc\":
-        \"this_should_be_the_error_document\"}]}",
+        \"this_should_be_the_too_large_error_document\"}]}",
     {ok, _, _, ResultBody} = test_request:post(Url ++ "/_bulk_docs/",
         [?CONTENT_JSON, ?AUTH], NewDoc),
     ResultJson = ?JSON_DECODE(ResultBody),

src/couch/src/couch_doc.erl

@@ -128,7 +128,7 @@ doc_to_json_obj(#doc{id=Id,deleted=Del,body=Body,revs={Start, RevIds},
 from_json_obj_validate(EJson) ->
     MaxSize = config:get_integer("couchdb", "max_document_size", 4294967296),
     Doc = from_json_obj(EJson),
-    case erlang:external_size(Doc#doc.body) =< MaxSize of
+    case couch_ejson_size:encoded_size(Doc#doc.body) =< MaxSize of
         true ->
              validate_attachment_sizes(Doc#doc.atts),
              Doc;

src/couch/src/couch_ejson_size.erl

@@ -0,0 +1,99 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+-module(couch_ejson_size).
+
+-export([encoded_size/1]).
+
+
+%% Compound objects
+
+encoded_size({[]}) ->
+    2;  % opening { and closing }
+
+encoded_size({KVs}) ->
+    % Would add 2 because opening { and closing }, but then inside the LC
+    % would accumulate an extra , at the end so subtract 2 - 1
+    1 + lists:sum([encoded_size(K) + encoded_size(V) + 2 || {K,V} <- KVs]);
+
+encoded_size([]) ->
+    2;  % opening [ and closing ]
+
+encoded_size(List) when is_list(List) ->
+    % 2 is for [ and ] but inside LC would accumulate an extra , so subtract
+    % 2 - 1
+    1 + lists:sum([encoded_size(V) + 1 || V <- List]);
+
+%% Floats.
+
+encoded_size(0.0) ->
+    3;
+
+encoded_size(1.0) ->
+    3;
+
+encoded_size(Float) when is_float(Float), Float < 0.0 ->
+    encoded_size(-Float) + 1;
+
+encoded_size(Float) when is_float(Float), Float < 1.0 ->
+    if
+        Float =< 1.0e-300 -> 3;  % close enough to 0.0
+        Float =< 1.0e-100 -> 6;  % Xe-YYY
+        Float =< 1.0e-10  -> 5;  % Xe-YY
+        Float =< 0.01     -> 4;  % Xe-Y, 0.0X
+        true              -> 3   % 0.X
+    end;
+
+encoded_size(Float) when is_float(Float) ->
+    if
+        Float >= 1.0e100  -> 5;  % XeYYY
+        Float >= 1.0e10   -> 4;  % XeYY
+        true              -> 3   % XeY, X.Y
+    end;
+
+%% Integers
+
+encoded_size(0) ->
+    1;
+
+encoded_size(Integer) when is_integer(Integer), Integer < 0 ->
+    encoded_size(-Integer) + 1;
+
+encoded_size(Integer) when is_integer(Integer) ->
+    if
+        Integer < 10    -> 1;
+        Integer < 100   -> 2;
+        Integer < 1000  -> 3;
+        Integer < 10000 -> 4;
+        true           -> trunc(math:log10(Integer)) + 1
+    end;
+
+%% Strings
+
+encoded_size(Binary) when is_binary(Binary) ->
+    2 + byte_size(Binary);
+
+%% Special terminal symbols as atoms
+
+encoded_size(null) ->
+    4;
+
+encoded_size(true) ->
+    4;
+
+encoded_size(false) ->
+    5;
+
+%% Other atoms
+
+encoded_size(Atom) when is_atom(Atom) ->
+    encoded_size(atom_to_binary(Atom, utf8)).

src/couch/test/couch_ejson_size_tests.erl

@@ -0,0 +1,72 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+-module(couch_ejson_size_tests).
+
+-include_lib("eunit/include/eunit.hrl").
+
+-define(HWAIR, $\x{10348}).  % 4 byte utf8 encoding
+-define(EURO, $\x{20ac}).  % 3 byte utf8 encoding
+-define(CENT, $\x{a2}).  % 2 byte utf8 encoding
+
+
+ejson_size_test_() ->
+    [?_assertEqual(R, couch_ejson_size:encoded_size(Input)) || {R, Input} <- [
+        {1, 1}, {1, 1}, {2, -1}, {1, 9}, {2, 10}, {3, -10},
+        {2, 11}, {2, 99}, {3, 100}, {3, 999}, {4, 1000}, {4, 9999},
+        {5, 10000},
+
+        {3, 0.0}, {3, 0.1}, {3, 1.0}, {4, -1.0}, {3, 1.0e9},
+        {4, 1.0e10}, {5, 1.0e-10}, {5, 1.0e-99}, {6, 1.0e-100}, {3, 1.0e-323},
+
+        {2, arr_nested(0)}, {22, arr_nested(10)}, {2002, arr_nested(1000)},
+        {9, obj_nested(0)}, {69, obj_nested(10)}, {6009, obj_nested(1000)},
+
+        {4, null}, {4, true}, {5, false},
+
+        {3, str(1, $x)}, {4, str(1, ?CENT)}, {5, str(1, ?EURO)},
+        {6, str(1, ?HWAIR)}, {3, str(1, $\x{1})}, {12, str(10, $x)},
+        {22, str(10, ?CENT)}, {32, str(10, ?EURO)}, {42, str(10, ?HWAIR)},
+        {12, str(10, $\x{1})}
+   ]].
+
+
+%% Helper functions
+
+arr_nested(MaxDepth) ->
+    arr_nested(MaxDepth, 0).
+
+
+obj_nested(MaxDepth) ->
+    obj_nested(MaxDepth, 0).
+
+
+obj(N, K, V) ->
+    {[{K, V} || _ <- lists:seq(1, N)]}.
+
+
+str(N, C) ->
+    unicode:characters_to_binary([C || _ <- lists:seq(1, N)]).
+
+
+arr_nested(MaxDepth, MaxDepth) ->
+    [];
+
+arr_nested(MaxDepth, Depth) ->
+    [arr_nested(MaxDepth, Depth + 1)].
+
+
+obj_nested(MaxDepth, MaxDepth) ->
+    obj(1, <<"k">>, <<"v">>);
+
+obj_nested(MaxDepth, Depth) ->
+    {[{<<"k">>, obj_nested(MaxDepth, Depth + 1)}]}.