Avoid re-compiling filter view functions

[nickva]

Filter view functions feature re-uses view map function for filtering _changes feeds. Instead of accumulating emitted KVs, it uses custom emit() function which just toggles a flag. However, in order to use this optimisation, the function is compiled first with the regular emit function, then the function source is queried with a non-portable toSource() method, and re-compiled again with a new sandbox where emit is overridden.

Instead of reparsing and re-compiling it, pass the sandbox to the compile function and compile filter views with that correct sandbox to start with. Moreover, this helps remove another non-portable function call.

[jaydoane]

Not a JS expert by any means, but this seems like a nice optimization, and I'm assuming there are many tests in CI that exercise this code already.

[nickva]

Good point @jaydoane. I knew about the new chttpd tests but I double-checked to verify we had a bunch more as well:

couchdb/src/chttpd/test/eunit/chttpd_changes_test.erl

Lines 485 to 493 in 1381631

	t_view_filter_no_match({_, DbUrl}) ->
	DDocId = "_design/views",
	ViewFun = <<"function(doc) {if (doc._id == 'docX') {emit(1, 1);}}">>,
	DDoc = #{<<"views">> => #{<<"v">> => #{<<"map">> => ViewFun}}},
	DDocUrl = DbUrl ++ "/" ++ DDocId,
	{_, #{<<"rev">> := Rev, <<"ok">> := true}} = req(put, DDocUrl, DDoc),
	Params = "?filter=_view&view=views/v",
	?assertEqual({8, 0, []}, changes(DbUrl, Params)),
	{200, #{}} = req(delete, DDocUrl ++ "?rev=" ++ binary_to_list(Rev)).

couchdb/src/couch_replicator/test/eunit/couch_replicator_filtered_tests.erl

Lines 119 to 139 in 1407a51

	should_succeed_with_view({_Ctx, {Source, Target}}) ->
	create_docs(Source),
	RepObject =
	{[
	{<<"source">>, db_url(Source)},
	{<<"target">>, db_url(Target)},
	{<<"filter">>, <<"_view">>},
	{<<"query_params">>,
	{[
	{<<"view">>, <<"filter_ddoc/mammals">>}
	]}}
	]},
	replicate(RepObject),
	FilterFun = fun(_DocId, #doc{body = {Props}}) ->
	couch_util:get_value(<<"class">>, Props) == <<"mammal">>
	end,
	{TargetDocCount, AllReplies} = compare_dbs(Source, Target, FilterFun),
	% Target DB has proper number of docs
	?assertEqual(1, TargetDocCount),
	% All the docs filtered as expected
	?assert(lists:all(fun(Valid) -> Valid end, AllReplies)).

couchdb/test/elixir/test/changes_test.exs

Lines 110 to 118 in adf1714

	@tag :with_db
	test "map function filtered changes", context do
	db_name = context[:db_name]
	create_filters_view(db_name)
	create_doc(db_name, %{_id: "blah", bop: "plankton"})
	resp = Couch.get("/#{db_name}/_changes?filter=_view&view=changes_filter/blah")
	assert length(resp.body["results"]) == 1
	assert Enum.at(resp.body["results"], 0)["id"] == "blah"
	end