Add support for custom xref queries

The custom queries are configured in rebar.config via the tuple
{xref_queries, [{query(), query_result()},...]}.  The implementation
passes the query() string to xref:q and compares the return value with
query_result(). It will result in an error if they do not match.

The following configuration, for example, is the same as running the
xref check undefined_function_calls. It additionally filters
ejabberd_logger:*_msg/4 from the result as these functions are generated
on execution by ejabberd and not available at compile time.

{xref_queries, [{"(XC - UC) || (XU - X - B -
                 (\"ejabberd_logger\":\".*_msg\"/\"4\"))",[]}]}.

This patch also modifies the build process of this package by running a
custom query instead of doing a diff against a static xref_warning file.

Makefile

@@ -11,11 +11,8 @@ debug:
 
 check: debug xref dialyzer
 
-xref: xref_warnings
-	@diff -U0 xref_reference xref_warnings
-
-xref_warnings:
-	-@./rebar xref > xref_warnings
+xref:
+	-@./rebar xref
 
 dialyzer: dialyzer_warnings
 	@diff -U0 dialyzer_reference dialyzer_warnings

rebar.config

@@ -3,4 +3,8 @@
 
 {app_bin, ["priv/rebar"]}.
 {erl_opts, [warnings_as_errors]}.
-{xref_checks, [undefined_function_calls]}.
+{xref_checks, []}.
+{xref_queries,
+  [{"(XC - UC) || (XU - X - B
+      - (\"escript\":\"foldl\"/\"3\")
+      - (\"abnfc\":\"file\"/\"2\"))",[]}]}.

rebar.config.sample

@@ -151,5 +151,15 @@
 %% == xref ==
 
 {xref_warnings, false}.
+
 %% xref checks to run
 {xref_checks, [exports_not_used, undefined_function_calls]}.
+
+%% Optional custom xref queries (xref manual has details) specified as
+%%     {xref_queries, [{query_string(), expected_query_result()},...]}
+%% The following for example removes all references to ejabberd:*_msg/4
+%% functions from undefined external function calls as those are in a
+%% generated module
+{xref_queries,
+  [{"(XC - UC) || (XU - X - B"
+    " - (\"ejabberd_logger\":\".*_msg\"/\"4\"))",[]}]}.

src/rebar_xref.erl

@@ -77,13 +77,19 @@ xref(Config, _) ->
             false ->
                 true
         end,
+
+    %% Run custom queries
+    QueryChecks = rebar_config:get(Config, xref_queries, []),
+    QueryNoWarn = lists:all(fun check_query/1, QueryChecks),
+
     %% Restore the original code path
     true = code:set_path(OrigPath),
 
     %% Stop xref
     stopped = xref:stop(xref),
 
-    case lists:all(fun(NoWarn) -> NoWarn end, [ExportsNoWarn, UndefNoWarn]) of
+    case lists:all(fun(NoWarn) -> NoWarn end,
+                   [ExportsNoWarn, UndefNoWarn, QueryNoWarn]) of
         true ->
             ok;
         false ->
@@ -115,6 +121,17 @@ check_undefined_function_calls() ->
       end, UndefinedCalls),
     UndefinedCalls =:= [].
 
+check_query({Query, Value}) ->
+    {ok, Answer} = xref:q(xref, Query),
+    case Answer =:= Value of
+        false ->
+            ?CONSOLE("Query ~s~n answer ~p~n did not match ~p~n",
+                     [Query, Answer, Value]),
+            false;
+        _     ->
+            true
+    end.
+
 code_path() ->
     [P || P <- code:get_path(),
           filelib:is_dir(P)] ++ [filename:join(rebar_utils:get_cwd(), "ebin")].