Fix infinite loop in merl_transform

This can happen when a syntactically incorrect text is passed to a
merl:qquote/2,/3 call.

The parse transform optimizes calls to some functions in merl by
converting strings into templates at compile time. If this evaluation
fails (in eval_call/4 - for example because of a sytanx error in the
parsed text) the original function call should be kept unchanged.
However in case of qquote/3 the call is converted into a combination of
quote/2 and subst/2, but upon failure the original qquote/3 call is
substituted into the wrong place. E.g.:

this expression
  merl:qquote(Pos, Text, Env)

is first converted to
  merl:subst(merl:quote(Pos, Text), Env)

then if evaluating the quote call fails into
  merl:subst(merl:qquote(Pos, Text, Env), Env)

and the expansion is run again on the internal qquote/3 argument
resulting in an infinite loop.

This is now fixed so in case of failure the original qquote/3 call is kept.

1 files changed, 19 insertions, 2 deletions

diff --git a/lib/syntax_tools/test/merl_SUITE.erl b/lib/syntax_tools/test/merl_SUITE.erl
index 945972d405..52bbd9b3b8 100644
--- a/lib/syntax_tools/test/merl_SUITE.erl
+++ b/lib/syntax_tools/test/merl_SUITE.erl
@@ -29,12 +29,14 @@
 	 init_per_group/2,end_per_group/2]).
 
 %% Test cases
--export([merl_smoke_test/1]).
+-export([merl_smoke_test/1,
+         transform_parse_error_test/1]).
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
 all() ->
-    [merl_smoke_test].
+    [merl_smoke_test,
+     transform_parse_error_test].
 
 groups() -> 
     [].
@@ -84,6 +86,21 @@ merl_smoke_test(Config) when is_list(Config) ->
                    end)),
     ok.
 
+transform_parse_error_test(_Config) ->
+    ?assertEqual("merl:quote(\"{\")",
+                 f(merl_transform:parse_transform(
+                     [?Q("merl:quote(\"{\")")], []))),
+    ?assertEqual("merl:quote(2, \"{\")",
+                 f(merl_transform:parse_transform(
+                     [?Q("merl:quote(2, \"{\")")], []))),
+    ?assertEqual("merl:qquote(\"{\", [{var, V}])",
+                 f(merl_transform:parse_transform(
+                     [?Q("merl:qquote(\"{\", [{var, V}])")], []))),
+    ?assertEqual("merl:qquote(2, \"{\", [{var, V}])",
+                 f(merl_transform:parse_transform(
+                     [?Q("merl:qquote(2, \"{\", [{var, V}])")], []))),
+    ok.
+
 %% utilities
 
 f(Ts) when is_list(Ts) ->