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, 15 insertions, 0 deletions

diff --git a/lib/syntax_tools/src/merl_tests.erl b/lib/syntax_tools/src/merl_tests.erl
index c1aae3100e..27db594050 100644
--- a/lib/syntax_tools/src/merl_tests.erl
+++ b/lib/syntax_tools/src/merl_tests.erl
@@ -48,6 +48,21 @@ parse_error_test_() ->
                    f(merl:quote("{")))
     ].
 
+transform_parse_error_test_() ->
+    [?_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}])")], [])))
+    ].
+
 term_test_() ->
     [?_assertEqual(tuple, erl_syntax:type(merl:term({}))),
      ?_assertEqual("{foo, 42}", f(merl:term({foo, 42})))