feat(tpl): make escaping mechanism more foolproof

Treat "${$}" as literal "$". This allows to template express
strings, for example, of the form "${some_var_value}" where
`some_var_value` is interpolated from bindings.

apps/emqx_auth/src/emqx_authn/emqx_authn_utils.erl

@@ -133,7 +133,7 @@ handle_disallowed_placeholders(Template, Source) ->
                 allowed => #{placeholders => ?ALLOWED_VARS},
                 notice =>
                     "Disallowed placeholders will be rendered as is."
-                    " However, consider using `$${...}` escaping for literal `${...}` where"
+                    " However, consider using `${$}` escaping for literal `$` where"
                     " needed to avoid unexpected results."
             }),
             Result = prerender_disallowed_placeholders(Template),
@@ -153,7 +153,7 @@ prerender_disallowed_placeholders(Template) ->
             % parse as a literal string.
             case lists:member(Name, ?ALLOWED_VARS) of
                 true -> "${" ++ Name ++ "}";
-                false -> "$${" ++ Name ++ "}"
+                false -> "${$}{" ++ Name ++ "}"
             end
         end
     }),

apps/emqx_auth/src/emqx_authz/emqx_authz_utils.erl

@@ -136,7 +136,7 @@ handle_disallowed_placeholders(Template, Source, Allowed) ->
                 allowed => #{placeholders => Allowed},
                 notice =>
                     "Disallowed placeholders will be rendered as is."
-                    " However, consider using `$${...}` escaping for literal `${...}` where"
+                    " However, consider using `${$}` escaping for literal `$` where"
                     " needed to avoid unexpected results."
             }),
             Result = prerender_disallowed_placeholders(Template, Allowed),
@@ -156,7 +156,7 @@ prerender_disallowed_placeholders(Template, Allowed) ->
             % parse as a literal string.
             case lists:member(Name, Allowed) of
                 true -> "${" ++ Name ++ "}";
-                false -> "$${" ++ Name ++ "}"
+                false -> "${$}{" ++ Name ++ "}"
             end
         end
     }),

apps/emqx_connector/src/emqx_connector_template.erl

@@ -42,7 +42,7 @@
 
 -type t() :: str() | {'$tpl', deeptpl()}.
 
--type str() :: [unicode:chardata() | placeholder()].
+-type str() :: [iodata() | byte() | placeholder()].
 -type deep() :: {'$tpl', deeptpl()}.
 
 -type deeptpl() ::
@@ -76,7 +76,8 @@
     var_trans => var_trans()
 }.
 
--define(RE_PLACEHOLDER, "\\$(\\$?)\\{[.]?([a-zA-Z0-9._]*)\\}").
+-define(RE_PLACEHOLDER, "\\$\\{[.]?([a-zA-Z0-9._]*)\\}").
+-define(RE_ESCAPE, "\\$\\{(\\$)\\}").
 
 %% @doc Parse a unicode string into a template.
 %% String might contain zero or more of placeholders in the form of `${var}`,
@@ -95,22 +96,21 @@ parse(String, Opts) ->
     RE =
         case Opts of
             #{strip_double_quote := true} ->
-                <<"((?|" ?RE_PLACEHOLDER "|\"" ?RE_PLACEHOLDER "\"))">>;
+                <<"((?|" ?RE_PLACEHOLDER "|\"" ?RE_PLACEHOLDER "\")|" ?RE_ESCAPE ")">>;
             #{} ->
-                <<"(" ?RE_PLACEHOLDER ")">>
+                <<"(" ?RE_PLACEHOLDER "|" ?RE_ESCAPE ")">>
         end,
     Splits = re:split(String, RE, [{return, binary}, group, trim, unicode]),
     Components = lists:flatmap(fun parse_split/1, Splits),
     Components.
 
-parse_split([Part, _PH, <<>>, Var]) ->
+parse_split([Part, _PH, Var, <<>>]) ->
     % Regular placeholder
     prepend(Part, [{var, unicode:characters_to_list(Var), parse_accessor(Var)}]);
-parse_split([Part, _PH = <<B1, $$, Rest/binary>>, <<"$">>, _]) ->
-    % Escaped literal, take all but the second byte, which is always `$`.
-    % Important to make a whole token starting with `$` so the `unparse/11`
-    % function can distinguish escaped literals.
-    prepend(Part, [<<B1, Rest/binary>>]);
+parse_split([Part, _Escape, <<>>, <<"$">>]) ->
+    % Escaped literal `$`.
+    % Use single char as token so the `unparse/1` function can distinguish escaped `$`.
+    prepend(Part, [$$]);
 parse_split([Tail]) ->
     [Tail].
 
@@ -159,8 +159,8 @@ unparse(Template) ->
 
 unparse_part({var, Name, _Accessor}) ->
     render_placeholder(Name);
-unparse_part(Part = <<"${", _/binary>>) ->
-    <<"$", Part/binary>>;
+unparse_part($$) ->
+    <<"${$}">>;
 unparse_part(Part) ->
     Part.
 

apps/emqx_connector/test/emqx_connector_template_SUITE.erl

@@ -115,7 +115,7 @@ t_render_missing_bindings(_) ->
     ).
 
 t_unparse(_) ->
-    TString = <<"a:${a},b:${b},c:$${c},d:{${d.d1}}">>,
+    TString = <<"a:${a},b:${b},c:$${c},d:{${d.d1}},e:${$}{e},lit:${$}{$}">>,
     Template = emqx_connector_template:parse(TString),
     ?assertEqual(
         TString,
@@ -129,12 +129,14 @@ t_const(_) ->
     ),
     ?assertEqual(
         false,
-        emqx_connector_template:is_const(emqx_connector_template:parse(<<"a:${a},b:${b},c:$${c}">>))
+        emqx_connector_template:is_const(
+            emqx_connector_template:parse(<<"a:${a},b:${b},c:${$}{c}">>)
+        )
     ),
     ?assertEqual(
         true,
         emqx_connector_template:is_const(
-            emqx_connector_template:parse(<<"a:$${a},b:$${b},c:$${c}">>)
+            emqx_connector_template:parse(<<"a:${$}{a},b:${$}{b}">>)
         )
     ).
 
@@ -147,16 +149,16 @@ t_render_partial_ph(_) ->
     ).
 
 t_parse_escaped(_) ->
-    Bindings = #{a => <<"1">>, b => 1},
-    Template = emqx_connector_template:parse(<<"a:${a},b:$${b}">>),
+    Bindings = #{a => <<"1">>, b => 1, c => "VAR"},
+    Template = emqx_connector_template:parse(<<"a:${a},b:${$}{b},c:${$}{${c}},lit:${$}{$}">>),
     ?assertEqual(
-        <<"a:1,b:${b}">>,
+        <<"a:1,b:${b},c:${VAR},lit:${$}">>,
         render_strict_string(Template, Bindings)
     ).
 
 t_parse_escaped_dquote(_) ->
     Bindings = #{a => <<"1">>, b => 1},
-    Template = emqx_connector_template:parse(<<"a:\"${a}\",b:\"$${b}\"">>, #{
+    Template = emqx_connector_template:parse(<<"a:\"${a}\",b:\"${$}{b}\"">>, #{
         strip_double_quote => true
     }),
     ?assertEqual(
@@ -299,7 +301,7 @@ t_render_tmpl_deep(_) ->
     Bindings = #{a => <<"1">>, b => 1, c => 1.0, d => #{d1 => <<"hi">>}},
 
     Template = emqx_connector_template:parse_deep(
-        #{<<"${a}">> => [<<"${b}">>, "c", 2, 3.0, '${d}', {[<<"${c}">>, <<"$${d}">>], 0}]}
+        #{<<"${a}">> => [<<"$${b}">>, "c", 2, 3.0, '${d}', {[<<"${c}">>, <<"${$}{d}">>], 0}]}
     ),
 
     ?assertEqual(
@@ -308,12 +310,12 @@ t_render_tmpl_deep(_) ->
     ),
 
     ?assertEqual(
-        #{<<"1">> => [<<"1">>, "c", 2, 3.0, '${d}', {[<<"1.0">>, <<"${d}">>], 0}]},
+        #{<<"1">> => [<<"$1">>, "c", 2, 3.0, '${d}', {[<<"1.0">>, <<"${d}">>], 0}]},
         emqx_connector_template:render_strict(Template, Bindings)
     ).
 
 t_unparse_tmpl_deep(_) ->
-    Term = #{<<"${a}">> => [<<"$${b}">>, "c", 2, 3.0, '${d}', {[<<"${c}">>], 0}]},
+    Term = #{<<"${a}">> => [<<"$${b}">>, "c", 2, 3.0, '${d}', {[<<"${c}">>], <<"${$}{d}">>, 0}]},
     Template = emqx_connector_template:parse_deep(Term),
     ?assertEqual(Term, emqx_connector_template:unparse(Template)).