Compojure: Trap 500 URL Decoding Error - clojure

Question

I have a web service in Compojure with one route that looks like this:
"/myapp/dosomething/:input"
This works well when :input is something normal for the app to handle, such as a word, a string of digits etc, but when garbage is put in, such as
GET /myapp/dosomething/%25%24%25%5E%24%25%5E%25%24%5E
I get a 500 error. My question is, how to I trap this, and return 400 instead?
HTTP ERROR 500
Problem accessing /myapp/dosomething/%25%24%25%5E%24%25%5E%25%24%5E. Reason:
Server Error
Caused by:
java.lang.IllegalArgumentException: URLDecoder: Illegal hex characters in escape (%) pattern - For input string: "$%"
at java.net.URLDecoder.decode(URLDecoder.java:192)
at clout.core$path_decode.invoke(core.clj:33)
at clout.core$path_decode.invoke(core.clj:31)
at clojure.core$map$fn__4207.invoke(core.clj:2485)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:60)
at clojure.lang.RT.seq(RT.java:484)
at clojure.core$seq.invoke(core.clj:133)
at clojure.core$map$fn__4211.invoke(core.clj:2490)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:60)
at clojure.lang.RT.seq(RT.java:484)
at clojure.core$seq.invoke(core.clj:133)
at clojure.core.protocols$seq_reduce.invoke(protocols.clj:30)
at clojure.core.protocols$fn__6026.invoke(protocols.clj:54)
at clojure.core.protocols$fn__5979$G__5974__5992.invoke(protocols.clj:13)
at clojure.core$reduce.invoke(core.clj:6177)
at clout.core$assoc_keys_with_groups.invoke(core.clj:54)
at clout.core.CompiledRoute.route_matches(core.clj:84)
at compojure.core$if_route$fn__472.invoke(core.clj:38)
at compojure.core$if_method$fn__465.invoke(core.clj:24)
at compojure.core$routing$fn__490.invoke(core.clj:106)
at clojure.core$some.invoke(core.clj:2443)
at compojure.core$routing.doInvoke(core.clj:106)
at clojure.lang.RestFn.applyTo(RestFn.java:139)
at clojure.core$apply.invoke(core.clj:619)
at compojure.core$routes$fn__494.invoke(core.clj:111)
at ring.middleware.keyword_params$wrap_keyword_params$fn__710.invoke(keyword_params.clj:27)
at ring.middleware.nested_params$wrap_nested_params$fn__749.invoke(nested_params.clj:65)
at ring.middleware.params$wrap_params$fn__682.invoke(params.clj:55)
at ring.middleware.multipart_params$wrap_multipart_params$fn__777.invoke(multipart_params.clj:103)
at ring.middleware.flash$wrap_flash$fn__1064.invoke(flash.clj:14)
at ring.middleware.session$wrap_session$fn__1055.invoke(session.clj:40)
at ring.middleware.cookies$wrap_cookies$fn__986.invoke(cookies.clj:160)
at vinws_chrome.servlet$_service$fn__116.invoke(servlet.clj:1)
at ring.util.servlet$make_service_method$fn__54.invoke(servlet.clj:145)

Answer 1

This issue occurs in war files generated by the Lein-Ring plugin, and has been recently fixed in Lein-Ring 0.8.6 as a result of this report.
The cause has to do with the difference in how Java Servlets and Ring deal with the path-info field. The Java Servlet specification has the context path url-encoded, but the path info is decoded. Ring treats both the :context and :path-info keys as url-encoded.