net/url: avoid failure on bad proxy environment variable

Related to #2663

pkgs/net-test/tests/net/bad-proxy-envvar.rkt

@@ -0,0 +1,11 @@
+#lang racket/base
+(parameterize ([current-environment-variables
+                (environment-variables-copy
+                 (current-environment-variables))]
+               [current-namespace (make-base-namespace)]
+               [current-logger (make-logger)])
+  ;; There are not a lot of strings that `url->string` rejects, but
+  ;; this one of them:
+  (putenv "http_proxy" "http://example.com:8080 ")
+  ;; Dynamic load so that we get to change the logger first
+  ((dynamic-require 'net/url 'current-proxy-servers)))

racket/collects/net/url.rkt

@@ -29,6 +29,12 @@
                                 "https"
                                 "git"))
 
+;; Like `string->url`, but returns #f for an error, intended for use
+;; with pattern matching.
+(define (string->url* str)
+  (with-handlers ([url-exception? (lambda (exn) #f)])
+    (string->url str)))
+
 ;; env->c-p-s-entries: (listof (listof string)) -> (listof (list string string num))
 ;;
 ;; "http" protocol is proxied by http proxy
@@ -41,11 +47,11 @@
     (match (getenv envvar)
       [#f #f]
       ["" null]
-      [(app string->url
+      [(app string->url*
             (url (and proxying-scheme "http") #f (? string? host) (? integer? port)
                  _ (list) (list) #f))
        (list (list proxied-scheme host port))]
-      [(app string->url
+      [(app string->url*
             (url (and proxying-scheme "http") _ (? string? host) (? integer? port)
                  _ _ _ _))
        (log-net/url-warning "~s contains somewhat invalid proxy URL format" envvar)