Report stderr output of transport proxies

Transport proxies may communicate useful information over stderr.
Thus far stderr has been ignored. We should at the very least let
the user know that we have some stderr output (NOTICE), and log it
at INFO level. For now, we assume transport proxy configuration
won't happen over stderr; but the output is meaningful.

Fixes #9957.

src/or/transports.c

@@ -597,7 +597,10 @@ configure_proxy(managed_proxy_t *mp)
 {
   int configuration_finished = 0;
   smartlist_t *proxy_output = NULL;
+  smartlist_t *proxy_err_output = NULL;
+
   enum stream_status stream_status = 0;
+  enum stream_status err_stream_status = 0;
 
   /* if we haven't launched the proxy yet, do it now */
   if (mp->conf_state == PT_PROTO_INFANT) {
@@ -614,6 +617,31 @@ configure_proxy(managed_proxy_t *mp)
   proxy_output =
     tor_get_lines_from_handle(tor_process_get_stdout_pipe(mp->process_handle),
                               &stream_status);
+  proxy_err_output =
+    tor_get_lines_from_handle(tor_process_get_stderr_pipe(mp->process_handle),
+                              &err_stream_status);
+
+  /* Handle lines from stderr. We have to do this before checking stdout's
+   * <b>stream_status</b>, because we may jump to <em>done</em>, etc. during
+   * that check. We want to first log stderr if there's anything to log. */
+  if (proxy_err_output) {
+    log_notice(LD_GENERAL, "Managed proxy '%s' is giving some output over "
+               "the standard error pipe. This usually means that it has "
+               "encountered some error that I won't be able to understand. "
+               "You should check its log, or enable INFO-level tor log to "
+               "see what it is saying.", mp->argv[0]);
+    log_info(LD_GENERAL, "Managed proxy '%s' reports a possible error over "
+             "stderr - stderr output follows:", mp->argv[0]);
+    SMARTLIST_FOREACH_BEGIN(proxy_err_output, const char *, line) {
+      /* Should we expect any output from stderr to contribute to the proxy
+       * config process? (We assume not.) */
+      log_info(LD_GENERAL, "Managed proxy '%s' says (over stderr): '%s'",
+               mp->argv[0], line);
+      /* (If we want to just output this stream in one go, we may use
+       * get_string_from_pipe() - but this works, too.) */
+    } SMARTLIST_FOREACH_END(line);
+  }
+
   if (!proxy_output) { /* failed to get input from proxy */
     if (stream_status != IO_STREAM_EAGAIN) { /* bad stream status! */
       mp->conf_state = PT_PROTO_BROKEN;
@@ -647,6 +675,11 @@ configure_proxy(managed_proxy_t *mp)
     smartlist_free(proxy_output);
   }
 
+  if (proxy_err_output) {
+    SMARTLIST_FOREACH(proxy_err_output, char *, cp, tor_free(cp));
+    smartlist_free(proxy_err_output);
+  }
+
   return configuration_finished;
 }