Do not forward the 'tried=' CGI within the proxy interface.

This may cause the proxy to respond with 'file not found', causing
upstream redirectors to believe the file is not present behind
the proxy at all - even if it's just 'not found' because the only
source was excluded.

src/XrdPss/XrdPss.cc

@@ -973,9 +973,26 @@ const char *XrdPssSys::P2CGI(int &cgilen, char *cbuff, int cblen,
    if (!Cgi1) {cgilen = strlen(Cgi2); return Cgi2;}
    if (!Cgi2) return Cgi1;
 
-// Compose the two cgi elements together
-//
-   cgilen = snprintf(cbuff, cblen, "%s&%s", Cgi1, Cgi2);
+// Strip out any 'tried='
+// Then, Compose the two cgi elements together
+   const char * tried_loc = strstr(Cgi1, "tried=");
+   if (tried_loc != NULL) {
+      const char *next_amp = strchr(tried_loc, '&');
+      size_t bytes_to_copy = tried_loc-Cgi1;
+      if (bytes_to_copy >= static_cast<size_t>(cblen)) {return NULL;}
+      memcpy(cbuff, Cgi1, bytes_to_copy);
+      cgilen = bytes_to_copy;
+      if (next_amp)
+      {
+          cgilen += snprintf(cbuff+bytes_to_copy, cblen-bytes_to_copy, "%s&%s", next_amp, Cgi2);
+      }
+      else
+      {
+          cgilen += snprintf(cbuff+bytes_to_copy, cblen-bytes_to_copy, "&%s", Cgi2);
+      }
+   } else {
+      cgilen = snprintf(cbuff, cblen, "%s&%s", Cgi1, Cgi2);
+   }
    return (cgilen >= cblen ? 0 : cbuff);
 }