Warn for bad combination of cookie options

Setting CookieAuthFileGroupReadable but without setting CookieAuthFile makes
no sense, because unix directory permissions for the data directory prevent
the group from accessing the file anyways.

changes/bug1843

@@ -0,0 +1,4 @@
+  o Minor features:
+    - Warn when CookieAuthFileGroupReadable is set but CookieAuthFile is
+      not. This would lead to a cookie that is still not group readable.
+      Closes bug 1843. Suggested by katmagic.

src/or/config.c

@@ -3501,6 +3501,13 @@ options_validate(or_options_t *old_options, or_options_t *options,
              "upgrade your Tor controller as soon as possible.");
   }
 
+  if (options->CookieAuthFileGroupReadable && !options->CookieAuthFile) {
+    log_warn(LD_CONFIG, "You set the CookieAuthFileGroupReadable but did "
+             "not configure a the path for the cookie file via "
+             "CookieAuthFile. This means your cookie will not be group "
+             "readable.");
+  }
+
   if (options->UseEntryGuards && ! options->NumEntryGuards)
     REJECT("Cannot enable UseEntryGuards with NumEntryGuards set to 0");