Reject torrc if RunAsDaemon is given with relative paths.

The chdir() call in RunAsDaemon makes the behavior here surprising,
and either way of trying to resolve the surprise seems sure to
startle a significant fraction of users.  Instead, let's refuse to
guess, and refuse these configurations.

Closes ticket 22731.

changes/bug22731

@@ -0,0 +1,5 @@
+  o Minor features (relay, configuration):
+    - Reject attempts to use relative file paths when RunAsDaemon is set.
+      Previously, Tor would accept these, but the directory-changing step
+      of RunAsDaemon would give strange and/or confusing results.
+      Closes ticket 22731.

src/or/config.c

@@ -2843,8 +2843,10 @@ options_validate_cb(void *old_options, void *options, void *default_options,
  * an absolute path that <b>filepath</b> will resolve to.
  *
  * In case <b>filepath</b> is absolute, do nothing.
+ *
+ * Return 1 if there were relative paths; 0 otherwise.
  */
-static void
+static int
 warn_if_option_path_is_relative(const char *option,
                                 char *filepath)
 {
@@ -2853,39 +2855,45 @@ warn_if_option_path_is_relative(const char *option,
     COMPLAIN("Path for %s (%s) is relative and will resolve to %s."
              " Is this what you wanted?", option, filepath, abs_path);
     tor_free(abs_path);
+    return 1;
   }
+  return 0;
 }
 
 /** Scan <b>options</b> for occurances of relative file/directory
  * path and log a warning whenever it is found.
+ *
+ * Return 1 if there were relative paths; 0 otherwise.
  */
-static void
+static int
 warn_about_relative_paths(or_options_t *options)
 {
   tor_assert(options);
+  int n = 0;
 
-  warn_if_option_path_is_relative("CookieAuthFile",
-                                  options->CookieAuthFile);
-  warn_if_option_path_is_relative("ExtORPortCookieAuthFile",
-                                  options->ExtORPortCookieAuthFile);
-  warn_if_option_path_is_relative("DirPortFrontPage",
-                                  options->DirPortFrontPage);
-  warn_if_option_path_is_relative("V3BandwidthsFile",
-                                  options->V3BandwidthsFile);
-  warn_if_option_path_is_relative("ControlPortWriteToFile",
-                                  options->ControlPortWriteToFile);
-  warn_if_option_path_is_relative("GeoIPFile",options->GeoIPFile);
-  warn_if_option_path_is_relative("GeoIPv6File",options->GeoIPv6File);
-  warn_if_option_path_is_relative("Log",options->DebugLogFile);
-  warn_if_option_path_is_relative("AccelDir",options->AccelDir);
-  warn_if_option_path_is_relative("DataDirectory",options->DataDirectory);
-  warn_if_option_path_is_relative("PidFile",options->PidFile);
+  n += warn_if_option_path_is_relative("CookieAuthFile",
+                                       options->CookieAuthFile);
+  n += warn_if_option_path_is_relative("ExtORPortCookieAuthFile",
+                                       options->ExtORPortCookieAuthFile);
+  n += warn_if_option_path_is_relative("DirPortFrontPage",
+                                       options->DirPortFrontPage);
+  n += warn_if_option_path_is_relative("V3BandwidthsFile",
+                                       options->V3BandwidthsFile);
+  n += warn_if_option_path_is_relative("ControlPortWriteToFile",
+                                       options->ControlPortWriteToFile);
+  n += warn_if_option_path_is_relative("GeoIPFile",options->GeoIPFile);
+  n += warn_if_option_path_is_relative("GeoIPv6File",options->GeoIPv6File);
+  n += warn_if_option_path_is_relative("Log",options->DebugLogFile);
+  n += warn_if_option_path_is_relative("AccelDir",options->AccelDir);
+  n += warn_if_option_path_is_relative("DataDirectory",options->DataDirectory);
+  n += warn_if_option_path_is_relative("PidFile",options->PidFile);
 
   for (config_line_t *hs_line = options->RendConfigLines; hs_line;
        hs_line = hs_line->next) {
     if (!strcasecmp(hs_line->key, "HiddenServiceDir"))
-      warn_if_option_path_is_relative("HiddenServiceDir",hs_line->value);
+      n += warn_if_option_path_is_relative("HiddenServiceDir",hs_line->value);
   }
+  return n != 0;
 }
 
 /* Validate options related to single onion services.
@@ -2989,7 +2997,11 @@ options_validate(or_options_t *old_options, or_options_t *options,
    * Always use the value of UseEntryGuards, not UseEntryGuards_option. */
   options->UseEntryGuards = options->UseEntryGuards_option;
 
-  warn_about_relative_paths(options);
+  if (warn_about_relative_paths(options) && options->RunAsDaemon) {
+    REJECT("You have specified at least one relative path (see above) "
+           "with the RunAsDaemon option. RunAsDaemon is not compatible "
+           "with relative paths.");
+  }
 
   if (server_mode(options) &&
       (!strcmpstart(uname, "Windows 95") ||