fix root path detection under thin/passenger

Fixed how the Sinatra application's path is set. Previously it
was derived exclusively from $0, but this was causing problems
when running Sinatra under Rack because $0 is the web server
(e.g., thin, passenger, etc) and thus Sinatra would incorrectly
set things like the default views path. This fix adds a new
locate_app_file method that uses a number of techniques to guess the
Sinatra application's path.

ChangeLog

@@ -2,6 +2,9 @@
 
   * BUG: use_in_file_templates! failes with CR/LF (#45)
 
+  * BUG: Sinatra detects the app file and root path when run under
+    thin/passenger.
+
 = 0.3.2
 
   * BUG: Static and send_file read entire file into String before

lib/sinatra.rb

@@ -923,7 +923,8 @@ module Sinatra
     # file, before any DSL related functions are invoked.
     def self.default_options
       return @default_options unless @default_options.nil?
-      root = File.expand_path(File.dirname($0))
+      app_file = locate_app_file
+      root = File.expand_path(File.dirname(app_file))
       @default_options = {
         :run => true,
         :port => 4567,
@@ -934,7 +935,7 @@ module Sinatra
         :public => root + '/public',
         :sessions => false,
         :logging => true,
-        :app_file => $0,
+        :app_file => app_file,
         :raise_errors => false
       }
       load_default_options_from_command_line!
@@ -957,6 +958,16 @@ module Sinatra
       end.parse!(ARGV.dup.select { |o| o !~ /--name/ })
     end
 
+    # Use heuristics to locate the application file.
+    def self.locate_app_file
+      caller[1..-1].reverse.each do |path|
+        path = path.split(':', 2)[0]
+        next if path =~ /sinatra\.rb$/ || path == '(__DSL__)'
+        return path
+      end
+      $0
+    end
+
     # Determine whether the application is in the process of being
     # reloaded.
     def reloading?