Work around Readline clash

Only limited debugging functionality is available to things that involve
the Readline formula, but it is better than crashing outright.

c.f. Homebrew/homebrew#15776.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/debrew.rb

@@ -1,9 +1,10 @@
-require 'irb'
-begin
-  require 'continuation' # needed on 1.9
-rescue LoadError
+def can_use_readline?
+  not ENV['HOMEBREW_NO_READLINE']
 end
 
+require 'irb' if can_use_readline?
+require 'continuation' if RUBY_VERSION.to_f >= 1.9
+
 class Menu
   attr_accessor :prompt
   attr_accessor :entries
@@ -75,7 +76,7 @@ module IRB
       irb_at_exit
     end
   end
-end
+end if can_use_readline?
 
 class Exception
   attr_accessor :continuation
@@ -92,7 +93,7 @@ def has_debugger?
     true
   rescue LoadError
     false
-  end
+  end if can_use_readline?
 end
 
 def debrew(exception, formula=nil)
@@ -121,7 +122,7 @@ def debrew(exception, formula=nil)
             end
           }
         end
-      end
+      end if can_use_readline?
       menu.choice(:shell) do
         puts "When you exit this shell, you will return to the menu."
         interactive_shell formula

Library/Homebrew/formula_installer.rb

@@ -73,6 +73,12 @@ class FormulaInstaller
       needed_deps = []
       needed_reqs = []
 
+      # HACK: If readline is present in the dependency tree, it will clash
+      # with the stdlib's Readline module when the debugger is loaded
+      if f.recursive_deps.any? { |d| d.name == "readline" } and ARGV.debug?
+        ENV['HOMEBREW_NO_READLINE'] = '1'
+      end
+
       ARGV.filter_for_dependencies do
         needed_deps = f.recursive_deps.reject{ |d| d.installed? }
         needed_reqs = f.recursive_requirements.reject { |r| r.satisfied? }