Add Pry.critical_section to break pry inside pry [Fixes #729]

3 files changed, 42 insertions, 8 deletions

diff --git a/lib/pry/pry_class.rb b/lib/pry/pry_class.rb
index 2d2be5a2..6f87cd57 100644
--- a/lib/pry/pry_class.rb
+++ b/lib/pry/pry_class.rb
@@ -128,6 +128,13 @@ class Pry
   #   Pry.start(Object.new, :input => MyInput.new)
   def self.start(target=nil, options={})
     return if ENV['DISABLE_PRY']
+
+    if in_critical_section?
+      output.puts "ERROR: Pry started inside Pry."
+      output.puts "This can happen if you have a binding.pry inside a #to_s or #inspect function."
+      return
+    end
+
     target = Pry.binding_for(target || toplevel_binding)
     initial_session_setup
 
@@ -416,6 +423,17 @@ class Pry
     @toplevel_binding.eval('private')
     @toplevel_binding
   end
+
+  def self.in_critical_section?
+    @critical_section.to_i > 0
+  end
+
+  def self.critical_section(&block)
+    @critical_section = @critical_section.to_i + 1
+    yield
+  ensure
+    @critical_section -= 1
+  end
 end
 
 Pry.init
diff --git a/lib/pry/pry_instance.rb b/lib/pry/pry_instance.rb
index 9bc81f29..21b3acbe 100644
--- a/lib/pry/pry_instance.rb
+++ b/lib/pry/pry_instance.rb
@@ -253,7 +253,9 @@ class Pry
     target = Pry.binding_for(target)
     result = re(target)
 
-    show_result(result)
+    Pry.critical_section do
+      show_result(result)
+    end
   end
 
   # Perform a read-eval
@@ -374,11 +376,12 @@ class Pry
     completion_proc = Pry.config.completer.build_completion_proc(target, self,
                                                         instance_eval(&custom_completions))
 
+    safe_completion_proc = proc{ |*a| Pry.critical_section{ completion_proc.call(*a) } }
 
     indentation = Pry.config.auto_indent ? @indent.current_prefix : ''
 
     begin
-      val = readline("#{current_prompt}#{indentation}", completion_proc)
+      val = readline("#{current_prompt}#{indentation}", safe_completion_proc)
 
     # Handle <Ctrl+C> like Bash, empty the current input buffer but do not quit.
     # This is only for ruby-1.9; other versions of ruby do not let you send Interrupt
@@ -659,13 +662,15 @@ class Pry
                        :eval_string    => eval_string,
                        :cont           => !eval_string.empty?)
 
-    # If input buffer is empty then use normal prompt
-    if eval_string.empty?
-      generate_prompt(Array(prompt).first, c)
+    Pry.critical_section do
+      # If input buffer is empty then use normal prompt
+      if eval_string.empty?
+        generate_prompt(Array(prompt).first, c)
 
-    # Otherwise use the wait prompt (indicating multi-line expression)
-    else
-      generate_prompt(Array(prompt).last, c)
+      # Otherwise use the wait prompt (indicating multi-line expression)
+      else
+        generate_prompt(Array(prompt).last, c)
+      end
     end
   end
 
diff --git a/test/test_pry.rb b/test/test_pry.rb
index 781027c1..aee20530 100644
--- a/test/test_pry.rb
+++ b/test/test_pry.rb
@@ -38,6 +38,17 @@ describe Pry do
     end
   end
 
+  describe "Pry.critical_section" do
+    it "should prevent Pry being called" do
+      output = StringIO.new
+      Pry.output = output
+      Pry.critical_section do
+        Pry.start
+      end
+      output.string.should =~ /Pry started inside Pry/
+    end
+  end
+
   describe "Pry.binding_for" do
 
     # regression test for burg's bug (see git history)