Return with an error message if pry is started inside signal handler (#2206)

Mutex#synchronize will throw an error if called from code passed to
Signal.trap. This makes it impossible to start a working Pry session
inside a signal handler.

Currently, calling binding.pry inside signal handling code raises an
exception because Pry uses mutexes to solve various thread-safety
issues.

This commit updates Pry.start to detect whether we'll have the ability
to use Mutex#synchronize. If not, we'll print an error message and
return instead of raising an error.

https://github.com/pry/pry/issues/2191

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/pry/pry_class.rb b/lib/pry/pry_class.rb
index 651cf67e..f5f8db6a 100644
--- a/lib/pry/pry_class.rb
+++ b/lib/pry/pry_class.rb
@@ -168,6 +168,12 @@ you can add "Pry.config.windows_console_warning = false" to your pryrc.
       return
     end
 
+    unless mutex_available?
+      output.puts "ERROR: Unable to obtain mutex lock."
+      output.puts "This can happen if binding.pry is called from a signal handler"
+      return
+    end
+
     options[:target] = Pry.binding_for(target || toplevel_binding)
     initial_session_setup
     final_session_setup
@@ -378,6 +384,13 @@ Readline version #{Readline::VERSION} detected - will not auto_resize! correctly
   ensure
     Thread.current[:pry_critical_section] -= 1
   end
+
+  def self.mutex_available?
+    Mutex.new.synchronize { true }
+  rescue ThreadError
+    false
+  end
+  private_class_method :mutex_available?
 end
 
 Pry.init