Print a warning and exit when the target process can't be hijacked du…

…e to the main thread being blocked waiting for another thread to join. Still trying to find a workaround for this.

lib/hijack/gdb.rb

@@ -9,8 +9,16 @@ def initialize(pid)
       wait
     end
 
+    def backtrace
+      @backtrace ||= exec('bt').reverse
+    end
+
     def attached_to_ruby_process?
-      exec('bt').any? {|line| line =~ /ruby_run/}
+      backtrace.any? {|line| line =~ /ruby_run/}
+    end
+
+    def main_thread_blocked_by_join?
+      backtrace.any? {|line| line =~ /rb_thread_join/}
     end
 
     def eval(cmd)

lib/hijack/payload.rb

@@ -6,6 +6,11 @@ def self.inject(pid)
         puts "\n=> #{pid} doesn't appear to be a Ruby process!"
         exit 1
       end
+      if gdb.main_thread_blocked_by_join?
+        puts "\n=> Unable to hijack #{pid} because the main thread is blocked waiting for another thread to join."
+        puts "=> Check that you are using the most recent version of hijack, a newer version may have solved this shortcoming."
+        exit 1
+      end
       gdb.eval(payload(pid))
       gdb.detach
     end