Conc::Map - override #inspect(), fix iteration error on MRI.

Provide custom version of Map#inspect. Hide implementation details
and fix potential "insert during iteration" MRI errors.

Author: thedarkone

lib/concurrent/map.rb

@@ -202,6 +202,15 @@ def marshal_load(hash)
 
     undef :freeze
 
+    # @!visibility private
+    DEFAULT_OBJ_ID_STR_WIDTH = (2**50).class == Fixnum ? 14 : 7 # we want to look "native", 7 for 32-bit, 14 for 64-bit
+    # override default #inspect() method: firstly, we don't want to be spilling our guts (i-vars), secondly, MRI backend's
+    # #inspect() call on its @backend i-var will bump @backend's iter level while possibly yielding GVL
+    def inspect
+      id_str = (object_id << 1).to_s(16).rjust(DEFAULT_OBJ_ID_STR_WIDTH, '0')
+      "#<#{self.class.name}:0x#{id_str} entries=#{size} default_proc=#{@default_proc.inspect}>"
+    end
+
     private
     def raise_fetch_no_key
       raise KeyError, 'key not found'

spec/concurrent/map_spec.rb

@@ -826,6 +826,15 @@ def key # assert_collision_resistance expects to be able to call .key to get the
       expect { Marshal.dump(Concurrent::Map.new {}) }.to raise_error(TypeError)
     end
 
+    it '#inspect' do
+      regexp = /\A#<Concurrent::Map:0x[0-9a-f]+ entries=[0-9]+ default_proc=.*>\Z/i
+      expect(Concurrent::Map.new.inspect).to match(regexp)
+      expect((Concurrent::Map.new {}).inspect).to match(regexp)
+      map = Concurrent::Map.new
+      map[:foo] = :bar
+      expect(map.inspect).to match(regexp)
+    end
+
     private
 
     def with_or_without_default_proc(&block)