[ruby/irb] Make save-history extension safe for concurrent use

This makes the save-history extension check for modifications to
the history file before saving it.  If the history file was modified
after the history was loaded and before it was saved, append only
the new history lines to the history file.

This can result in more lines in the history file than SAVE_HISTORY
allows.  However, that will be fixed the next time irb is run and
the history is saved.

Fixes [Bug #13654]

https://github.com/ruby/irb/commit/041ef53845

lib/irb/ext/save-history.rb

@@ -81,6 +81,8 @@ module IRB
             end
           }
         end
+        @loaded_history_lines = history.size
+        @loaded_history_mtime = File.mtime(history_file)
       end
     end
 
@@ -105,13 +107,21 @@ module IRB
           raise
         end
 
-        open(history_file, "w:#{IRB.conf[:LC_MESSAGES].encoding}", 0600) do |f|
+        if File.exist?(history_file) && @loaded_history_mtime &&
+           File.mtime(history_file) != @loaded_history_mtime
+          history = history[@loaded_history_lines..-1]
+          append_history = true
+        end
+
+        open(history_file, "#{append_history ? 'a' : 'w'}:#{IRB.conf[:LC_MESSAGES].encoding}", 0600) do |f|
           hist = history.map{ |l| l.split("\n").join("\\\n") }
+          unless append_history
             begin
               hist = hist.last(num) if hist.size > num and num > 0
             rescue RangeError # bignum too big to convert into `long'
               # Do nothing because the bignum should be treated as inifinity
             end
+          end
           f.puts(hist)
         end
       end

test/irb/test_history.rb

@@ -127,6 +127,37 @@ module TestIRB
       INPUT
     end
 
+    def test_history_concurrent_use
+      omit "Skip Editline" if /EditLine/n.match(Readline::VERSION)
+      IRB.conf[:SAVE_HISTORY] = 1
+      assert_history(<<~EXPECTED_HISTORY, <<~INITIAL_HISTORY, <<~INPUT) do |history_file|
+        exit
+        5
+        exit
+      EXPECTED_HISTORY
+        1
+        2
+        3
+        4
+      INITIAL_HISTORY
+        5
+        exit
+      INPUT
+        assert_history(<<~EXPECTED_HISTORY2, <<~INITIAL_HISTORY2, <<~INPUT2)
+        exit
+      EXPECTED_HISTORY2
+        1
+        2
+        3
+        4
+      INITIAL_HISTORY2
+        5
+        exit
+      INPUT2
+        File.utime(File.atime(history_file), File.mtime(history_file) + 2, history_file)
+      end
+    end
+
     private
 
     def assert_history(expected_history, initial_irb_history, input)
@@ -143,6 +174,11 @@ module TestIRB
         io = TestInputMethod.new
         io.class::HISTORY.clear
         io.load_history
+        if block_given?
+          history = io.class::HISTORY.dup
+          yield IRB.rc_file("_history")
+          io.class::HISTORY.replace(history)
+        end
         io.class::HISTORY.concat(input.split)
         io.save_history