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]

Author: jeremyevans

lib/irb/ext/save-history.rb

@@ -81,6 +81,8 @@ def load_history
             end
           }
         end
+        @loaded_history_lines = history.size
+        @loaded_history_mtime = File.mtime(history_file)
       end
     end
 
@@ -105,12 +107,20 @@ def save_history
           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") }
-          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
+          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

test/irb/test_history.rb

@@ -127,6 +127,37 @@ def test_history_save_minus_as_infinity
       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 @@ def assert_history(expected_history, initial_irb_history, input)
         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