Watcher: No need to keep polling mtimes if already stale

lib/spring/watcher/polling.rb

@@ -33,7 +33,7 @@ def start
             Thread.current.abort_on_exception = true
 
             begin
-              loop do
+              until stale?
                 Kernel.sleep latency
                 check_stale
               end
@@ -42,6 +42,8 @@ def start
                 "poller: aborted: #{e.class}: #{e}\n  #{e.backtrace.join("\n  ")}"
               end
               raise
+            ensure
+              @poller = nil
             end
           }
         end
@@ -55,6 +57,10 @@ def stop
         end
       end
 
+      def running?
+        @poller && @poller.alive?
+      end
+
       def subjects_changed
         computed = compute_mtime
         debug { "subjects_changed: mtime #{@mtime} -> #{computed}" }

test/unit/watcher_test.rb

@@ -6,4 +6,55 @@ class PollingWatcherTest < Spring::Test::WatcherTest
   def watcher_class
     Spring::Watcher::Polling
   end
+
+  test "skips staleness checks if already stale" do
+    class << watcher
+      attr_reader :checked_when_stale_count
+      attr_reader :checked_when_not_stale_count
+
+      def check_stale
+        @checked_when_stale_count = 0 unless defined? @checked_when_stale_count
+        @checked_when_not_stale_count = 0 unless defined? @checked_when_not_stale_count
+
+        if stale?
+          @checked_when_stale_count += 1
+        else
+          @checked_when_not_stale_count += 1
+        end
+
+        super
+      end
+
+      # Wait for the poller thread to finish.
+      def join
+        @poller.join if @poller
+      end
+    end
+
+    # Track when we're marked as stale.
+    on_stale_count = 0
+    watcher.on_stale { on_stale_count += 1 }
+
+    # Add a file to watch and start polling.
+    file = "#{@dir}/omg"
+    touch file, Time.now - 2.seconds
+    watcher.add file
+    watcher.start
+    assert watcher.running?
+
+    # First touch bumps mtime and marks as stale.
+    touch file, Time.now - 1.second
+    Timeout.timeout(1) { watcher.join }
+    assert !watcher.running?
+    assert_equal 0, watcher.checked_when_stale_count
+    assert_equal 1, watcher.checked_when_not_stale_count
+    assert_equal 1, on_stale_count
+
+    # Second touch skips mtime check because it's already stale.
+    touch file, Time.now
+    sleep 1
+    assert_equal 0, watcher.checked_when_stale_count
+    assert_equal 1, watcher.checked_when_not_stale_count
+    assert_equal 1, on_stale_count
+  end
 end