watch cmd: Update out-of-date bundles right away.

src/webassets/script.py

@@ -231,7 +231,15 @@ def check_for_changes():
             return changed_bundles
 
         try:
-            self.log.info("Watching %d bundles for changes..." % len(self.environment))
+            # Before starting to watch for changes, also recognize changes
+            # made while we did not run, and apply those immediately.
+            for bundle in self.environment:
+                print 'Bringing up to date: %s' % bundle.output
+                bundle.build(force=False)
+
+            self.log.info("Watching %d bundles for changes..." %
+                          len(self.environment))
+
             while True:
                 changed_bundles = check_for_changes()
                 built = []

tests/test_script.py

@@ -254,3 +254,26 @@ def test_same_file_multiple_bundles(self):
         # Both output files have been updated.
         assert self.get('out') == 'foo'
         assert self.get('out2') == 'foo'
+
+    def test_initial_build(self):
+        """The watch command also detects changes that were made while it was
+        not running, and applies those right away on start.
+        """
+        # Register a bundle to watch
+        bundle = self.mkbundle('in', output='out')
+        self.env.register('test', bundle)
+
+        # Mark the input file has changed before we even run the command.
+        now = self.setmtime('in')
+        self.setmtime('out', mtime=now-100)
+
+        # Assert initial state
+        assert self.get('out') == 'bar'
+
+        # Run the watch command for a while, but don't make any changes.
+        with self:
+            time.sleep(0.2)
+
+        # Output file has been updated, not due to a change detected by watch,
+        # but because watch recognized the initial requirement for a build.
+        assert self.get('out') == 'foo'