Docserver: Update Future.Then() to be more Promise-like

BUG=306341
NOTRY=True

Review URL: https://codereview.chromium.org/417163004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287482 0039d316-1c4b-4281-b951-d872f2087c98

8 files changed, 96 insertions, 41 deletions

diff --git a/chrome/common/extensions/docs/server2/caching_file_system.py b/chrome/common/extensions/docs/server2/caching_file_system.py
index 59d8030..c0ff5d0 100644
--- a/chrome/common/extensions/docs/server2/caching_file_system.py
+++ b/chrome/common/extensions/docs/server2/caching_file_system.py
@@ -63,14 +63,12 @@ class CachingFileSystem(FileSystem):
     if dir_stat is not None:
       return Future(value=make_stat_info(dir_stat))
 
-    dir_stat_future = self._MemoizedStatAsyncFromFileSystem(dir_path)
-    def resolve():
-      dir_stat = dir_stat_future.Get()
+    def next(dir_stat):
       assert dir_stat is not None  # should have raised a FileNotFoundError
       # We only ever need to cache the dir stat.
       self._stat_object_store.Set(dir_path, dir_stat)
       return make_stat_info(dir_stat)
-    return Future(callback=resolve)
+    return self._MemoizedStatAsyncFromFileSystem(dir_path).Then(next)
 
   @memoize
   def _MemoizedStatAsyncFromFileSystem(self, dir_path):
@@ -93,18 +91,17 @@ class CachingFileSystem(FileSystem):
     # with a value.
     stat_futures = {}
 
-    def swallow_file_not_found_error(future):
-      def resolve():
-        try: return future.Get()
-        except FileNotFoundError: return Nnone
-      return Future(callback=resolve)
+    def handle(error):
+      if isinstance(error, FileNotFoundError):
+        return None
+      raise error
 
     for path in paths:
       stat_value = cached_stat_values.get(path)
       if stat_value is None:
         stat_future = self.StatAsync(path)
         if skip_not_found:
-          stat_future = swallow_file_not_found_error(stat_future)
+          stat_future = stat_future.Then(lambda x: x, handle)
       else:
         stat_future = Future(value=stat_value)
       stat_futures[path] = stat_future
@@ -120,19 +117,16 @@ class CachingFileSystem(FileSystem):
       # Everything was cached and up-to-date.
       return Future(value=fresh_data)
 
-    # Read in the values that were uncached or old.
-    read_futures = self._file_system.Read(
-        set(paths) - set(fresh_data.iterkeys()),
-        skip_not_found=skip_not_found)
-    def resolve():
-      new_results = read_futures.Get()
+    def next(new_results):
       # Update the cache. This is a path -> (data, version) mapping.
       self._read_object_store.SetMulti(
           dict((path, (new_result, stat_futures[path].Get().version))
                for path, new_result in new_results.iteritems()))
       new_results.update(fresh_data)
       return new_results
-    return Future(callback=resolve)
+    # Read in the values that were uncached or old.
+    return self._file_system.Read(set(paths) - set(fresh_data.iterkeys()),
+                                  skip_not_found=skip_not_found).Then(next)
 
   def GetIdentity(self):
     return self._file_system.GetIdentity()
diff --git a/chrome/common/extensions/docs/server2/chroot_file_system.py b/chrome/common/extensions/docs/server2/chroot_file_system.py
index af521d8..2ed612f 100644
--- a/chrome/common/extensions/docs/server2/chroot_file_system.py
+++ b/chrome/common/extensions/docs/server2/chroot_file_system.py
@@ -32,13 +32,11 @@ class ChrootFileSystem(FileSystem):
       prefixed = posixpath.join(self._root, path)
       prefixed_paths[prefixed] = path
       return prefixed
-    future_result = self._file_system.Read(
-        tuple(prefix(path) for path in paths),
-        skip_not_found=skip_not_found)
-    def resolve():
+    def next(results):
       return dict((prefixed_paths[path], content)
-                  for path, content in future_result.Get().iteritems())
-    return Future(callback=resolve)
+                  for path, content in results.iteritems())
+    return self._file_system.Read(tuple(prefix(path) for path in paths),
+                                  skip_not_found-skip_not_found).Then(next)
 
   def Refresh(self):
     return self._file_system.Refresh()
diff --git a/chrome/common/extensions/docs/server2/fake_url_fetcher.py b/chrome/common/extensions/docs/server2/fake_url_fetcher.py
index dbfaa55..21b6cf6 100644
--- a/chrome/common/extensions/docs/server2/fake_url_fetcher.py
+++ b/chrome/common/extensions/docs/server2/fake_url_fetcher.py
@@ -129,11 +129,10 @@ class MockURLFetcher(object):
 
   def FetchAsync(self, url, **kwargs):
     self._fetch_async_count += 1
-    future = self._fetcher.FetchAsync(url, **kwargs)
-    def resolve():
+    def next(result):
       self._fetch_resolve_count += 1
-      return future.Get()
-    return Future(callback=resolve)
+      return result
+    return self._fetcher.FetchAsync(url, **kwargs).Then(next)
 
   def CheckAndReset(self,
                     fetch_count=0,
diff --git a/chrome/common/extensions/docs/server2/file_system.py b/chrome/common/extensions/docs/server2/file_system.py
index 99e33c5..1e5b567 100644
--- a/chrome/common/extensions/docs/server2/file_system.py
+++ b/chrome/common/extensions/docs/server2/file_system.py
@@ -106,13 +106,12 @@ class FileSystem(object):
       return Future(value=True)
 
     parent, base = SplitParent(path)
-    list_future = self.ReadSingle(ToDirectory(parent))
-    def resolve():
-      try:
-        return base in list_future.Get()
-      except FileNotFoundError:
+    def handle(error):
+      if isinstance(error, FileNotFoundError):
         return False
-    return Future(callback=resolve)
+      raise error
+    return self.ReadSingle(ToDirectory(parent)).Then(lambda l: base in l,
+                                                     handle)
 
   def Refresh(self):
     '''Asynchronously refreshes the content of the FileSystem, returning a
diff --git a/chrome/common/extensions/docs/server2/future.py b/chrome/common/extensions/docs/server2/future.py
index 4b09073..51c2842 100644
--- a/chrome/common/extensions/docs/server2/future.py
+++ b/chrome/common/extensions/docs/server2/future.py
@@ -7,6 +7,10 @@ import sys
 _no_value = object()
 
 
+def _DefaultErrorHandler(error):
+  raise error
+
+
 def All(futures, except_pass=None):
   '''Creates a Future which returns a list of results from each Future in
   |futures|.
@@ -61,12 +65,17 @@ class Future(object):
         self._exc_info is None):
       raise ValueError('Must have either a value, error, or callback.')
 
-  def Then(self, callback):
+  def Then(self, callback, error_handler=_DefaultErrorHandler):
     '''Creates and returns a future that runs |callback| on the value of this
-    future.
+    future, or runs optional |error_handler| if resolving this future results in
+    an exception.
     '''
     def then():
-      return callback(self.Get())
+      try:
+        val = self.Get()
+      except Exception as e:
+        return error_handler(e)
+      return callback(val)
     return Future(callback=then)
 
   def Get(self):
diff --git a/chrome/common/extensions/docs/server2/future_test.py b/chrome/common/extensions/docs/server2/future_test.py
index 440994b..1c188d1 100755
--- a/chrome/common/extensions/docs/server2/future_test.py
+++ b/chrome/common/extensions/docs/server2/future_test.py
@@ -159,6 +159,63 @@ class FutureTest(unittest.TestCase):
                  except_pass=(ValueError,))
     self.assertRaises(ValueError, race.Get)
 
+  def testThen(self):
+    def assertIs42(val):
+      self.assertEquals(val, 42)
+
+    then = Future(value=42).Then(assertIs42)
+    # Shouldn't raise an error.
+    then.Get()
+
+    # Test raising an error.
+    then = Future(value=41).Then(assertIs42)
+    self.assertRaises(AssertionError, then.Get)
+
+    # Test setting up an error handler.
+    def handle(error):
+      if isinstance(error, ValueError):
+        return 'Caught'
+      raise error
+
+    def raiseValueError():
+      raise ValueError
+
+    def raiseException():
+      raise Exception
+
+    then = Future(callback=raiseValueError).Then(assertIs42, handle)
+    self.assertEquals(then.Get(), 'Caught')
+    then = Future(callback=raiseException).Then(assertIs42, handle)
+    self.assertRaises(Exception, then.Get)
+
+    # Test chains of thens.
+    addOne = lambda val: val + 1
+    then = Future(value=40).Then(addOne).Then(addOne).Then(assertIs42)
+    # Shouldn't raise an error.
+    then.Get()
+
+    # Test error in chain.
+    then = Future(value=40).Then(addOne).Then(assertIs42).Then(addOne)
+    self.assertRaises(AssertionError, then.Get)
+
+    # Test handle error in chain.
+    def raiseValueErrorWithVal(val):
+      raise ValueError
+
+    then = Future(value=40).Then(addOne).Then(raiseValueErrorWithVal).Then(
+        addOne, handle).Then(lambda val: val + ' me')
+    self.assertEquals(then.Get(), 'Caught me')
+
+    # Test multiple handlers.
+    def myHandle(error):
+      if isinstance(error, AssertionError):
+        return 10
+      raise error
+
+    then = Future(value=40).Then(assertIs42).Then(addOne, handle).Then(addOne,
+                                                                       myHandle)
+    self.assertEquals(then.Get(), 10)
+
 
 if __name__ == '__main__':
   unittest.main()
diff --git a/chrome/common/extensions/docs/server2/mock_file_system.py b/chrome/common/extensions/docs/server2/mock_file_system.py
index e15f58c..52a84bd 100644
--- a/chrome/common/extensions/docs/server2/mock_file_system.py
+++ b/chrome/common/extensions/docs/server2/mock_file_system.py
@@ -42,16 +42,15 @@ class MockFileSystem(FileSystem):
     from |_updates|, if any.
     '''
     self._read_count += 1
-    future_result = self._file_system.Read(paths, skip_not_found=skip_not_found)
-    def resolve():
+    def next(result):
       self._read_resolve_count += 1
-      result = future_result.Get()
       for path in result.iterkeys():
         update = self._GetMostRecentUpdate(path)
         if update is not None:
           result[path] = update
       return result
-    return Future(callback=resolve)
+    return self._file_system.Read(paths,
+                                  skip_not_found=skip_not_found).Then(next)
 
   def Refresh(self):
     return self._file_system.Refresh()
diff --git a/chrome/common/extensions/docs/server2/mock_file_system_test.py b/chrome/common/extensions/docs/server2/mock_file_system_test.py
index f2fe7d4..813751f 100755
--- a/chrome/common/extensions/docs/server2/mock_file_system_test.py
+++ b/chrome/common/extensions/docs/server2/mock_file_system_test.py
@@ -60,7 +60,7 @@ class MockFileSystemTest(unittest.TestCase):
     future = fs.Read(['notfound.html', 'apps/'])
     self.assertTrue(*fs.CheckAndReset(read_count=1))
     self.assertRaises(FileNotFoundError, future.Get)
-    self.assertTrue(*fs.CheckAndReset(read_resolve_count=1))
+    self.assertTrue(*fs.CheckAndReset(read_resolve_count=0))
 
     fs.Stat('404.html')
     fs.Stat('404.html')