Add Python util to read from a data pipe into a bytearray

mojo/python/mojo_utils/data_pipe_utils.py

Index: mojo/python/mojo_utils/data_pipe_utils.py
diff --git a/mojo/python/mojo_utils/data_pipe_utils.py b/mojo/python/mojo_utils/data_pipe_utils.py
new file mode 100644
index 0000000000000000000000000000000000000000..4299edb76e9391bfa1f2ec4f7752ca330a7a3082
--- /dev/null
+++ b/mojo/python/mojo_utils/data_pipe_utils.py
@@ -0,0 +1,77 @@
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import itertools
+import sys
+
+import mojo_system
+from mojo_bindings import promise
+
+class DataPipeCopyException(Exception):
+  def __init__(self, *args, **kwargs):
+    Exception.__init__(self, *args, **kwargs)
+    self.__traceback__ = sys.exc_info()[2]
+
+
+def CopyFromDataPipe(data_pipe, deadline):
+  """
+  Returns a Promise that operates as follows:
+  - If |data_pipe| is successfully read from, the promise resolves with the
+    bytes that were read.
+  - Otherwise, the promise rejects with an exception whose message contains the
+    status from the attempted read.
+  """
+  class DataPipeCopyHelper():
+    def __init__(self, data_pipe, deadline, resolve, reject):
+      self.data_pipe = data_pipe
+      self.original_deadline = deadline
+      self.start_time = mojo_system.GetTimeTicksNow()
+      self.resolve = resolve
+      self.reject = reject
+      self.buffer_size = 1024
+      self.data = bytearray(self.buffer_size)
+      self.index = 0
+
+    def _ComputeCurrentDeadline(self):
+      if (self.original_deadline == 0 or
+          self.original_deadline == mojo_system.DEADLINE_INDEFINITE):
+        return self.original_deadline
+      elapsed_time = mojo_system.GetTimeTicksNow() - self.start_time
+      return self.original_deadline - elapsed_time

[qsr, 2015/03/03 17:10:36]

You probably want a max(0, self.original_deadline - elapsed_time), and so you
probably do not need to special case original_deadline == 0

[blundell, 2015/03/04 10:28:52]

Done.

+
+    def CopyFromDataPipeAsync(self, result):
+      while result == mojo_system.RESULT_OK:
+        assert self.index <= len(self.data)
+        if self.index == len(self.data):
+          self.buffer_size *= 2
+          self.data.extend(itertools.repeat(0, self.buffer_size))
+
+        # Careful! Have to construct a memoryview object here as otherwise the
+        # slice operation will create a copy of |data| and hence not write into
+        # |data| as desired.
+        result, read_bytes = self.data_pipe.ReadData(
+            memoryview(self.data)[self.index:])
+        if read_bytes:
+          self.index += len(read_bytes)
+        del read_bytes
+
+      if result == mojo_system.RESULT_SHOULD_WAIT:
+        data_pipe.AsyncWait(mojo_system.HANDLE_SIGNAL_READABLE,
+                            self._ComputeCurrentDeadline(),
+                            self.CopyFromDataPipeAsync)
+        return
+
+      # Treat a failed precondition as EOF.
+      if result == mojo_system.RESULT_FAILED_PRECONDITION:
+        self.resolve(self.data[:self.index])
+        return
+
+      self.reject(DataPipeCopyException("Result: %d" % result))
+
+
+  def GenerationMethod(resolve, reject):
+    helper = DataPipeCopyHelper(data_pipe, deadline, resolve, reject)
+    helper.CopyFromDataPipeAsync(mojo_system.RESULT_OK)
+
+  return promise.Promise(GenerationMethod)