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

mojo/python/mojo_utils/data_pipe_utils.py

+# 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 mojo_system
+
+def BlockingCopyFromDataPipeIntoString(data_pipe, deadline):
+  """
+  Reads from |data_pipe| until either (1) there is no more data to read or
+  (2) an error is encountered.
+  Returns a tuple of the result status and output string. If the status is not
+  mojo_system.RESULT_OK, the returned string will be None.
+  """
+  BUFFER_SIZE = 1000

[qsr, 2015/02/25 15:22:35]

Use a power of 2, usual values are 1 to 4k

[blundell, 2015/02/26 16:33:53]

Done.

+  input_bytes = bytearray(BUFFER_SIZE)
+  output_bytes = bytearray()

[qsr, 2015/02/25 15:22:35]

Instead of transferring in BUFFER_SIZE chunk, because you keep everything in
memory in the end, you can probably keep your buffer, double its size at each
iteration and fill it by passing views to the ReadData method.

[blundell, 2015/02/26 16:33:53]

Done.

+  while True:
+    status, read_bytes = data_pipe.ReadData(input_bytes)
+    if status == mojo_system.RESULT_OK:
+      output_bytes = output_bytes + read_bytes
+      continue
+
+    if status == mojo_system.RESULT_SHOULD_WAIT:
+      status, _ = data_pipe.Wait(mojo_system.HANDLE_SIGNAL_READABLE, deadline)
+
+      # If the wait results in RESULT_OK, try to read more bytes.
+      if status == mojo_system.RESULT_OK:
+        continue

[qsr, 2015/02/25 15:22:35]

What happen is you get another result here? In particular, if this is not
RESULT_FAILED_PRECONDITION

[blundell, 2015/02/26 16:33:53]

Any result other than OK or FAILED_PRECONDITION is a failure, i.e., should end
up at line 34.

[qsr, 2015/02/26 16:45:19]

 Hum, misread the alignment. Ok.

+
+    # Treat a failed precondition as EOF.
+    if status == mojo_system.RESULT_FAILED_PRECONDITION:
+      return (mojo_system.RESULT_OK, output_bytes.decode("utf-8"))

[qsr, 2015/02/25 15:22:35]

This method being a generic utility method, it should probably return a
bytearray instead.

[blundell, 2015/02/26 16:33:52]

Done.

+
+    return (status, None)