[Telemetry] Add capability for values to reference external files.

tools/telemetry/telemetry/util/file_handle.py

Index: tools/telemetry/telemetry/util/file_handle.py
diff --git a/tools/telemetry/telemetry/util/file_handle.py b/tools/telemetry/telemetry/util/file_handle.py
new file mode 100644
index 0000000000000000000000000000000000000000..2eb8b8dae33ac54dbf4fec71a2241acff291e7f4
--- /dev/null
+++ b/tools/telemetry/telemetry/util/file_handle.py
@@ -0,0 +1,66 @@
+# Copyright 2014 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 os
+
+_next_file_id = 0
+
+class FileHandle(object):
+  def __init__(self, tf, path=None):
+    """Constructs a FileHandle object.
+
+    This constructor should not be used by the user; rather it is preferred to
+    use the module-level GetRelPath and FromTempFile functions.
+
+    Args:
+      tempfile: An instance of a temporary file object.
+      path: A path; should not be passed if tempfile is and vice-versa.
+    """
+    # We should never build a file handle from nothing.
+    assert (path is None) != (tf is None)
+    self._tf = tf
+    self._path = path

[nduca, 2014/09/16 20:56:24]

self._path -> self._absolute_path

+
+    global _next_file_id
+    self._id = _next_file_id
+    _next_file_id += 1
+
+  @property
+  def id(self):
+    return self._id

[nednguyen, 2014/09/16 16:15:04]

I am still not convinced that we need the id for this API. As discussed before,
the logic of assigning these ids can be delayed until all the files are about to
be written to json.

[nduca, 2014/09/16 20:56:24]

as coded, this i think this is the right approach. i agree with what ethan has
done here. if ids are assigned at output time, then Value.AsDict() needs to have
access to this id mapping. We've got precidence for  this approach: this is how
pages work

+
+  def GetRelPath(self, path='.'):

[nduca, 2014/09/16 20:56:24]

i think path=None is the right behavior here

rename path->start to match os.path.relpath

[nduca, 2014/09/16 20:56:24]

i think path=None is the right behavior here

+    """Returns the path to the pointed-to file relative to the given path.
+
+    Args:
+      path: A string representing a path.
+    Returns:
+      A string giving the relative path from path to this file.
+    """
+    if self._tf:
+      name = self._tf.name
+    else:
+      name = self._path
+
+    return os.path.relpath(name, path)
+
+  @staticmethod
+  def FromTempFile(tf):
+    """Constructs a FileHandle pointing to a temporary file.
+
+    Returns:
+      A FileHandle referring to a named temporary file.
+    """
+    return FileHandle(tf)
+
+  @staticmethod
+  def FromPath(path):
+    """Constructs a FileHandle from a path.
+
+    Args:
+      path: A string giving the path to a file.
+    Returns:
+      A FileHandle referring to the file at the specified path.
+    """
+    return FileHandle(None, path)

[nduca, 2014/09/16 20:56:24]

pass in os.path.abspath(path)