Implement stream_read_file for the Swift storage engine

Note that Swift doesn't seem to have a file-like interface, so we need to wrap the generator we get back from it.

Fixes #210

util/generatorfile.py

@@ -0,0 +1,76 @@
+def _complain_ifclosed(closed):
+  if closed:
+    raise ValueError, "I/O operation on closed file"
+
+class GeneratorFile(object):
+  """ File-like object which wraps a Python generator to produce the file contents.
+      Modeled on StringIO and comments on the file-like interface copied from there.
+  """
+  def __init__(self, generator):
+    self._generator = generator
+    self._closed = False
+    self._buf = ''
+
+  def __iter__(self):
+    return self
+
+  def next(self):
+    """A file object is its own iterator, for example iter(f) returns f
+    (unless f is closed). When a file is used as an iterator, typically
+    in a for loop (for example, for line in f: print line), the next()
+    method is called repeatedly. This method returns the next input line,
+    or raises StopIteration when EOF is hit.
+    """
+    _complain_ifclosed(self._closed)
+    r = self.read()
+    if not r:
+      raise StopIteration
+    return r
+
+  def readline(self):
+    buf = []
+    while True:
+      c = self.read(size=1)
+      buf.append(c)
+      if c == '\n' or c == '':
+        return ''.join(buf)
+
+  def flush(self):
+    _complain_ifclosed(self._closed)
+
+  def read(self, size=-1):
+    """Read at most size bytes from the file
+    (less if the read hits EOF before obtaining size bytes).
+
+    If the size argument is negative or omitted, read all data until EOF
+    is reached. The bytes are returned as a string object. An empty
+    string is returned when EOF is encountered immediately.
+    """
+    _complain_ifclosed(self._closed)
+    buf = self._buf
+    while size < 0 or len(buf) < size:
+      try:
+        buf = buf + self._generator.next()
+      except StopIteration:
+        break
+
+    returned = ''
+    if size >= 1:
+      self._buf = buf[size:]
+      returned = buf[:size]
+    else:
+      self._buf = ''
+      returned = buf
+
+    return returned
+
+
+  def close(self):
+    self._closed = True
+    del self._buf
+
+  def __enter__(self):
+    return self
+
+  def __exit__(self, type, value, traceback):
+    self._closed = True