def GetDownloadStartByte(src_obj_metadata,
dst_url,
api_selector,
start_byte,
existing_file_size,
component_num=None):
"""Returns the download starting point.
The methodology of this function is the same as in
ReadOrCreateDownloadTrackerFile, with the difference that we are not
interested here in possibly creating a tracker file. In case there is no
tracker file, this means the download starting point is start_byte.
Args:
src_obj_metadata: Metadata for the source object. Must include etag and
generation.
dst_url: Destination URL for tracker file.
api_selector: API to use for this operation.
start_byte: The start byte of the byte range for this download.
existing_file_size: Size of existing file for this download on disk.
component_num: The component number, if this is a component of a parallel
download, else None.
Returns:
download_start_byte: The first byte that still needs to be downloaded.
"""
assert src_obj_metadata.etag
tracker_file_name = None
if src_obj_metadata.size < ResumableThreshold():
# There is no tracker file for small downloads; this means we start from
# scratch.
return start_byte
if component_num is None:
tracker_file_type = TrackerFileType.DOWNLOAD
else:
tracker_file_type = TrackerFileType.DOWNLOAD_COMPONENT
tracker_file_name = GetTrackerFilePath(dst_url,
tracker_file_type,
api_selector,
component_num=component_num)
tracker_file = None
# Check to see if we already have a matching tracker file.
try:
tracker_file = open(tracker_file_name, 'r')
if tracker_file_type is TrackerFileType.DOWNLOAD:
etag_value = tracker_file.readline().rstrip('\n')
if etag_value == src_obj_metadata.etag:
return existing_file_size
elif tracker_file_type is TrackerFileType.DOWNLOAD_COMPONENT:
component_data = json.loads(tracker_file.read())
if (component_data['etag'] == src_obj_metadata.etag
and component_data['generation']
== src_obj_metadata.generation):
return component_data['download_start_byte']
except (IOError, ValueError):
# If the file does not exist, there is not much we can do at this point.
pass
finally:
if tracker_file:
tracker_file.close()
# There wasn't a matching tracker file, which means our starting point is
# start_byte.
return start_byte
def ReadOrCreateDownloadTrackerFile(src_obj_metadata,
dst_url,
logger,
api_selector,
start_byte,
existing_file_size,
component_num=None):
"""Checks for a download tracker file and creates one if it does not exist.
The methodology for determining the download start point differs between
normal and sliced downloads. For normal downloads, the existing bytes in
the file are presumed to be correct and have been previously downloaded from
the server (if a tracker file exists). In this case, the existing file size
is used to determine the download start point. For sliced downloads, the
number of bytes previously retrieved from the server cannot be determined
from the existing file size, and so the number of bytes known to have been
previously downloaded is retrieved from the tracker file.
Args:
src_obj_metadata: Metadata for the source object. Must include etag and
generation.
dst_url: Destination URL for tracker file.
logger: For outputting log messages.
api_selector: API to use for this operation.
start_byte: The start byte of the byte range for this download.
existing_file_size: Size of existing file for this download on disk.
component_num: The component number, if this is a component of a parallel
download, else None.
Returns:
tracker_file_name: The name of the tracker file, if one was used.
download_start_byte: The first byte that still needs to be downloaded.
"""
assert src_obj_metadata.etag
tracker_file_name = None
if src_obj_metadata.size < ResumableThreshold():
# Don't create a tracker file for a small downloads; cross-process resumes
# won't work, but restarting a small download is inexpensive.
return tracker_file_name, start_byte
download_name = dst_url.object_name
if component_num is None:
tracker_file_type = TrackerFileType.DOWNLOAD
else:
tracker_file_type = TrackerFileType.DOWNLOAD_COMPONENT
download_name += ' component %d' % component_num
tracker_file_name = GetTrackerFilePath(dst_url,
tracker_file_type,
api_selector,
component_num=component_num)
tracker_file = None
# Check to see if we already have a matching tracker file.
try:
tracker_file = open(tracker_file_name, 'r')
if tracker_file_type is TrackerFileType.DOWNLOAD:
etag_value = tracker_file.readline().rstrip('\n')
if etag_value == src_obj_metadata.etag:
return tracker_file_name, existing_file_size
elif tracker_file_type is TrackerFileType.DOWNLOAD_COMPONENT:
component_data = json.loads(tracker_file.read())
if (component_data['etag'] == src_obj_metadata.etag
and component_data['generation']
== src_obj_metadata.generation):
return tracker_file_name, component_data['download_start_byte']
logger.warn(
'Tracker file doesn\'t match for download of %s. Restarting '
'download from scratch.' % download_name)
except (IOError, ValueError) as e:
# Ignore non-existent file (happens first time a download
# is attempted on an object), but warn user for other errors.
if isinstance(e, ValueError) or e.errno != errno.ENOENT:
logger.warn(
'Couldn\'t read download tracker file (%s): %s. Restarting '
'download from scratch.' % (tracker_file_name, str(e)))
finally:
if tracker_file:
tracker_file.close()
# There wasn't a matching tracker file, so create one and then start the
# download from scratch.
if tracker_file_type is TrackerFileType.DOWNLOAD:
_WriteTrackerFile(tracker_file_name, '%s\n' % src_obj_metadata.etag)
elif tracker_file_type is TrackerFileType.DOWNLOAD_COMPONENT:
WriteDownloadComponentTrackerFile(tracker_file_name, src_obj_metadata,
start_byte)
return tracker_file_name, start_byte