luci-py: Refactor file writing code to allow file objects.

client/cipd.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Fetches CIPD client and installs packages."""
 
 __version__ = '0.2'
 
 import collections
 import contextlib
@@ skipping 350 matching lines @@
     version_cache = isolateserver.DiskCache(
         unicode(os.path.join(cache_dir, 'versions')),
         isolateserver.CachePolicies(0, 0, 300),
         hashlib.sha1)
     with version_cache:
       version_cache.cleanup()
       # Convert |version| to a string that may be used as a filename in disk
       # cache by hashing it.
       version_digest = hashlib.sha1(version).hexdigest()
       try:
-        instance_id = version_cache.read(version_digest)
+        with version_cache.getfileobj(version_digest) as f:
+          instance_id = f.read()
       except isolateserver.CacheMiss:
         instance_id = resolve_version(
             service_url, package_name, version, timeout=timeoutfn())
         version_cache.write(version_digest, instance_id)
 
   # instance_cache is {instance_id -> client binary} mapping.
   # It is bounded by 5 client versions.
   instance_cache = isolateserver.DiskCache(
       unicode(os.path.join(cache_dir, 'clients')),
       isolateserver.CachePolicies(0, 0, 5),
       hashlib.sha1)
   with instance_cache:
     instance_cache.cleanup()
     if instance_id not in instance_cache:
       logging.info('Fetching CIPD client %s:%s', package_name, instance_id)
       fetch_url = get_client_fetch_url(
           service_url, package_name, instance_id, timeout=timeoutfn())
       _fetch_cipd_client(instance_cache, instance_id, fetch_url, timeoutfn)
 
     # A single host can run multiple swarming bots, but ATM they do not share
     # same root bot directory. Thus, it is safe to use the same name for the
     # binary.
     binary_path = unicode(os.path.join(cache_dir, 'cipd' + EXECUTABLE_SUFFIX))
     if fs.isfile(binary_path):
       file_path.remove(binary_path)
-    instance_cache.link(instance_id, binary_path, 0511, False)  # -r-x--x--x
+
+    with instance_cache.getfileobj(instance_id) as f:
+      isolateserver.putfile(f, binary_path, 0511)  # -r-x--x--x
 
     yield CipdClient(binary_path)
 
 
 def parse_package_args(packages):
   """Parses --cipd-package arguments.
 
   Assumes |packages| were validated by validate_cipd_options.
 
   Returns:
     A map {path: [(package, version)]}.
   """
   result = collections.defaultdict(list)
   for pkg in packages:
     path, name, version = pkg.split(':', 2)
     path = path.replace('/', os.path.sep)
     if not name:
       raise Error('Invalid package "%s": package name is not specified' % pkg)
     if not version:
       raise Error('Invalid package "%s": version is not specified' % pkg)
     result[path].append((name, version))
   return result