swarming: custom cipd package paths

client/isolateserver.py

Index: client/isolateserver.py
diff --git a/client/isolateserver.py b/client/isolateserver.py
index bde5fc89f8cc46a88e3360841a7df30e6c5d0f15..36edb0b4d68310b0ce3eb9e7ce5058dfd1914e91 100755
--- a/client/isolateserver.py
+++ b/client/isolateserver.py
@@ -116,6 +116,10 @@ class Aborted(Error):
   pass
 
 
+class AlreadyExists(Error):
+  """File already exists."""
+
+
 def file_read(path, chunk_size=isolated_format.DISK_FILE_CHUNK, offset=0):
   """Yields file content in chunks of |chunk_size| starting from |offset|."""
   with fs.open(path, 'rb') as f:
@@ -221,6 +225,8 @@ def create_symlinks(base_directory, files):
       logging.warning('Ignoring symlink %s', filepath)
       continue
     outfile = os.path.join(base_directory, filepath)
+    if os.path.exists(outfile):

[M-A Ruel, 2016/06/15 17:28:27]

I'm not super excited about adding another system call here, could we trap the
exception generated by os.symlink()?

import errno

try:
  os.symlink(...)
except OSError as e:
  if e.errno == errno.EEXIST:
    raise AlreadyExists(...)
  raise

tested to work on both Ubuntu and OSX

[nodir, 2016/06/15 17:53:42]

Done

+      raise AlreadyExists('File %s already exists.' % outfile)
     # os.symlink() doesn't exist on Windows.
     os.symlink(properties['l'], outfile)  # pylint: disable=E1101
 
@@ -1967,8 +1973,10 @@ def fetch_isolated(isolated_hash, storage, cache, outdir):
 
           # Link corresponding files to a fetched item in cache.
           for filepath, props in remaining.pop(digest):
-            cache.hardlink(
-                digest, os.path.join(outdir, filepath), props.get('m'))
+            dest = os.path.join(outdir, filepath)
+            if os.path.exists(dest):

[M-A Ruel, 2016/06/15 17:28:27]

this one is harder to skip since the actual execution is implementation
dependent.

[nodir, 2016/06/15 17:53:42]

Acknowledged.

+              raise AlreadyExists('File %s already exists' % dest)
+            cache.hardlink(digest, dest, props.get('m'))
 
           # Report progress.
           duration = time.time() - last_update