[gsutil] Fix race when downloading gsutil

If gsutil is not downloaded, and if gsutil (or
download_from_google_storage) is called concurrently with n>2, it's
possible that those processes are doing the same work. There is also a
critical point where gsutil.py can fail with:

Destination path '/depot_tools_path/external_bin/gsutil/gsutil_4.68/d'
already exists error.

To avoid this problem, use FS locking around code that manipulates with
files.

R=jojwang@google.com

Bug: 338040708
Change-Id: Ib83aaa1e09628f878e512d79f2fa5221c2bcfd37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5502531
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>

lockfile.py

@@ -80,14 +80,16 @@ def _try_lock(lockfile):
 def _lock(path, timeout=0):
     """_lock returns function to release the lock if locking was successful.
 
-    _lock also implements simple retry logic."""
+    _lock also implements simple retry logic.
+    NOTE: timeout value doesn't include time it takes to aquire lock, just
+    overall sleep time."""
     elapsed = 0
+    sleep_time = 0.1
     while True:
         try:
             return _try_lock(path + '.locked')
         except (OSError, IOError) as e:
             if elapsed < timeout:
-                sleep_time = min(10, timeout - elapsed)
                 logging.info(
                     'Could not create git cache lockfile; '
                     'will retry after sleep(%d).', sleep_time)