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Fix bug with lock upgrades.

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- Closing and re-opening to upgrade to write will lose all existing read
  locks on this process.
  - If we didn't allow ranges, sleeping until no reads would work.
  - With ranges, we may never be able to take some legal write locks
    without invalidating all reads. e.g., if a write lock has distinct
    range from all reads, it should just work, but we'd have to close the
    file, reopen, and re-take reads.

- It's easier to just check whether the file is writable in the first
  place and open for writing from the start.

- Lock now only opens files read-only if we *can't* write them.
This commit is contained in:
Todd Gamblin 2016-10-06 00:31:19 -07:00
parent a024c6df95
commit 3d8d8d3644
1 changed files with 15 additions and 11 deletions
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26
lib/spack/llnl/util/lock.py
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@ -91,21 +91,25 @@ def _lock(self, op, timeout=_default_timeout):
start_time = time.time()
while (time.time() - start_time) < timeout:
try:
# If this is already open read-only and we want to
# upgrade to an exclusive write lock, close first.
if self._file is not None:
if op == fcntl.LOCK_EX and self._file.mode == 'r':
self._file.close()
self._file = None
# If we could write the file, we'd have opened it 'r+'.
# Raise an error when we attempt to upgrade to a write lock.
if op == fcntl.LOCK_EX:
if self._file and self._file.mode == 'r':
raise LockError(
"Can't take exclusive lock on read-only file: %s"
% self.path)
# Open reader locks read-only if possible.
# lock doesn't exist, open RW + create if it doesn't exist.
# Create file and parent directories if they don't exist.
if self._file is None:
self._ensure_parent_directory()
os_mode, fd_mode = os.O_RDONLY, 'r'
if op == fcntl.LOCK_EX or not os.path.exists(self.path):
os_mode, fd_mode = (os.O_RDWR | os.O_CREAT), 'r+'
# Prefer to open 'r+' to allow upgrading to write
# lock later if possible. Open read-only if we can't
# write the lock file at all.
os_mode, fd_mode = (os.O_RDWR | os.O_CREAT), 'r+'
if os.path.exists(self.path) and not os.access(
self.path, os.W_OK):
os_mode, fd_mode = os.O_RDONLY, 'r'
fd = os.open(self.path, os_mode)
self._file = os.fdopen(fd, fd_mode)