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Fix race conditions in FileSystemBytecodeCache
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David Vitek authored and davidism committed Apr 25, 2022
1 parent 466a200 commit 746bb95
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Showing 2 changed files with 49 additions and 6 deletions.
1 change: 1 addition & 0 deletions CHANGES.rst
Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ Unreleased

- Add parameters to ``Environment.overlay`` to match ``__init__``.
:issue:`1645`
- Handle race condition in ``FileSystemBytecodeCache``. :issue:`1654`


Version 3.1.1
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54 changes: 48 additions & 6 deletions src/jinja2/bccache.py
Original file line number Diff line number Diff line change
Expand Up @@ -79,7 +79,7 @@ def load_bytecode(self, f: t.BinaryIO) -> None:
self.reset()
return

def write_bytecode(self, f: t.BinaryIO) -> None:
def write_bytecode(self, f: t.IO[bytes]) -> None:
"""Dump the bytecode into the file or file like object passed."""
if self.code is None:
raise TypeError("can't write empty bucket")
Expand Down Expand Up @@ -262,13 +262,55 @@ def _get_cache_filename(self, bucket: Bucket) -> str:
def load_bytecode(self, bucket: Bucket) -> None:
filename = self._get_cache_filename(bucket)

if os.path.exists(filename):
with open(filename, "rb") as f:
bucket.load_bytecode(f)
# Don't test for existence before opening the file, since the
# file could disappear after the test before the open.
try:
f = open(filename, "rb")
except (FileNotFoundError, IsADirectoryError, PermissionError):
# PermissionError can occur on Windows when an operation is
# in progress, such as calling clear().
return

with f:
bucket.load_bytecode(f)

def dump_bytecode(self, bucket: Bucket) -> None:
with open(self._get_cache_filename(bucket), "wb") as f:
bucket.write_bytecode(f)
# Write to a temporary file, then rename to the real name after
# writing. This avoids another process reading the file before
# it is fully written.
name = self._get_cache_filename(bucket)
f = tempfile.NamedTemporaryFile(
mode="wb",
dir=os.path.dirname(name),
prefix=os.path.basename(name),
suffix=".tmp",
delete=False,
)

def remove_silent() -> None:
try:
os.remove(f.name)
except OSError:
# Another process may have called clear(). On Windows,
# another program may be holding the file open.
pass

try:
with f:
bucket.write_bytecode(f)
except BaseException:
remove_silent()
raise

try:
os.replace(f.name, name)
except OSError:
# Another process may have called clear(). On Windows,
# another program may be holding the file open.
remove_silent()
except BaseException:
remove_silent()
raise

def clear(self) -> None:
# imported lazily here because google app-engine doesn't support
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