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Use caching for the etag using stored hash if available #182

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Merged
merged 6 commits into from
Jan 21, 2025
Merged

Use caching for the etag using stored hash if available #182

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67849af
Use caching for the etag
blink1073 Jan 15, 2025
17e7424
Fix handling of sha sum
blink1073 Jan 15, 2025
794c9d4
fix test
blink1073 Jan 15, 2025
00a02fb
Use a fixed size cache to store sha sums
blink1073 Jan 16, 2025
006e7bc
address review
blink1073 Jan 17, 2025
ed52375
add md5 test
blink1073 Jan 21, 2025
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24 changes: 18 additions & 6 deletions flask_pymongo/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,7 @@
__all__ = ("PyMongo", "ASCENDING", "DESCENDING", "BSONObjectIdConverter", "BSONProvider")

import hashlib
from collections import OrderedDict
from mimetypes import guess_type
from typing import Any

Expand Down Expand Up @@ -66,6 +67,8 @@ def __init__(
) -> None:
self.cx: MongoClient | None = None
self.db: Database | None = None
self._hash_cache = OrderedDict()
self._hash_limit = 128

if app is not None:
self.init_app(app, uri, *args, **kwargs)
Expand Down Expand Up @@ -183,12 +186,21 @@ def get_upload(filename):
response.headers["Content-Disposition"] = f"attachment; filename={filename}"
response.content_length = fileobj.length
response.last_modified = fileobj.upload_date
# Compute the sha1 sum of the file for the etag.
pos = fileobj.tell()
raw_data = fileobj.read()
fileobj.seek(pos)
digest = hashlib.sha1(raw_data).hexdigest()
response.set_etag(digest)

# GridFS does not manage its own checksum, so we manage our own using its
# metadata storage, to be used for the etag.
sha1_sum = self._hash_cache.get(str(fileobj._id))
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This is problematic because although _id is unique, _ids can be reused over time by deleting the old file and uploading a new one with the same _id. In that case this cache becomes out of date.

if sha1_sum is None:
# Compute the checksum of the file for the etag.
pos = fileobj.tell()
raw_data = fileobj.read()
fileobj.seek(pos)
sha1_sum = hashlib.sha1(raw_data).hexdigest()
while len(self._hash_cache) >= self._hash_limit:
self._hash_cache.popitem()
self._hash_cache[str(fileobj._id)] = sha1_sum
response.set_etag(sha1_sum)
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Should we check for an existing md5 and use that hash here? That way it would be compatible with existing flask-pymongo databases and apps won't pay the cost of recomputing the hash every time when reading existing data.

Alternatively we could update the fileobj and add the sha1 hash here so that the cost is only paid on the first read.

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This conflicts with the version parameter of send_file, since we'd end up creating new versions to add the shasum if it doesn't exist, and the ordering would get all garbled

I believe the file "version" concept is for the file data itself. Also, I do feel more strongly that we should continue to use the md5 field if it's available for backwards compat. So the full read logic would be:

  1. Use sha hash from file metadata if available,
  2. Fallback to using md5 hash from file metadata if available,
  3. Fallback to recompute sha hash.

This should be good enough since step 3 only happens when reading files that were not uploaded via save_file() and that's probably a niche use case. If that's an incorrect assumption we can revisit the idea later on.

Regardless of the read_file impl details, send_file should add the hash automatically so that future reads are cheap. One clever way would be to add a wrapper class around fileobj that hashes the data as it's read:

class Wrapper():
    def __init__(self, file):
        self.file = file
        self.hash = hashlib.sha1()
        
    def read(self, n):
        data = self.file.read(n)
        if data:
            self.hash.update(data)
        return data

def save_file(...)
        storage = GridFS(db_obj, base)
        hashingfile = Wrapper(fileobj)
        with storage.new_file(filename=filename, content_type=content_type, **kwargs) as grid_file:
            grid_file.write(hashingfile)
            grid_file.sha1 = hashingfile.hash.hexdigest()
            return grid_file._id


response.cache_control.max_age = cache_for
response.cache_control.public = True
response.make_conditional(request)
Expand Down
2 changes: 1 addition & 1 deletion tests/util.py
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Original file line number Diff line number Diff line change
Expand Up @@ -33,5 +33,5 @@ def setUp(self):
def tearDown(self):
assert self.mongo.cx is not None
self.mongo.cx.drop_database(self.dbname)

self.mongo.cx.close()
super().tearDown()
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