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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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29 changes: 22 additions & 7 deletions flask_pymongo/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -183,12 +183,13 @@ 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)

# Get or compute the sha1 sum for the etag.
metadata = fileobj.metadata
sha1_sum = metadata and metadata.get("sha1_sum")
sha1_sum = sha1_sum or self._compute_sha(fileobj)
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 Expand Up @@ -237,5 +238,19 @@ def save_upload(filename):
db_obj = self.db
assert db_obj is not None, "Please initialize the app before calling save_file!"
storage = GridFS(db_obj, base)
id = storage.put(fileobj, filename=filename, content_type=content_type, **kwargs)

# GridFS does not manage its own hash, so we manage our own using its
# metadata storage, to be used for the etag.
sha1_sum = self._compute_sha(fileobj)
metadata = dict(sha1_sum=sha1_sum)
id = storage.put(
fileobj, filename=filename, content_type=content_type, metadata=metadata, **kwargs
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Hmm I just had another thought. GridOut is seekable but fileobj on save_file() here can be any file-like object and IIRC not every such object is seekable, is that true?. If so we may need to upload the file and compute the hash in one pass.

)
return id

def _compute_sha(self, fileobj: Any) -> str:
"""Compute the sha sum of a file object."""
pos = fileobj.tell()
raw_data = fileobj.read()
fileobj.seek(pos)
return hashlib.sha1(raw_data).hexdigest()
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Instead of reading the whole file, it's better to iterate the file data in chunks and calculate the hash:

hash = hashlib.sha1()
while True:
    chunk = fileobj.readchunk()
    if not chunk:
        break
    hash.update(chunk)
sha1_sum = hash.hexdigest()

That way we don't have to assemble the entire file in memory at once. However, I suggest we don't implement this optimization now and defer it as future work because there are too many other questions in flight that would change this code.

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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