Fix FsspecFileIO.get_fs thread safety
#2495
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AlexWendland
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Sep 22, 2025
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kevinjqliu
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`FsspecFileIO.get_fs` can be called by multiple threads when `ExecutorFactory` is used (for example by `DataScan.plan_files`). The base class of `fsspec` filesystem objects, `fsspec.spec.AbstractFileSystem`, internally caches instances through the `fsspec.spec._Cached` metaclass. The caching key used includes `threading.get_ident()`, making entries thread-local: https://github.com/fsspec/filesystem_spec/blame/f84b99f0d1f079f990db1a219b74df66ab3e7160/fsspec/spec.py#L71 The `FsspecFileIO.get_fs` LRU cache (around `FsspecFileIO._get_fs`) breaks the thread-locality of the filesystem instances as it will return the same instance for different threads. One consequence of this is that for `s3fs.S3FileSystem`, HTTP connection pooling no longer occurs per thread (as is normal with `aiobotocore`), as the `aiobotocore` client object (containing the `aiohttp.ClientSession`) is stored on the `s3fs.S3FileSystem`. This change addresses this by making the `FsspecFileIO.get_fs` cache thread-local.
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Removed some unnecessary events ^ |
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The existing S3 remote signing hook function (`s3v4_rest_signer`) uses `requests.post` to submit `POST` requests to the REST signing endpoint, but this internally creates a new `requests.Session` for every request, preventing any reuse of connections. In my profiling I saw this add overhead from repeated loading of CA certs and reestablishing of TLS connections. This change makes the signing function a callable object that wraps a `request.Session`, using this for `POST`ing, therefore achieving connection reuse. Signer callables are stored on the hook internals of the `aiobotocore` client inside the `s3fs.S3FileSystem` instance, so use and lifetime will match that of those instances. The `s3fs.S3FileSystem` instances are guaranteed thread-local since: apache#2495.
Fokko
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Thanks @lwfitzgerald for working on this 🙌 This looks great!
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Rationale for this change
FsspecFileIO.get_fscan be called by multiple threads whenExecutorFactoryis used (for example byDataScan.plan_files).The base class of
fsspecfilesystem objects,fsspec.spec.AbstractFileSystem, internally caches instances through thefsspec.spec._Cachedmetaclass. The caching key used includesthreading.get_ident(), making entries thread-local:https://github.com/fsspec/filesystem_spec/blob/f84b99f0d1f079f990db1a219b74df66ab3e7160/fsspec/spec.py#L71
The
FsspecFileIO.get_fsLRU cache (aroundFsspecFileIO._get_fs) breaks the thread-locality of the filesystem instances as it will return the same instance for different threads.One consequence of this is that for
s3fs.S3FileSystem, HTTP connection pooling no longer occurs per thread (as is normal withaiobotocore), as theaiobotocoreclient object (containing theaiohttp.ClientSession) is stored on thes3fs.S3FileSystem.This change addresses this by making the
FsspecFileIO.get_fscache thread-local.Are these changes tested?
Tested locally. Unit test included covering the caching behaviour.
Are there any user-facing changes?
Yes - S3 HTTP connection pooling now occurs per-thread, matching the behaviour of
aiobotocorewhen it used in the recommended way with an event loop per thread.