Per-catalog auth state is shared: commit_table mutates the session, and cached S3FileSystem instances share a signer

[sungwy]

Two places where authentication state configured for one catalog or table persists onto another.

1. commit_table mutates the shared session's headers

headers = self._session.headers            # the live session mapping, not a copy
if table_token := table.config.get(TOKEN):
    headers[AUTHORIZATION_HEADER] = f"{BEARER_PREFIX} {table_token}"

self._session.headers is the session's own mapping, so assigning into it persists the table-scoped token on the session. Every subsequent request from that RestCatalog carries it, including requests for other tables.

2. fsspec-cached S3FileSystem instances share one signer registration

fs = S3FileSystem(**s3_fs_kwargs)
for event_name, event_function in register_events.items():
    fs.s3.meta.events.unregister(event_name, unique_id=1925)
    fs.s3.meta.events.register_last(event_name, event_function, unique_id=1925)

The signer is registered as a botocore event handler after construction, under a fixed unique_id. It is not part of s3_fs_kwargs, and fsspec caches filesystem instances by constructor arguments (skip_instance_cache is not set). Two catalogs whose anon / client_kwargs / config_kwargs match therefore receive the same S3FileSystem object, and the second unregister + register_last replaces the first catalog's signer with its own.

Note the per-thread lru_cache in get_fs is not involved — _thread_locals is an instance attribute, so that cache is already per-FileIO. The sharing comes from fsspec's instance cache.

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Issue investigation generated via claude, reviewed by Sung, Kevin, Fokko.