enable_url_table returns a handle on a different session, unlike every other with_* derivation

从 crates/core/src/context.rs:425-428 开始，将 enable_url_table 与 set_session_query_planner (1729-1740) 以及 with_python_udf_inlining (1571-1588) 进行比较。运行 issue 的 Python reproduction，并检查 examples/create-context.py:41 以及列出的 FFI 和 Upgrade Guide 部分。完成的标准是：重复调用是安全的，handles 共享同一个 session 和 state，绑定到 FFI 的组件保持有效，并且所引用的文档和 docstring 描述了共享行为。

Describe the bug

SessionContext.enable_url_table is the only derivation in the binding that returns a handle on a different session than the one it was called on, and it is the only one that leaves two live handles reporting the same session_id with divergent state. Every other with_* method returns a handle wrapping the same underlying session.

The root cause is one line. Upstream's SessionContext::enable_url_table consumes self (datafusion/core/src/execution/context/mod.rs:414):

pub fn enable_url_table(self) -> Self {
    let current_catalog_list = Arc::clone(self.state.read().catalog_list());
    let factory = Arc::new(DynamicListTableFactory::new(SessionStore::new()));
    let catalog_list = Arc::new(DynamicFileCatalog::new(
        current_catalog_list,
        Arc::clone(&factory) as Arc<dyn UrlTableFactory>,
    ));
    let session_id = self.session_id.clone();
    let ctx: SessionContext = self
        .into_state_builder()
        .with_session_id(session_id)
        .with_catalog_list(catalog_list)
        .build()
        .into();
    factory.session_store().with_state(ctx.state_weak_ref());
    ctx
}

Taking self by value means the old and new contexts are never meant to coexist, which is exactly why carrying session_id over is correct there. The binding at crates/core/src/context.rs:425 takes &self and clones instead:

ctx: Arc::new(self.ctx.as_ref().clone().enable_url_table()),

That manufactures the coexistence upstream's signature prevents, and everything else is downstream of it: the must-not-outlive caveat at crates/core/src/context.rs:428, the "one exception" paragraph at docs/source/contributor-guide/ffi.md:475-477, and the state divergence below.

To Reproduce

from datafusion import SessionContext

a = SessionContext()
b = a.enable_url_table()
print("same session_id:", a.session_id() == b.session_id())

a.sql("SET datafusion.execution.batch_size = 111").collect()
print("a", a.sql("SHOW datafusion.execution.batch_size").collect()[0].column(1).to_pylist())
print("b", b.sql("SHOW datafusion.execution.batch_size").collect()[0].column(1).to_pylist())

same session_id: True
a ['111']
b ['8192']

Two live sessions with independent SessionState, both reporting one id.

The second half of the problem is not reproducible without a built FFI extension, but it is what the existing caveat comments are about: any FFI codec, query planner, or task-context provider handed out by the receiver holds an FFI_TaskContextProvider bound weakly to the receiver's Arc<SessionContext>. The returned context is a different allocation, so ctx = ctx.enable_url_table() drops the last strong reference and those components fail at query time with TaskContextProvider went out of scope over FFI boundary. examples/create-context.py:41 is written as ctx = ctx.enable_url_table() — exactly the pattern the caveat forbids. Harmless in that example because it installs no FFI components, but it is the shape users copy.

Expected behavior

enable_url_table should behave like with_logical_extension_codec, with_physical_extension_codec, and with_python_udf_inlining: return a handle on the same underlying session, so session_id stays unique to a session, state cannot diverge, and FFI components bound to any handle in the lineage stay valid.

The pattern already exists in the same file. set_session_query_planner (crates/core/src/context.rs:1729-1740) mutates through state_ref() rather than deriving a new context, including the load-bearing with_session_id carry-over:

let state_ref = self.ctx.state_ref();
let factory = Arc::new(DynamicListTableFactory::new(SessionStore::new()));
let catalog_list = Arc::new(DynamicFileCatalog::new(
    Arc::clone(state_ref.read().catalog_list()),
    Arc::clone(&factory) as Arc<dyn UrlTableFactory>,
));
{
    let mut guard = state_ref.write();
    *guard = SessionStateBuilder::new_from_existing(guard.clone())
        .with_session_id(guard.session_id().to_string())
        .with_catalog_list(catalog_list)
        .build();
}
factory.session_store().with_state(Arc::downgrade(&state_ref));

The returned handle then shares Arc::clone(&self.ctx) like every sibling.

Two details for whoever picks this up:

• Needs an idempotence guard. DynamicFileCatalog::new wraps whatever catalog list is current, so calling enable_url_table twice nests wrappers. Upstream has the same wart, but mutating in place makes it easier to hit. with_python_udf_inlining (crates/core/src/context.rs:1571-1588) already establishes the pattern of skipping the state rebuild when the call would change nothing.
• This is an api change. ctx = ctx.enable_url_table() keeps working, but the original handle also gains url tables afterwards. That is the same trade with_logical_extension_codec already documents — it takes effect on the shared session even if the returned context is discarded — so this is bringing enable_url_table in line rather than introducing a new surprise. Needs a section in docs/source/user-guide/upgrade-guides.md, plus updates to the docstring at python/datafusion/context.py:589, the "one exception" paragraph at docs/source/contributor-guide/ffi.md:475-477, and the shared-derivation list at docs/source/contributor-guide/ffi.md:481-486.

Additional context

Found while reviewing #1679. That PR's _derive_for_extensions currently forks state the same way, so the "one method that mints a second Arc<SessionContext>" comment at crates/core/src/context.rs:428 is temporarily inaccurate. If #1679 lands with that fork removed, enable_url_table is once again the only site and that comment becomes true again.

The two are independent in both directions and should not be chained: #1679 does not need this fixed, and this does not need #1679.