aio-libs / multidict / 35971910018

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Pass the entry identity to the ForEach visitor (#1552)

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## What do these changes do?

The `ForEach` visitor received the key and the value but not the
identity,
the canonical key a multidict actually looks entries up by. On a
`CIMultiDict` that is the internal lower-cased form, so a visitor
wanting
to group or compare entries by it had to derive it again itself,
duplicating work the walk had already done.

`MultiDict_ItemVisitor` now takes
`(void *user_data, PyObject *identity, PyObject *key, PyObject *value)`.
`user_data` stays first so existing call sites keep their shape, and the
identity goes next to the key and value it belongs with rather than at
the
end. `MultiDict_CyItemVisitor` gains it in the same position, keeping
its
own `user_data`-last order. The internal `md_item_visitor_t` in
`_multilib/walk.h` is widened to match, along with `md_walk_all`,
`md_walk_with_hash` and their two in-tree visitors.

The two walks source it differently. `md_walk_with_hash` passes the
identity it was already handed, which every matched entry compares equal
to by the walk's own predicate, so the keyed path does no extra work.
`md_walk_all` reads `entry->identity` and holds a reference across the
visitor call, the same way it already does for the value, since
`_md_ensure_key()` and the visitor can both run Python code that frees
the entry.

The capsule is unreleased: it landed in #1480, after v6.9.1, and
`MultiDict_CAPI_VERSION` is still 1, so no client can have been built
against the narrower signature. That is why the typedef changes in place
instead of growing a parallel entry point at the end of the struct with
a
version bump.

## Are there changes in behavior for the user?

No, for Python-level users. A C or Cython client of the capsule has to
take the extra parameter, but the capsule has not shipped in a release
yet, so there is nothing to migrate.

## Is it a substantial burden for the maintainers to support this?... (continued)

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