Second of four; stacked on #38046. Mechanical relocation plus one design change
that the relocation makes possible. **Review by checking the identity proofs at
the bottom** -- nothing here is meant to change behaviour.
## The declarations move
`ServerArgs` carried all 487 declarations in one 4,462-line file, each tagged
with an `NS("...")` marker naming the namespace it belongs to -- structure
supplied by annotation, in a file a namespace away from the
`arg_groups/*_hook.py` that resolves it.
They move to `arg_groups/fields/`: one module per top-level namespace, one class
per leaf namespace (21 of them, `exec.moe` becomes `exec_.py::ExecMoe`). The
class carries the `_NS_PATH` it stands for, so the module a field is declared in
*is* its namespace and the marker is redundant -- `namespace_of` reads the
declaring class instead. `NS` stays for the one case a class cannot express: a
single ad-hoc dataclass whose fields span namespaces, which is what the
config-bag tests build.
Two things travel with the declarations. The `*_CHOICES` lists and the
`add_*_choices` adders that extend them move to `arg_groups/choices.py`, since
the fields naming them can no longer import from `server_args` without a cycle;
`server_args` re-exports all of them, because out-of-tree plugins have always
reached them there. And five fields whose only annotation element was the
namespace marker become plain annotations -- `A` is `Annotated`, which needs two
arguments, so stripping the marker would have left them invalid.
`server_args.py` goes from 4,458 lines to about 1,000.
## The record is assembled, not inherited
Inheriting the namespace classes would make the record's contents a property of
which classes happen to appear in a base list. That is correct only while every
namespace declares nothing but operator input, and it stops being correct the
moment a derived field is declared: `attn_tp_size` belongs in `parallel.py`
next to the leaves it is derived from, and inheriting `Parallel` would put it on
the record -- where it is neither input nor safe, since the record is what
crosses a process boundary and a derived width pickled to a subprocess is a
stamp that elastic scale-up will not refresh.
`collect_input_fields` takes the classes that declare input and returns their
annotations, defaults and namespaces. Each source's annotations are resolved in
its own module and handed on as type objects; carried across as text they would
be re-evaluated where they land, and the composing module deliberately imports
none of the names the declarations use. A namespace can now declare both halves
side by side, and which half reaches the record is one readable call rather than
an invariant spread across a base-class list. Nothing is registered on the
derived side yet -- this is what makes it possible.
`ServerArgs` is still one flat dataclass with 494 attributes, so
`server_args.tp_size`, `ServerArgs(model_path=..., tp_size=8)`, pickling to a
subprocess and every existing call site are untouched.
### Field order is a contract, so it is written down
A dataclass turns field order into a positional constructor signature, and
collecting whole namespaces groups fields that used to be interleaved. Keeping
`model_path` first is not enough: `ServerArgs("dummy", "/tmp/tokenizer")` would
set `load_format="/tmp/tokenizer"` and leave `tokenizer_path=None`, which then
selects an invalid model loader -- silently, at a call site that did not change.
So `arg_groups/field_order.py` records the order the record had before the
split, and `collect_input_fields` orders what it collects by it. A field the
record declares that the frozen order does not name goes after it, in
declaration order -- the only backward-compatible place for a new field anyway,
so a new declaration needs no edit there. The list is a compatibility record and
nothing else reads it; the namespace a field belongs to is still the module it
is declared in.
## Verification
Four ways, all against the base commit:
| check | result |
|---|---|
| `namespace_of` map, field by field | 494 / 494, **0 differences** |
| CLI surface (options, defaults, choices, actions) | 507 / 507, **0 differences** |
| field order, name by name | 494 / 494, **identical to the base** |
| resolution result, 24 launch shapes x 489 fields | **0 differences** |
| names importable from `sglang.srt.server_args` | nothing lost |
Plus a full registered-unit sweep (648 files) against the stack's merge-base:
19 failures on both sides, the same 19, none of them config.
Cheng Wan
ed82def55f