🐛 fix(schema): declare prefix in the labeled factor group

SchemaStore compiles the published schema with ajv strict mode, which
refuses a `required` naming a property the schema never declares. The
labeled factor group has said `not: {required: [prefix]}` since April to
keep a bare range dict from reading as a label, and ajv had been
satisfied by `prefix` reaching it from an inlined sibling. Adding the
range and values shapes to its `additionalProperties` in 4.61 changed
how ajv inlines, the property stopped arriving, and the release sync PR
failed to compile.

Declaring `prefix` inside the `not` with an empty schema names it for
strict mode while constraining nothing, so validation is unchanged. The
new test walks the committed schema for the same defect, since neither
the freshness check nor the tombi lint compiles the schema this way.

Author: gaborbernat

docs/changelog/4051.bugfix.rst

@@ -0,0 +1,2 @@
+Declare ``prefix`` inside the labeled factor group's ``not`` clause of the generated JSON Schema, so the published
+schema compiles under the strict mode SchemaStore validates with - by :user:`gaborbernat`.

src/tox/session/cmd/schema.py

@@ -223,7 +223,8 @@ def gen_schema(state: State) -> int:
                 "type": "object",
                 "minProperties": 1,
                 "maxProperties": 1,
-                "not": {"required": ["prefix"]},
+                # ``properties`` declares the key so ajv strict mode can see it; ``{}`` adds no constraint of its own
+                "not": {"properties": {"prefix": {}}, "required": ["prefix"]},
                 "additionalProperties": {
                     "oneOf": [
                         {"type": "array", "items": {"type": "string"}},

src/tox/tox.schema.json

@@ -804,6 +804,9 @@
       "minProperties": 1,
       "maxProperties": 1,
       "not": {
+        "properties": {
+          "prefix": {}
+        },
         "required": ["prefix"]
       },
       "additionalProperties": {

tests/session/cmd/test_schema.py

@@ -11,6 +11,8 @@
 import pytest
 
 if TYPE_CHECKING:
+    from collections.abc import Iterator
+
     from tox.pytest import MonkeyPatch, ToxProjectCreator
 
 
@@ -143,6 +145,29 @@ def test_schema_freshness(
     )
 
 
+def _subschemas(node: Any, pointer: str = "#") -> Iterator[tuple[str, dict[str, Any]]]:
+    if isinstance(node, dict):
+        yield pointer, node
+        for key, value in node.items():
+            yield from _subschemas(value, f"{pointer}/{key}")
+    elif isinstance(node, list):
+        for index, value in enumerate(node):
+            yield from _subschemas(value, f"{pointer}/{index}")
+
+
+def test_schema_required_properties_are_declared(committed_schema: dict[str, Any]) -> None:
+    # SchemaStore compiles the published schema with ajv strict mode, which rejects a required property that the
+    # schema never declares, so catch it here rather than in a release-time sync PR (see tox-dev/tox#4051)
+    undeclared = [
+        f"{pointer} requires {name!r}"
+        for pointer, schema in _subschemas(committed_schema)
+        if isinstance(schema.get("required"), list)
+        for name in schema["required"]
+        if name not in schema.get("properties", {})
+    ]
+    assert undeclared == []
+
+
 def test_schema_allows_deps_array(committed_schema: dict[str, Any]) -> None:
     deps_schema = committed_schema["properties"]["env_run_base"]["properties"]["deps"]