v3.2: Support nested multipart with nested Encoding Objects

src/oas.md

@@ -1641,6 +1641,9 @@ These fields MAY be used either with or without the RFC6570-style serialization
 | ---- | :----: | ---- |
 | <a name="encoding-content-type"></a>contentType | `string` | The `Content-Type` for encoding a specific property. The value is a comma-separated list, each element of which is either a specific media type (e.g. `image/png`) or a wildcard media type (e.g. `image/*`). Default value depends on the property type as shown in the table below. |
 | <a name="encoding-headers"></a>headers | Map[`string`, [Header Object](#header-object) \| [Reference Object](#reference-object)] | A map allowing additional information to be provided as headers. `Content-Type` is described separately and SHALL be ignored in this section. This field SHALL be ignored if the media type is not a `multipart`. |
+| <a name="encoding-encoding"></a>encoding | Map[`string`, [Encoding Object](#encoding-object)] | Applies nested Encoding Objects in the same manner as the [Media Type Object](#media-type-object)'s `encoding` field. |
+| <a name="encoding-prefix-encoding"></a>prefixEncoding | [[Encoding Object](#encoding-object)] | Applies nested Encoding Objects in the same manner as the [Media Type Object](#media-type-object)'s `prefixEncoding` field. |
+| <a name="encoding-item-encoding"></a>itemEncoding | [Encoding Object](#encoding-object) | Applies nested Encoding Objects in the same manner as the [Media Type Object](#media-type-object)'s `itemEncoding` field. |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
 
@@ -1674,6 +1677,12 @@ See also [Appendix C: Using RFC6570 Implementations](#appendix-c-using-rfc6570-b
 Note that the presence of at least one of `style`, `explode`, or `allowReserved` with an explicit value is equivalent to using `schema` with `in: "query"` Parameter Objects.
 The absence of all three of those fields is the equivalent of using `content`, but with the media type specified in `contentType` rather than through a Media Type Object.
 
+##### Nested Encoding
+
+Nested formats requiring encoding, most notably nested `multipart/mixed`, can be supported with this Object's `encoding`, `prefixEncoding`, and / or `itemEncoding` fields.
+Implementations MUST support one level of nesting, and MAY support additional levels.
+If supporting additional levels, any limits on nesting levels MUST be documented.
+
 ##### Encoding the `x-www-form-urlencoded` Media Type
 
 To work with content using form url encoding via [RFC1866](https://tools.ietf.org/html/rfc1866), use the `application/x-www-form-urlencoded` media type in the [Media Type Object](#media-type-object).
@@ -1869,6 +1878,30 @@ requestBody:
 
 As seen in the [Encoding Object's `contentType` field documentation](#encoding-content-type), the empty schema for `items` indicates a media type of `application/octet-stream`.
 
+###### Example: Nested `multipart/mixed`
+
+This defines a two-part `multipart/mixed` where the first part is JSON and the second part is a nested `multipart/mixed` document.
+The nested parts are JSON, plain text, and a PNG image.
+
+```yaml
+multipart/mixed:
+  schema:
+    type: array
+    prefixItems:
+    - type: array
+    - prefixItems:
+      - type: object
+      - type: string
+      - {}
+  prefixEncoding:
+    - {} # Accept the default application/json
+    - contentType: multipart/mixed
+      prefixEncoding:
+      - contentType: application/xml
+      - {} # Accept the default text/plain
+      - contentType: image/png
+```
+
 #### Responses Object
 
 A container for the expected responses of an operation.

src/schemas/validation/schema.yaml

@@ -561,9 +561,24 @@ $defs:
       allowReserved:
         default: false
         type: boolean
+      encoding:
+        type: object
+        additionalProperties:
+          $ref: '#/$defs/encoding'
+      prefixEncoding:
+        type: array
+        items:
+          $ref: '#/$defs/encoding'
+      itemEncoding:
+        $ref: '#/$defs/encoding'
     allOf:
       - $ref: '#/$defs/specification-extensions'
       - $ref: '#/$defs/styles-for-form'
+      - dependentSchemas:
+          encoding:
+            properties:
+              prefixEncoding: false
+              itemEncoding: false
     unevaluatedProperties: false
 
   responses:

tests/schema/fail/encoding-enc-item-exclusion.yaml

@@ -0,0 +1,12 @@
+openapi: 3.2.0
+info:
+  title: API
+  version: 1.0.0
+components:
+  requestBodies:
+    content:
+      multipart/mixed:
+        prefixEncoding:
+        - contentType: multipart/mixed
+          encoding: {}
+          prefixEncoding: []

tests/schema/fail/encoding-enc-prefix-exclusion.yaml

@@ -0,0 +1,12 @@
+openapi: 3.2.0
+info:
+  title: API
+  version: 1.0.0
+components:
+  requestBodies:
+    content:
+      multipart/mixed:
+        prefixEncoding:
+        - contentType: multipart/mixed
+          encoding: {}
+          itemEncoding: []

tests/schema/pass/media-type-examples.yaml

@@ -117,6 +117,10 @@ paths:
                   type: string
                 forCoverage2:
                   type: string
+                nested1:
+                  type: object
+                nested2:
+                  type: array
             encoding:
               addresses:
                 # require XML Content-Type in utf-8 encoding
@@ -138,3 +142,12 @@ paths:
               forCoverage2:
                 style: spaceDelimited
                 explode: true
+              nested1:
+                contentType: multipart/form-data
+                encoding:
+                  inner: {}
+              nested2:
+                contentType: multipart/mixed
+                prefixEncoding:
+                - {}
+                itemEncoding: {}