[Bug] Fix multipart schema inference for allOf/anyOf/oneOf of primitive types and non-binary arrays (#391)

[Bug] Fix multipart schema inference for allOf/anyOf/oneOf of primitive types and non-binary arrays

### Motivation

As I started testing the multipart generation on real-world projects, I discovered two bugs:
- allOf/anyOf/oneOf of primitive types (such as string) were encoded as JSON instead of a raw string (aka HTTPBody), which was wrong
- arrays of non-binary and arrays of binary elements were treated inconsistently

### Modifications

Fixed the bug by refactoring the inferrence logic a bit.

### Result

Now e.g. an anyOf of a string still gets encoded as a primitive type, not JSON.

### Test Plan

Added a unit test for this logic with a few test cases, easier to debug this way.


Reviewed by: simonjbeaumont

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#391

Author: czechboy0

Sources/_OpenAPIGeneratorCore/Translator/Multipart/MultipartContentInspector.swift

@@ -257,31 +257,52 @@ extension FileTranslator {
             default: return .infer(.primitive)
             }
         }
-        let repetitionKind: MultipartPartInfo.RepetitionKind
-        let candidateSource: MultipartPartInfo.ContentTypeSource
-        switch try schema.dereferenced(in: components) {
-        case .null, .not: return nil
-        case .boolean, .number, .integer:
-            repetitionKind = .single
-            candidateSource = .infer(.primitive)
-        case .string(_, let context):
-            repetitionKind = .single
-            candidateSource = try inferStringContent(context)
-        case .object, .all, .one, .any, .fragment:
-            repetitionKind = .single
-            candidateSource = .infer(.complex)
-        case .array(_, let context):
-            repetitionKind = .array
-            if let items = context.items {
-                switch items {
-                case .null, .not: return nil
-                case .boolean, .number, .integer: candidateSource = .infer(.primitive)
-                case .string(_, let context): candidateSource = try inferStringContent(context)
-                case .object, .all, .one, .any, .fragment, .array: candidateSource = .infer(.complex)
-                }
-            } else {
+        func inferAllOfAnyOfOneOf(_ schemas: [DereferencedJSONSchema]) throws -> MultipartPartInfo.ContentTypeSource? {
+            // If all schemas are primitive, the allOf/anyOf/oneOf is also primitive.
+            // These cannot be binary, so only primitive vs complex.
+            for schema in schemas {
+                guard let (_, kind) = try inferSchema(schema) else { return nil }
+                guard case .infer(.primitive) = kind else { return kind }
+            }
+            return .infer(.primitive)
+        }
+        func inferSchema(_ schema: DereferencedJSONSchema) throws -> (
+            MultipartPartInfo.RepetitionKind, MultipartPartInfo.ContentTypeSource
+        )? {
+            let repetitionKind: MultipartPartInfo.RepetitionKind
+            let candidateSource: MultipartPartInfo.ContentTypeSource
+            switch schema {
+            case .null, .not: return nil
+            case .boolean, .number, .integer:
+                repetitionKind = .single
+                candidateSource = .infer(.primitive)
+            case .string(_, let context):
+                repetitionKind = .single
+                candidateSource = try inferStringContent(context)
+            case .object, .fragment:
+                repetitionKind = .single
                 candidateSource = .infer(.complex)
+            case .all(of: let schemas, _), .one(of: let schemas, _), .any(of: let schemas, _):
+                repetitionKind = .single
+                guard let value = try inferAllOfAnyOfOneOf(schemas) else { return nil }
+                candidateSource = value
+            case .array(_, let context):
+                repetitionKind = .array
+                if let items = context.items {
+                    switch items {
+                    case .null, .not: return nil
+                    case .boolean, .number, .integer: candidateSource = .infer(.primitive)
+                    case .string(_, let context): candidateSource = try inferStringContent(context)
+                    case .object, .all, .one, .any, .fragment, .array: candidateSource = .infer(.complex)
+                    }
+                } else {
+                    candidateSource = .infer(.complex)
+                }
             }
+            return (repetitionKind, candidateSource)
+        }
+        guard let (repetitionKind, candidateSource) = try inferSchema(schema.dereferenced(in: components)) else {
+            return nil
         }
         let finalContentTypeSource: MultipartPartInfo.ContentTypeSource
         if let encoding, let contentType = encoding.contentType {
@@ -301,9 +322,23 @@ extension FileTranslator {
             let resolvedSchema: JSONSchema
             if isOptional { resolvedSchema = baseSchema.optionalSchemaObject() } else { resolvedSchema = baseSchema }
             return (info, resolvedSchema)
+        } else if repetitionKind == .array {
+            let isOptional = try typeMatcher.isOptional(schema, components: components)
+            guard case .array(_, let context) = schema.value else {
+                preconditionFailure("Array repetition should always use an array schema.")
+            }
+            let elementSchema: JSONSchema = context.items ?? .fragment
+            let resolvedSchema: JSONSchema
+            if isOptional {
+                resolvedSchema = elementSchema.optionalSchemaObject()
+            } else {
+                resolvedSchema = elementSchema
+            }
+            return (info, resolvedSchema)
         }
         return (info, schema)
     }
+
     /// Parses the names of component schemas used by multipart request and response bodies.
     ///
     /// The result is used to inform how a schema is generated.

Tests/OpenAPIGeneratorCoreTests/Translator/Multipart/Test_MultipartContentInspector.swift

@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import OpenAPIKit
+@testable import _OpenAPIGeneratorCore
+
+class Test_MultipartContentInspector: Test_Core {
+    func testSerializationStrategy() throws {
+        let translator = makeTypesTranslator()
+        func _test(
+            schemaIn: JSONSchema,
+            encoding: OpenAPI.Content.Encoding? = nil,
+            source: MultipartPartInfo.ContentTypeSource,
+            repetition: MultipartPartInfo.RepetitionKind,
+            schemaOut: JSONSchema,
+            file: StaticString = #file,
+            line: UInt = #line
+        ) throws {
+            let (info, actualSchemaOut) = try XCTUnwrap(
+                translator.parseMultipartPartInfo(schema: schemaIn, encoding: encoding, foundIn: "")
+            )
+            XCTAssertEqual(info.repetition, repetition, file: file, line: line)
+            XCTAssertEqual(info.contentTypeSource, source, file: file, line: line)
+            XCTAssertEqual(actualSchemaOut, schemaOut, file: file, line: line)
+        }
+        try _test(schemaIn: .object, source: .infer(.complex), repetition: .single, schemaOut: .object)
+        try _test(schemaIn: .array(items: .object), source: .infer(.complex), repetition: .array, schemaOut: .object)
+        try _test(
+            schemaIn: .string,
+            source: .infer(.primitive),
+            repetition: .single,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+        try _test(
+            schemaIn: .integer,
+            source: .infer(.primitive),
+            repetition: .single,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+        try _test(
+            schemaIn: .boolean,
+            source: .infer(.primitive),
+            repetition: .single,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+        try _test(
+            schemaIn: .string(allowedValues: ["foo"]),
+            source: .infer(.primitive),
+            repetition: .single,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+        try _test(
+            schemaIn: .array(items: .string),
+            source: .infer(.primitive),
+            repetition: .array,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+        try _test(
+            schemaIn: .any(of: .string, .string(allowedValues: ["foo"])),
+            source: .infer(.primitive),
+            repetition: .single,
+            schemaOut: .string(contentEncoding: .binary)
+        )
+    }
+}