External File Loading for Swift OpenAPI Generator

ExternalLoader.swift

import Foundation
import OpenAPIKit
import Yams

struct NonFileURLError: Swift.Error {
    let url: URL
}

public struct GeneratorExternalLoader: ExternalLoader {
    public typealias Message = Void

    public static func load<T>(_ url: URL) async throws -> (T, [Message]) where T: Decodable {
//        guard url.isFileURL else { throw NonFileURLError(url: url) }
        let fileUrl = URL(string: url.absoluteString, relativeTo: URL.currentDirectory())!

        let (data, _) = try await URLSession.shared.data(from: fileUrl)

        let decodedValue = try YAMLDecoder().decode(T.self, from: data)
        let augmentedValue: T
        // we want to keep track of where a reference was loaded from. Vendor
        // Extensions are a nice place for this information.
        if var extendable = decodedValue as? any VendorExtendable {
            extendable.vendorExtensions["x-source-url"] = AnyCodable(url)
            augmentedValue = extendable as! T
        } else {
            augmentedValue = decodedValue
        }
        return (augmentedValue, [])
    }

    public static func componentKey<T>(type: T.Type, at url: URL) throws -> OpenAPIKit.OpenAPI.ComponentKey {
        // unverified whether this will always result in a valid components key.
        // needs more thinking.
        let pathExtension = url.pathExtension
        let urlString = url.pathComponents.dropFirst().joined(separator: "_").replacingOccurrences(of: ".", with: "_")
        let componentKeyString: String
        if pathExtension.isEmpty {
            componentKeyString = urlString
        } else {
            componentKeyString = String(urlString.dropLast(pathExtension.count + 1))
        }
        return try .forceInit(rawValue: componentKeyString)
    }
}

FilterCommand.diff

diff --git a/Sources/swift-openapi-generator/FilterCommand.swift b/Sources/swift-openapi-generator/FilterCommand.swift
index bb689b5..27881ba 100644
--- a/Sources/swift-openapi-generator/FilterCommand.swift
+++ b/Sources/swift-openapi-generator/FilterCommand.swift
@@ -43,10 +43,11 @@ struct _FilterCommand: AsyncParsableCommand {
         let configData = try Data(contentsOf: config)
         let config = try YAMLDecoder().decode(_UserConfig.self, from: configData)
         let documentInput = try InMemoryInputFile(absolutePath: docPath, contents: Data(contentsOf: docPath))
-        let document = try timing(
+        var document = try timing(
             "Parsing document",
             YamsParser.parseOpenAPIDocument(documentInput, diagnostics: StdErrPrintingDiagnosticCollector())
         )
+        try await document.externallyDereference(with: GeneratorExternalLoader.self, depth: 1)
         guard let documentFilter = config.filter else {
             FileHandle.standardError.write("warning: No filter config provided\n")
             FileHandle.standardOutput.write(try encode(document, outputFormat))

Hello.yml

summary: hello

Main.yml

openapi: '3.2.0'
info:
    title: test
    version: '1.0.0'
paths:
    '/hello':
        $ref: './Hello.yml'

output.yml

openapi: 3.1.2
info:
  title: test
  version: 1.0.0
paths:
  /hello:
    $ref: '#/components/pathItems/Hello'
components:
  pathItems:
    Hello:
      summary: hello
      x-source-url: ./Hello.yml