Example showing Ktor OAuth2 usage as OAuth2 Flow for Compose Desktop

RefreshToken.kt

suspend fun refreshToken(): String? = withContext(Dispatchers.IO) {
        val callback = CompletableDeferred<OAuth2>()
        val server = embeddedServer(CIO, port = 0) {
            val port = async { engine.resolvedConnectors().first().port }
            authentication {
                oauth("oauth") {
                    @OptIn(ExperimentalCoroutinesApi::class)
                    urlProvider = { "http://localhost:${port.getCompleted()}/callback" }
                    providerLookup = {
                        OAuthServerSettings.OAuth2ServerSettings(
                            name = "google",
                            authorizeUrl = "https://accounts.google.com/o/oauth2/auth",
                            accessTokenUrl = "https://oauth2.googleapis.com/token",
                            requestMethod = HttpMethod.Post,
                            clientId = config.googleClientId,
                            clientSecret = config.clientSecret,
                            defaultScopes = listOf("email"),
                        )
                    }
                    client = httpClient
                }
            }
            routing {
                authenticate("oauth") {
                    get("/login") {}
                    get("/callback") {
                        val principal: OAuth2? = call.authentication.principal()
                        if (principal == null) {
                            callback.completeExceptionally(IllegalStateException("No OAuth2 principal"))
                            call.respondText(createErrorResponseHtml(), ContentType.Text.Html)
                        } else {
                            callback.complete(principal)
                            call.respondText(createSuccessResponseHtml(), ContentType.Text.Html)
                        }
                    }
                }
            }
        }

        try {
            server.startSuspend(wait = false)
            val port = server.engine.resolvedConnectors().first().port
            val response = httpClient.config {
                followRedirects = false
            }.get("http://localhost:$port/login")
            require(response.status == HttpStatusCode.Found) {
                "Server returned ${response.status} status code. Expected 302 Found for OAuth redirect."
            }
            val url = response.headers["Location"]!!
            Desktop.getDesktop().browse(URI(url))
            val oauth = callback.await()
            token = oauth
            oauth.extraParameters["id_token"]
        } finally {
            withContext(NonCancellable) { server.stopSuspend(1000, 5000) }
        }
    }