Ktor Google OAuth2

KtorOAuth.kt

import io.ktor.client.HttpClient
import io.ktor.client.call.body
import io.ktor.client.engine.cio.CIO
import io.ktor.client.request.get
import io.ktor.client.request.headers
import io.ktor.http.HttpHeaders
import io.ktor.http.HttpMethod
import io.ktor.http.HttpStatusCode
import io.ktor.server.application.call
import io.ktor.server.application.Application
import io.ktor.server.application.install
import io.ktor.server.auth.Authentication
import io.ktor.server.auth.OAuthAccessTokenResponse
import io.ktor.server.auth.OAuthServerSettings
import io.ktor.server.auth.authenticate
import io.ktor.server.auth.oauth
import io.ktor.server.auth.principal
import io.ktor.server.html.respondHtml
import io.ktor.server.response.respondRedirect
import io.ktor.server.response.respondText
import io.ktor.server.routing.get
import io.ktor.server.routing.routing
import io.ktor.server.sessions.Sessions
import io.ktor.server.sessions.cookie
import io.ktor.server.sessions.get
import io.ktor.server.sessions.sessions
import io.ktor.server.sessions.set
import kotlinx.html.a
import kotlinx.html.body
import kotlinx.html.p
import kotlinx.serialization.SerialName

fun Application.installAuth() {
  val httpClient = HttpClient(CIO) {
    install(io.ktor.client.plugins.ContentNegotiation) {
      json()
    }
  }

  install(Sessions) {
    cookie<UserSession>("user_session")
  }

  install(Authentication) {
    oauth("auth-oauth-google") {
      urlProvider = { "http://localhost:8080/callback" }
      providerLookup = {
        OAuthServerSettings.OAuth2ServerSettings(
          name = "google",
          authorizeUrl = "https://accounts.google.com/o/oauth2/auth",
          accessTokenUrl = "https://accounts.google.com/o/oauth2/token",
          requestMethod = HttpMethod.Post,
          clientId = System.getenv("GOOGLE_CLIENT_ID"),
          clientSecret = System.getenv("GOOGLE_CLIENT_SECRET"),
          defaultScopes = listOf("https://www.googleapis.com/auth/userinfo.profile")
        )
      }
      // Closeable
      client = httpClient
    }
  }

  routing {
    authenticate("auth-oauth-google") {
      get("/login") {
        // Redirects to 'authorizeUrl' automatically
      }

      get("/callback") {
        val principal: OAuthAccessTokenResponse.OAuth2? = call.principal()
        call.sessions.set(UserSession(principal?.accessToken.toString()))
        call.respondRedirect("/hello")
      }
    }

    get("/") {
      call.respondHtml(HttpStatusCode.OK) {
        body {
          p {
            a("/login") { +"Login with Google" }
          }
        }
      }
    }

    get("/hello") {
      val userSession: UserSession? = call.sessions.get()
      if (userSession != null) {
        val userInfo: UserInfo = httpClient.get("https://www.googleapis.com/oauth2/v2/userinfo") {
          headers {
            append(HttpHeaders.Authorization, "Bearer ${userSession.token}")
          }
        }.body()
        call.respondText("Hello, ${userInfo.name}!")
      } else {
        call.respondRedirect("/")
      }
    }
  }
}


data class UserSession(val token: String)

@kotlinx.serialization.Serializable
data class UserInfo(
  val id: String,
  val name: String,
  @SerialName("given_name") val givenName: String,
  @SerialName("family_name") val familyName: String,
  val picture: String,
  val locale: String
)