Wrap token obtain and refresh logic in a critical section

get_and_refresh_token_in_a_critical_section.kt

suspend fun obtainAccessToken(): String =
    KCriticalSection().synchronized {
        val accessToken = readAccessTokenFromStorageOrMemory() // might be a suspending call

        if (accessToken != null && System.currentTimeMillis() < accessToken.createdAt + accessToken.expiresIn) {
            return@synchronized accessToken.toString() // access token is valid
        }

        val refreshToken = readRefreshTokenFromSecureStorage() // normally a suspending call

        // normally suspending as it makes another API call
        val newAccessToken = performTokenRefresh(
            accessToken = accessToken,
            refreshToken = refreshToken
        )

        saveAccessTokenToStorageOrMemory(newAccessToken) // might be a suspending call
        return@synchronized  newAccessToken.toString()
    }