Custom baseUrl does not work when using ConfigCat Proxy

[croemmich]

Describe the bug

We're running the ConfigCat Proxy (v0.4.2) and have set a custom baseUrl. The initial config fetch works however subsequent fetches do not.

The issue lies in how preferences from the response are being handled:

kotlin-sdk/src/commonMain/kotlin/com/configcat/fetch/ConfigFetcher.kt

Lines 33 to 39 in ea7a1fb

	suspend fun fetch(eTag: String): FetchResponse {
	val currentUrl = baseUrl.value
	val response = fetchHTTPWithPreferenceHandling(currentUrl, eTag)
	val newUrl = response.entry.config.preferences?.baseUrl ?: currentUrl
	baseUrl.update { newUrl }
	return response
	}

response.entry.config.preferences?.baseUrl is https://cdn-global.configcat.com. As such, baseUrl.update { "https://cdn-global.configcat.com" } is being called, which breaks the client.

I'm not sure if this should be fixed in the client or if the proxy should be re-writing the baseUrl.

I also don't love that the redirect preference can force the baseUrl to update when using the proxy.

SDK version

com.configcat:configcat-kotlin-client-jvm:3.0.0

SDK configuration

ConfigCatClient("configcat-proxy/my-sdk") {
    baseUrl = "http://proxy-url"
}

Logs

2024-04-09 16:44:59 UTC [ERROR]: ConfigCat - [1100] Your SDK Key seems to be wrong. You can find the valid SDK Key at https://app.configcat.com/sdkkey. Received response: 403

Language/Framework version

Kotlin 1.9.23
Java 21

Platform

Mac/Linux

[z4kn4fein]

Hi @croemmich, thank you for reporting this issue! I'm going to create a fix here, because it's indeed a bug in the Kotlin SDK and let you know when it's published. Also, we'll discuss the possibility of removing the whole preferences section of the config JSON on the Proxy side to prevent any accidental redirection.

[croemmich]

Thanks @z4kn4fein!

[z4kn4fein]

Hi @croemmich, I've released the fix in 3.0.1 (it may take a little longer to be available on maven central).

Regarding the FORCE_REDIRECT value of the redirect parameter, I can confirm that it's not used currently anywhere in our system, and we don't plan to do so either. However, we'll definitely figure out something for making the Proxy usage an exception for any possible force redirection.

[croemmich]

Our team pulled it in and confirmed it's working. Thanks @z4kn4fein!