Empty users list in UsergroupsUsersUpdateRequest returns `invalid_arguments`

[rozza-sb]

The SDK converts the users list into a comma delimited list of user ID strings to confirm with the form request it is making. However, for an empty List this is captured as an empty string, which is not valid for form requests. Ideally I would like to be able to set a usergroup to have 0 members via an empty List in the request object.

From testing with curl, it seems that for an empty list a value of [] can be provided into the url encoded form data to represent no users in a usergroup. This could be used instead of an empty value when an empty Java list is provided into the users parameter of a UsergroupUsersUpdateRequest object (either directly or via a request builder).

Reproducible in:

The Slack SDK version

+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0
|    +--- com.slack.api:slack-api-model:1.48.0
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-model:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-client:1.48.0 (n)
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0
|    +--- com.slack.api:slack-api-model:1.48.0
|    +--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (*)
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-model:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0
|    +--- com.slack.api:slack-api-model:1.48.0
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-model:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0
|    +--- com.slack.api:slack-api-model:1.48.0
|    +--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (*)
+--- com.slack.api:slack-api-client:{strictly 1.48.0} -> 1.48.0 (c)
+--- com.slack.api:slack-api-model:{strictly 1.48.0} -> 1.48.0 (c)

Java Runtime version

openjdk version "21.0.3" 2024-04-16
OpenJDK Runtime Environment JBR-21.0.3+13-453.2-jcef (build 21.0.3+13-b453.2)
OpenJDK 64-Bit Server VM JBR-21.0.3+13-453.2-jcef (build 21.0.3+13-b453.2, mixed mode, sharing)

OS info

ProductName:    macOS
ProductVersion: 26.4
BuildVersion:   25E246

Steps to reproduce:

(Share the commands to run, source code, and project settings (e.g., pom.xml/build.gradle))

1. Create a UsergroupsUsersUpdateRequest object with the users property set as an empty List, and appropriate values for the other properties
2. Make the request with the above object - this can be done via either async or sync methods

Expected result:

The targeted usergroup would be updated to have 0 users, with an appropriate OK response object.

Actual result:

A not-OK response is returned, with the error invalid_arguments.

{"ok":false,"error":"invalid_arguments"}

Requirements

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[mwbrooks]

Hi @rozza-sb 👋🏻 Thanks for the detailed issue 🙇🏻

I agree, this appears to be a bug that should be fixed. A quick glance over the code and I think we would want to consider 3 use-cases for the empty users list:

1. users is null - the field is omitted entirely (current behaviour and correct)
2. users is a non-empty list - joins with commas (e.g. U123,U456) (current
   behaviour and correct)
3. users is an empty list - currently returns users= but should return users=[]

I'll leave this issue open and loop in a few maintainers to take a look at putting together a PR - you're also welcome to share a PR if you'd like @rozza-sb 😄

[rozza-sb]

Thanks for the quick reply @mwbrooks o/

That seems reasonable to me ^^ particularly with 1, as this is a required property and I would expect there to be a difference between omission (forgot to explicitly state the users) and an empty list. 2 and 3 make sense as well

I will try and spin up a PR for this - I'm hoping that the fix is just some conditions within the toForm method for UsergroupsUsersUpdateRequest :)

[rozza-sb]

PR opened - I'm not 100% sure on the testing, but the base fix is pretty simple and should cover the above bug!