Error: "invalid access of mutable storage in an 'isolated' function" for anonymous function

I am getting the compilation error invalid access of mutable storage in an 'isolated' function when attempting to update an array using an anonymous function with the forEach iterator.

The code:

isolated function searchEmailThread() returns string {
    gmail:MailThread[] threads = [];

    stream<gmail:MailThread,error?>|error threadList = gmail->listThreads(filter = {includeSpamTrash: false, labelIds: ["INBOX"]});
        
    if (threadList is stream<gmail:MailThread,error?>) {  
        error? threadItr = threadList.forEach(isolated function (gmail:MailThread thread) {
            lock {
                threads.push(thread);
            }
        }); 
        return threads.toString();
    } else {
        return "Failed to list threads";
    }
}

The variable gmail:MailThread[] threads cannot be defined as isolated as well. Is there an alternative approach to obtain and return the mail threads from the stream?

>Solution :

threads is a captured variable for the argument function passed to forEach and for that function to be isolated, threads should either be final and have a type that is a subtype of readonly|isolated object {} or it should be an isolated variable.

Since none of that is possible here, you could rewrite this to use a query action (or a query expression).

error? res = from gmail:MailThread thread in threadList 
do {
    threads.push(thread);
};

Also related to https://github.com/ballerina-platform/ballerina-spec/issues/602

Only module-level variables can be defined as isolated variables at the moment (related spec issue).