I'm trying to write a unit test for a trigger that captures the OwnerID of an Opportunity, and then populates a "Owner Details" field (giving us access to their entire user record) and a "Sales Manager" field (which is the user's manager).

I'm at 73% coverage, but the part that is not covered is the after update when the Opportunity Owner has changed. I've written into my test a scenario when the Opp Owner changes, but I'm getting the following error message:

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, CopyOwnerInformation: execution of AfterUpdate caused by: System.DmlException: Update failed. First exception on row 0 with id 006S0000006jI1iIAE; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, CopyOwnerInformation: maximum trigger depth exceeded Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i] Opportunity trigger event AfterUpdate for [006S0000006jI1i]: [] Trigger.CopyOwnerInformation: line 40, column 1: []

Here is the relevant code from my trigger. This is the part that I am trying to cover:

//Insert new owner details ID if it has changed
if(opp.Owner_Details__c == null || (opp.OwnerId != oldopp.OwnerID)){
List<Opportunity> OppsWithNewOwner = [SELECT ID, OwnerID, Owner_Details__c FROM Opportunity WHERE id in:trigger.new];

for (Integer i=0; i<OppsWithNewOwner.size(); i++){                                      
    OppswithNewOwner[i].Owner_Details__c = OppsWithNewOwner[i].OwnerID;}
    
    update OppsWithNewOwner;

    }

And here is my test class:

@isTest
private class Test_CopyOwnerInformation{
    static testMethod void CopyOwnerInformation(){
    
//set up test records
        
User user1 = new User();
        user1.firstname = 'Test User 1 First Name';
        user1.lastname = 'Test User 1 Last Name';
        user1.alias = 'user1';
        user1.email = 'testuser@unittest.com';
        user1.EmailEncodingKey = 'ISO-8859-1';
        user1.username = 'testuser1@unittest.com';
        user1.TimeZoneSidKey = 'America/Chicago';
        user1.LocaleSidKey = 'en_US';
        user1.ProfileId = '00e70000000sP2v';
        user1.LanguageLocaleKey = 'en_US';
        insert user1;
        
User manager2 = new User();
        manager2.firstname = 'Test Manager 2 First Name';
        manager2.lastname = 'Test Manager 2 Last Name';
        manager2.alias = 'mgr2';
        manager2.email = 'testmanager@unittest.com';
        manager2.EmailEncodingKey = 'ISO-8859-1';
        manager2.username = 'testmanager2@unittest.com';
        manager2.TimeZoneSidKey = 'America/Chicago';
        manager2.LocaleSidKey = 'en_US';
        manager2.ProfileId = '00e70000000sP2v';
        manager2.LanguageLocaleKey = 'en_US';
        insert manager2;
        
User user2 = new User();
        user2.firstname = 'Test User 2 First Name';
        user2.lastname = 'Test User 2 Last Name';
        user2.alias = 'user2';
        user2.email = 'testuser@unittest.com';
        user2.EmailEncodingKey = 'ISO-8859-1';
        user2.username = 'testuser2@unittest.com';
        user2.manager = manager2;
        user2.TimeZoneSidKey = 'America/Chicago';
        user2.LocaleSidKey = 'en_US';
        user2.ProfileId = '00e70000000sP2v';
        user2.LanguageLocaleKey = 'en_US';
        insert user2;
  
Account account = new Account();
        account.Name = 'Test Account';
        account.OwnerID = user1.id;
        account.web_site__c = 'test.com';
        account.industry = 'Entertainment';
        account.type = 'Advertiser';
        insert account;
        
Opportunity opportunity = new Opportunity();
        opportunity.AccountId = account.Id;
        opportunity.OwnerID = account.OwnerId;
        opportunity.Name = 'Test Opportunity';
        opportunity.StageName = 'Active';
        opportunity.CloseDate = System.today();
        opportunity.Amount = 1000.00;
        opportunity.Type = 'New Client';
        
//test  
    test.startTest();     
        insert opportunity;
    
//Validate that the Owner Details field was updated.
    opportunity = [SELECT OwnerId, Owner_Details__c, Owner_Details__r.ManagerID, SalesManager__c FROM Opportunity WHERE id = :opportunity.id];
    System.assertEquals(opportunity.OwnerId, opportunity.Owner_Details__c);
    System.assertEquals(opportunity.SalesManager__c, Opportunity.Owner_Details__r.ManagerID);

//Test that the Owner Details changes with an Opp Owner Change
   
        opportunity.OwnerId = user2.ID;
        update opportunity;
     test.stopTest();

//Validate that the Owner Details and Manager fields were updated.
    opportunity = [SELECT OwnerID, Owner_Details__c, Owner_Details__r.ManagerID, SalesManager__c FROM Opportunity WHERE id = :opportunity.id];
    System.assertEquals(opportunity.Owner_Details__c, user2.Id);
    System.assertEquals(opportunity.SalesManager__c, user2.Manager.Id);
    }
}

 Thank you for the help!