How to cover the catch block for after insert Trigger?

Question

When you create a Barrel, a Filling is also created. Is there any chance to make a fake error in unit test between Barrel and Filling to crush the upsert process and cover the catch block?

Helper:

public void insertFilling(List<Barrel__c> createdBarrels){
List<Filling__c> fillingsToInsert = new List<Filling__c>();
for(Barrel__c barrel : createdBarrels){
    if(barrel.FinishDateTime__c >= Date.TODAY()){     
        fillingsToInsert.add(
            new Filling__c(Barrel__c = barrel.Id, Barrel_ExternalID__c = barrel.Id)
        );
    }
}
if(!fillingsToInsert.isEmpty()){
    try{
        upsert fillingsToInsert Barrel_ExternalID__c;
    //I want to cover this part:
    } catch(Exception e){
        //email error notification
    }
}
}

Trigger:

trigger BarrelTrigger on Barrel__c (after insert) {
  TriggerHelper helper = new TriggerHelper();
  helper.insertFilling(Trigger.new);
}

Test:

    @isTest static void testBarrelInsertException(){
    Barrel__c barrel = new Barrel__c(
        Name = 'TestBarrel',
        Opportunity__c = null, //required field
        FinishDateTime__c = DateTime.NOW().AddDays(1)
    );
    //But I even can't insert it. I'm getting the 'REQUIRED_FIELD_MISSING' error:
    insert barrel;
    //And still no coverage.
}

Answer 1

The only way to cover a catch is if you cause your code inside the try to fail.

In your case you should get rid of the try/catch because it will never fail (or, let's say you introduce a new validation rule on the Filling it will always fail and you will need to change the trigger).

There another good answer that discuss the benefits of catching: When do we really need try/catch?