Queueable HttpCallout.TooManyCalloutsException: Too many callouts requested vs SFDC Limit: 0

Question

I have a Queueable class that performs a single callout. When it executes, it generates this error:

HttpCallout.TooManyCalloutsException: Too many callouts requested vs SFDC Limit: 0

public class MyQueueable implements System.Queueable{
    
    public void execute(QueueableContext qc) {
        system.debug('callout limits='+ Limits.getLimitCallouts()); // <= displays 0
        
        ... do a callout here
    }
}

• How can the SFDC callout limit be zero?
• Where is this documented?
• How do I resolve this?

Answer 1

Posting this as Q&A because if you actually Google (as of this writing):

"HttpCallout.TooManyCalloutsException: Too many callouts requested vs SFDC Limit: 0" 

you don't get any matches.

• The Apex Developer Guide on Queueable Apex is silent on this subject
• The Apex Developer Guide on Callout Limits is silent as well
• Various StackExchange and other posts that show examples of Queueables might hint at the solution but only if you know where to look
• The Apex Developer Guide on Schedulable Apex has an oblique reference that is relevant but only if you make the mental leap from how Batchables can do callouts (and, if you've never written a Batchable, you might not be aware of this):

Synchronous Web service callouts are not supported from scheduled Apex. To make asynchronous callouts, use Queueable Apex, implementing the Database.AllowsCallouts marker interface.

The solution is to add Database.AllowsCallouts to your Queueable class definition

public class MyQueueable implements System.Queueable, Database.AllowsCallouts {

  public void execute(QueueableContext qc) {
    system.debug('callout limits='+ Limits.getLimitCallouts()); // <= displays 100
    
    ... do a callout here
  }

}

Note that if an outer class implements Database.AllowsCallouts, this is not inherited by any inner class that implements System.Queueable