Ada: the select/else statement on protected entries. What does it do? - ada

Question

I have a doubt about Ada, and in particular about the select statement when used in conjunction with a protected entry. Let's consider the following code fragment:
select
Protected_Object.Some_Entry;
else
DoSomethingElse;
end select;
My question is simple: when the select statement is reached, what happens? In particular, what I want to know is: does the else branch get chosen only if Some_Entry's guard is closed or does it get chosen even if the guard is opened but the entry is "occupied" (ie: there's already a call to Some_Entry executing) and thus cannot be called immediately??

Answer 1

I believe that the else branch is chosen only if the entry's guard is closed (i.e. the barrier condition is false). The sequence of events for a protected entry call is given by RM 9.5.3(8):
A new protected action is started on the object.
The named entry is checked to see if it is open (i.e. the barrier condition is true); if open, the entry call is said to be selected immediately, and then the entry body is executed.
Starting a protected action may involve a short delay if another task is performing a protected action on the same object (9.5.1(4)). However, the intent is that this delay is always very short. If a protected subprogram or entry does anything that could block the program, this is considered an error (9.5.1(8-18)). Thus, waiting until another task releases a protected object is supposed to be a very short wait if at all; on a multi-processor system, it's perfectly acceptable to implement this wait by spinning (essentially while Protected_Object_Is_In_Use(Obj) loop null; end loop;) as opposed to waiting on a queue.
Thus, my reading of 9.5.3(8) is that the definition of "selected immediately" does not take into account the short wait needed if another task is engaged in a protected action on the object. If the task has to wait, it does so. If, once it is able to grab the object, it finds that the barrier is true, then the entry is "selected immediately". This may not quite fit our idea of what the English word "immediately" means, but it's how the term is defined.
Thus, for a conditional entry call, RM 9.7.3 says that the entry call is cancelled (and the else branch executed) if it is not selected immediately. Using the definition in 9.5.3(8), this means that the else branch is executed only if the barrier condition is false (after the task succeeds in grabbing the protected object).

Answer 2

The 'else' branch is chosen if Some_Entry is not immediately accessible (either because there is already another task accessing the protected object, or because the guard prevents calling the entry in the first place.
The goal of the 'else' branch is basically that your task does not stay blocked and thus might miss a timeout or an action to execute on a regular schedule. So when you use 'else', the select statement cannot be blocking (unless of course the entry itself, once called, takes forever).
Edit: As demonstrated below by #simonwright, this answer is incorrect: the 'else' branch is taken when the guard is False, otherwise the task will block on the protected object's entry. By design, such entries should perform their work in a very limited amount of time.