Re: Overriding discriminants perplexes GNAT 3.14p

[adam@irvine.com (Adam Beneschan)]

Dmitry A.Kazakov <mailbox@dmitry-kazakov.de> wrote in message news:<amc58u$4ebgo$1@ID-77047.news.dfncis.de>...

> 2. Circular requeue hangs at run-time under Linux:
> 
> package Objects is
>    protected Object1 is
>       entry Point1;
>       entry Point2;
>    end Object1;
>    protected Object2 is
>       entry Point1;
>    end Object2;
> end Objects;
> ------------
> package body Objects is
>    protected body Object1 is
>       entry Point1 when True is
>       begin
>          requeue Object2.Point1;
>       end Point1;
>       entry Point2 when True is
>       begin
>          null;
>       end Point2;
>    end Object1;
>    protected body Object2 is
>       entry Point1 when True is
>       begin
>          requeue Object1.Point2;
>       end Point1;
>    end Object2;
> end Objects;
> ------------
> with Ada.Text_IO;  use Ada.Text_IO;
> with Objects;
> 
> procedure Test is
> begin
>    Objects.Object1.Point1;
>    Put_Line ("That's OK");
> end Test;

I think GNAT's behavior is legitimate here.  Protected entry calls and
external requeues start new "protected actions" on the target
protected object (9.5.3(8), 9.5.4(11)).  Thus, when
Objects.Object1.Point1 is called, a new protected action is started on
Object1.  When this requeues to Object2.Point1, a new protected action
is started on Object2; however, the protected action on Object1 does
not complete (AI95-00148).  When Object2.Point1 then requeues to
Object1.Point2, it attempts to start a new protected action on
Object1.  However, 9.5.1(4) says that a new protected action cannot be
"started on a protected object while another protected action on the
same protected object is underway".  Thus, the requeue to
Object1.Point2 must wait until the already in-progress protected
action is completed, and that can never happen; the result is
deadlock.  Note that the second requeue is not necessarily
"queued"---see 9.5.1(18).

See also 9.5.1(8-16), which says that if a protected action performs
an external requeue on the same target object as that of a protected
action (which is what your program does, indirectly), it is a bounded
error.  Implementations may or may not detect this error; see
9.5.1(17).

In other words, it's your program that's in error here, not GNAT.

				-- Adam