Re: Select then abort can fail to abort the abortable part

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 20 Nov 2014 21:08:50 GMT, Jean François Martinez wrote:

> Context:  Gnat x86_64 on Linux
> 
> 
> Let's have that simple loop
> 
> for Ind in 1..6
> loop
>     Start:=Clock;
>     select
>         delay Ind * 10.0
>     then abort 
>        Ultimate_Prime;
>     end select; 
>     My_Duration:= Clock - Start;
> end loop;
> 
> Ultimate_Prime is function containing a nearly infinite loop trying to 
> find prime numbers.  
> 
> First version stores primes found.  This mandates a linked list but that 
> would create an unacceptable overhead so I use big chunks
> 
>      type Big_Array is array (Positive range 1..100_000) of Long_Mod;
>      type Chunk is record
>           Position: Positive  range Big_Array'Range;
>           Payload: Big_Array;
>      end record;
>      type T_Ptr_Chunk is access  T;
> 
>      package P_List_Chunks is
>        new Ada.Containers.Doubly_Linked_Lists(T_Ptr_Chunk);

You certainly should not abort any code changing dynamically allocated
structures. I don't know the implementation of
Ada.Containers.Doubly_Linked_Lists, but it is not protected against abort
in the middle of changing list links, you may get garbage in the result.

You should almost never use the asynchronous transfer of control. A clean
method of aborting a lengthy computation is by inserting inspection points
within loops and raising an exception when abort was requested or time was
expired.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de