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Date: Fri, 03 Oct 2008 11:18:39 +0200
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Subject: Re: Naming convention to identify functions with side effects
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Date: 2008-10-03T11:18:40+02:00
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Ole-Hjalmar Kristensen wrote:
> How would you implement the lock in youre case? I prefer not to mess
> around with semaphores, but use protected objects instead. In that
> case keeping the lock across the two calls is not so trivial.

In a simple, maybe simplistic way I'd hide the lock.

package News26 is

   subtype URL is String;

   type Web_Resource is tagged limited private;
     -- ... Observe the calling protocol, which is
     --  Load -> Is_Loaded -> Discard

   function Is_Loaded (Page: Web_Resource) return Boolean;
   procedure Load (Page: in out Web_Resource; Location: URL);
   procedure Discard (Page: in out Web_Resource);

private

   protected type Resource is
      entry Seize;
      procedure Release;
   private
      Busy: Boolean := False;
   end Resource;

   type Web_Resource is tagged limited
      record
         Lock: Resource;
         -- ...
      end record;

end News26;

Procedure Load will then Seize the Lock.

Another scheme could, I think, be using Dmitry's Page_Cache
administrating the sequencing of calls by slot-wise locking
or some such.