[Ada] improve implementation of suspension_objects

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on mainline.

Suspension_Objects were implemented using protected objects. It had two major
problems:
First, this was not compatible with the Ravenscar profile restrictions,
because these protected objects needed two different entries, and it is not
allowed by pragma Restrictions (Max_Protected_Entries => 1).
Second, the use of protected object for implementing suspension objects is
an overkill (although convenient for portability reasons) because all the
overhead associated to protected objects is not required for implementing
the relatively lightweight suspension objects.
Hence, the implementation has been changed so that the functionality provided
by Ada.Synchronous_Task_Control is implemented by a target-dependent package
(System.Task_Primitives.Operations). These low-level suspension objects are
implemented more efficiently (and compliant with the Ravenscar profile) using
a mutex and a condition variable (or their equivalent in the concrete
operating system), plus some internal data to reflect the state of the
suspension object.

Fix also an annoying behavior when using tasking applications with GNAT:
all signals were masked by all tasks by default. This means for example
that when using a tasking application, you cannot do a kill -TERM app
without using pragma Interrupt_State.
This change addresses this limitation by only setting up the signal mask
when needed, that is to say when the package Ada.Interrupts is used either
explicitly or implicitly (using e.g. pragma Attach_Handler).

2005-06-14  Jose Ruiz  <ruiz@adacore.com>
	    Arnaud Charlet  <charlet@adacore.com>

	* a-sytaco.ads, a-sytaco.adb (Suspension_Object): These objects are no
	longer protected objects. They have been replaced by lower-level
	suspension objects made up by a mutex and a condition variable (or
	their equivalent given a particular OS) plus some internal data to
	reflect the state of the suspension object.
	(Initialize, Finalize): Add this initialization procedure for
	Suspension_Object, which is a controlled type.
	(Finalize): Add the finalization procedure for Suspension_Object,
	which is a controlled type.

	* a-sytaco-vxworks.ads, a-sytaco-vxworks.adb: Remove this version of
	Ada.Synchronous_Task_Control because there is no longer a need for a
	VxWorks specific version of this package. Target dependencies
	has been moved to System.Task_Primitives.Operations.

	* s-osinte-mingw.ads (pCRITICAL_SECTION): Remove this type which is no
	longer needed.
	(InitializeCriticalSection, EnterCriticalSection,
	LeaveCriticalSection, DeleteCriticalSection): Replace the type
	pCriticalSection by an anonymous access type so that we avoid problems
	of accessibility to local objects.

	* s-taprop.ads, s-taprop-posix.adb, s-taprop-vxworks.adb,
	s-taprop-mingw.adb, s-taprop-vms.adb, s-taprop-solaris.adb,
	s-taprop-os2.adb, s-taprop-dummy.adb, s-taprop-hpux-dce.adb,
	s-taprop-linux.adb, s-taprop-irix.adb, s-taprop-irix-athread.adb,
	s-taprop-tru64.adb, s-taprop-lynxos.adb (Elaboration Code): No longer
	set the environment task mask here.
	(Current_State): Add this function that returns the state of the
	suspension object.
	(Set_False): Add this procedure that sets the state of the suspension
	object to False.
	(Set_True): Add this procedure that sets the state of the suspension
	object to True, releasing the task that was suspended, if any.
	(Suspend_Until_True): Add this procedure that blocks the calling task
	until the state of the object is True. Program_Error is raised if
	another task is already waiting on that suspension object.
	(Initialize): Add this procedure for initializing the suspension
	object. It initializes the mutex and the condition variable which are
	used for synchronization and queuing, and it sets the internal state
	to False.
	(Finalize): Add this procedure for finalizing the suspension object,
	destroying the mutex and the condition variable.

	* s-taspri-posix.ads, s-taspri-vxworks.ads, s-taspri-mingw.ads,
	s-taspri-vms.ads, s-taspri-solaris.ads, s-taspri-os2.ads,
	s-taspri-dummy.ads, s-taspri-hpux-dce.ads, s-taspri-linux.ads,
	s-taspri-tru64.ads, s-taspri-lynxos.ads (Suspension_Object): New object
	which provides a low-level abstraction (using operating system
	primitives) for Ada.Synchronous_Task_Control.
	This object is made up by a mutex (for ensuring mutual exclusion), a
	condition variable (for queuing threads until the condition is
	signaled), a Boolean (State) indicating whether the object is open,
	and a Boolean (Waiting) reflecting whether there is a task already
	suspended on this object.

	* s-intman.ads, s-intman-irix.adb, s-intman-irix-athread.adb,
	s-intman-dummy.adb, s-intman-solaris.adb, s-intman-vms.adb,
	s-intman-vms.ads, s-intman-mingw.adb,
	(Initialize_Interrupts): Removed, no longer used.

	* s-inmaop-posix.adb, s-inmaop-vms.adb, s-inmaop-dummy.adb,
	(Setup_Interrupt_Mask): New procedure.

	* s-intman-vxworks.ads, s-intman-vxworks.adb:  Update comments.

	* s-inmaop.ads (Setup_Interrupt_Mask): New procedure

	* s-interr.adb: Add explicit call to Setup_Interrupt_Mask now that
	this is no longer done in the body of s-taprop
	(Server_Task): Explicitely test for Pending_Action in case
	System.Parameters.No_Abort is True.

	* s-taasde.adb: Add explicit call to Setup_Interrupt_Mask now that this
	is no longer done in the body of s-taprop

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