ReplicatedPG: add Mutex to protect snapset_contexts

snapset_contexts_locks is added and locked each time snapset_contexts
or the SnapSetContext ref data member is needs to be accessed or
modified.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
do not use ReplicatedPG::AccessMode::wake

It is used each time an ObjectContext is dereferenced to signal all
waiters on mode.waiting_cond. Samuel Just & Sage Weil say this code
can be discarded because it is not actually used.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
ObjectContext is made compatible with SharedPtrRegistry

When creating a new object SharedPtrRegistry::lookup_or_create uses
the default ObjectContext constructor with no argument. The existing
ObjectContext constructor is modified to have no argument and the
initialization that was previously done within the constructor is done
by the caller (that only happens three times).

The ObjectContext::get method is removed: its only purpose is to
increment the ref.

The ObjectContext::registered data member is removed as well as all
the associated assert()

The ObjectContext::destructor_callback data member Context is added
and called by the destructor. It will allow the caller to perform
additional cleanup, if necessary.

All ObjectContext * data members are replaced with shared_ptr.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
ObjectContext * becomes ObjectContextRef

The map of hobject_t to ObjectContext is made a
SharedPtrRegistry owned by ReplicatedPG

    -  map<hobject_t, ObjectContext*> object_contexts;
    +  SharedPtrRegistry<hobject_t, ObjectContext> object_contexts;

All ObjectContext pointers are changed into ObjectContextRef, i.e.
shared_ptr.

In Watch.h std::tr1::shared_ptr<ObjectContext> is used instead
of ObjectContextRef because Watch.h is included before it is
defined.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
remove reference counting logic from RepliatedPG.{cc,h}

ObjectContext manual reference counting and managing the
object_contexts object involves calls to

* obc->ref++ and obc->get()
* put_object_context and put_object_contexts
* register_object_context
* assertions on obc->registered

They are all removed because SharedPtrRegistry provides the
same service.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
remove ReplicatedPG::lookup_object_context method

Both ReplicatedPG::lookup_object_context and
ReplicatedPG::_lookup_object_context methods are provided by
SharedPtrRegistry.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
replace map iterators with SharedPtrRegistry::get_next

SharedPtrRegistry does not provide an iterator equivalent to

    map<hobject_t, ObjectContext*>::iterator i

It is replaced with a thread safe get_next method roughly used
as follows:

    pair<hobject_t, ObjectContextRef> i;
    while (object_contexts.get_next(i.first, &i))

All occurences of the iterator are replaced with get_next style
traversal.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
add Context to cleanup the PG after an ObjectContext deletion

ReplicatedPG::C_PG_ObjectContext is added to encapsulate a
call to ReplicatedPG::object_context_destructor_callback method
which is reponsible for

  * manually de-allocating the SnapSetContext of the ObjectContext if
    any. It will eventually be managed by a SharedPtrRegistry.

ReplicatedPG::C_PG_ObjectContext must be added to the destructor_callback
member of ObjectContext immediately after it is created.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
replace object_contexts.find with object_contexts.lookup

The std::map equivalent of find is SharedPtrRegistry::lookup

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>
create ObjectContext with SharedPtrRegistry

All new ObjectContext are replaced with calls to
SharedPtrRegistry::lookup_or_create to ensure that they are all
registered. Because the constructor is invoked with no argument, care
is taken to always initialize the destructor_callback data member
immediately afterwards.

ReplicatedPG::get_object_context contains a redundant call to
get_snapset_context that is removed.

http://tracker.ceph.com/issues/5510 refs #5510

Signed-off-by: Loic Dachary <loic@dachary.org>