BLE: Enable getting an implicitly-created CCCD through GattCharacteristic::getDescriptor

[AGlass0fMilk]

Summary of changes

See #13581 for information on issue.

Resolves #13581 by allowing the GattServer to pass a pointer to an implicitly-created CCCD. This GattAttribute is dynamically allocated (using new) and ownership is transferred to the relevant GattCharacteristic, which takes care of deleteing the GattAttribute in its destructor.

I tried to make GattServer a friend class of GattCharacteristic so GattCharacteristic::_setImplicitCCCD could be private, but I wasn't able to get it to compile due to all the weird namespacing stuff going on with ble::GattServer and ble::impl::GattServer -- it boggled my mind so I just made this a public "internal" function.

Impact of changes

GattCharacteristic::getDescriptor and GattCharacteristic::getDescriptorCount now behave as expected when a GattCharacteristic has an implicitly-created CCCD (ie: the GattCharacteristic has indicate and/or notify as one of its properties).

Migration actions required

None

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@pan- @paul-szczepanek-arm

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[ciarmcom]

@AGlass0fMilk, thank you for your changes.
@pan- @paul-szczepanek-arm @ARMmbed/mbed-os-maintainers please review.

[pan-]

Good PR. Thanks @AGlass0fMilk .

I tried to make GattServer a friend class of GattCharacteristic so GattCharacteristic::_setImplicitCCCD could be private, but I wasn't able to get it to compile due to all the weird namespacing stuff going on with ble::GattServer and ble::impl::GattServer -- it boggled my mind so I just made this a public "internal" function.

In C++, friendship is not inherited. The solution is to:

• Forward declare ble::GattServer in GattCharacteristic.h
• Mark it friend in GattCharacteristic (friend ble::GattCharacteristic)
• Add a protected function in ble::GattServer that can be used by child to add the CCCD. setCharacteristicImplicitCCCD(GattCharacteristic&, GattAttribute*)

[AGlass0fMilk]

Good PR. Thanks @AGlass0fMilk .

I tried to make GattServer a friend class of GattCharacteristic so GattCharacteristic::_setImplicitCCCD could be private, but I wasn't able to get it to compile due to all the weird namespacing stuff going on with ble::GattServer and ble::impl::GattServer -- it boggled my mind so I just made this a public "internal" function.

In C++, friendship is not inherited. The solution is to:

* Forward declare `ble::GattServer` in `GattCharacteristic.h`

* Mark it friend in `GattCharacteristic` (`friend ble::GattCharacteristic`)

* Add a protected function in `ble::GattServer` that can be used by child to add the CCCD. `setCharacteristicImplicitCCCD(GattCharacteristic&, GattAttribute*)`

It turns out that ble::impl::GattServer is not actually a child class of ble::GattServer. So I simply had to forward declare ble::impl::GattServer and then declare it as a friend in the GattCharacteristic declaration.

My problems stemmed from me trying to forward declare only impl::GattServer... does not name a type 🙄

I moved the forward declaration outside of the GattCharacteristic class body, similar to what I saw in GattServer.h and it compiles fine now. A much cleaner implementation 👍

Let me know if anything else needs to be addressed.

[pan-]

Looks good, I request a minor change in the allocation of the CCCD attribute. After that it is good to go.

Pull request has been modified.

[pan-]

LGTM

[0xc0170]

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