Cellular: disconnect does not deactivate context (for some cases)

[boraozgen]

Description of defect

I use a Quectel BG95 modem (similar to BG96) to connect and disconnect to the cellular network in my application. I noticed that during disconnection, the PDP context is not deactivated. I looked into the code and found some curious logic. There are two parts I believe are buggy or I do not understand.

In AT_CellularContext::do_disconnect(), the value of _is_context_activated is checked in order to deactivate the context.

mbed-os/connectivity/cellular/source/framework/AT/AT_CellularContext.cpp

Line 691 in f83d100

if (_is_context_activated) {

However, this variable is not set in case an active context is found in AT_CellularContext::find_and_activate_context(). There, only _is_context_active is set.

mbed-os/connectivity/cellular/source/framework/AT/AT_CellularContext.cpp

Line 546 in f83d100

_is_context_active = true;

This leads to the issue that the context is not deactivated if it is found active. Is this desired? I think disconnect should deactivate the context in both cases.

Also, what is the difference between _is_context_active and _is_context_activated? Are they redundant?

When I changed this, it went one step further but the context was still not getting deactivated. Looking further, I found that the only deactivate_context() call is inside this check:

mbed-os/connectivity/cellular/source/framework/AT/AT_CellularContext.cpp

Lines 758 to 761 in f83d100

	if (_is_context_active && (rat < CellularNetwork::RAT_E_UTRAN || active_contexts_count > 1)) {
	_at.clear_error();
	deactivate_context();
	}

which checks for something really unclear (3G and older RAT?) or active_contexts_count > 1, which I again do not understand. For my case, I am connected using NB-IoT and I have only 1 active context, so this check was failing and my context was not deactivated. Maybe only the _at.clear_error() should be in the if block and deactivate_context() should be after the block. When I changed it in that way, it finally worked for me.

I can open a PR with my changes if needed.

Target(s) affected by this defect ?

Not target specific.

Toolchain(s) (name and version) displaying this defect ?

Not relevant.

What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.2.0
mbed-os-99.99.99

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

Not relevant.

How is this defect reproduced ?

Disconnect from cellular network using without activating (connect with a modem which has an active PDP context).
For the second issue, disconnect from a NB-IoT network with one active context.

[0xc0170]

@boraozgen Could you send a pull request proposing a fix? I can see a reference from the fix above