Bluetooth: Host: bt_le_set_auto_conn() doesn't work

[ivaniushkov]

Describe the bug
Zephyr Host has a function called bt_le_set_auto_conn() which is supposed to auto-reconnect when the ACL connection disconnected.
Unfortunately, the function doesn't work as expected.
There are at least two issues with the current implementation of this function:

• When bt_le_set_auto_conn() is called from the application, the application never gets disconnected callback. The following flow is triggered in this case: the connection state is set to BT_CONN_DISCONNECTED and right after that it is set to BT_CONN_SCAN_BEFORE_INITIATING, so notify_disconnected() handler is never called because it is only called from deferred_work()
• Another issue is that a device never reconnects to the Peripheral. Not sure exactly why it happens, but it looks like some parts of the auto-reconnect feature are missing in conn.c and scan.c

To Reproduce
I modified samples/bluetooth/central sample to reproduce the issue.
The changes include: removed disconnection, added bt_le_set_auto_conn() to device_found(), increased RSSI to -30dBm to connect only to the device located very close.
Steps to reproduce the behavior:

1. cd central_auto
2. west build -bnrf52840dk_nrf52840
3. west flash

It required additional device to be used as a Peripheral

I was using zephyr v3.7.99 but I think the bug is still presented on the v4.0

Expected behavior
The following messages are expected in the sample output:

Device found: 52:84:E1:BC:96:03 (random) (RSSI -30)
Connected: 52:84:E1:BC:96:03 (random)
Disconnected: 52:84:E1:BC:96:03, reason 0x08
Connected: 52:84:E1:BC:96:03 (random)

Observed behavior

Disconnected: 52:84:E1:BC:96:03, reason 0x08

message is never printed when Peripheral is disconnected. It happens because notify_disconnected() is never called.
Also, the second Connected: 52:84:E1:BC:96:03 (random) is not printed.

Suggested solution
I would suggest to just remove this function because the use-cases are unclear. Also it looks like it didn't work as expected for a long time

central_auto.zip

[ivaniushkov]

bt_conn_le_create_auto() function is similar to bt_le_set_auto_conn() but uses BT_FILTER_ACCEPT_LIST feature. I guess it may have the same problems

[jhedberg]

Bluetooth WG meeting: decided that it should be fine to remove for 4.1, however the removal should follow the documented process for stable API breakage: https://docs.zephyrproject.org/latest/develop/api/api_lifecycle.html#introducing-breaking-api-changes

[ivaniushkov]

I created RFC issue to deprecate the API:
#81597
Will create a PR later this week

[github-actions]

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[jhedberg]

Taken care of by #81757