Fixes the issue that BLE.scan() might hang the device.

[XuGuohui]

Problem

BLE may hang the thread that calls BLE.scan() due to unexpected scanning failure. Since the BLE.scan() is a blocking API waiting until a semaphore is given on scanning timeout event, in some condition the scanning procedure never timeout or a incorrect event is delivered from SoftDevice on scanning failure, in which case the semaphore cannot be taken any more.

Solution

Introduce a timer to guard the scanning procedure. For example, if the scanning timeout is set to 5s and it starts scanning, the timer will expire after 6s to stop scanning actively. Normally BLE scanning can timeout by SoftDevice and the timer will be stopped by then.

References

#1932

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

@XuGuohui Can we add a synthetic test for this? For example block the BLE thread or block BLE interrupt to simulate this behavior?

[XuGuohui]

@XuGuohui Can we add a synthetic test for this? For example block the BLE thread or block BLE interrupt to simulate this behavior?

A test is added to user/tests/wiring/ble_scanner_broadcaster/ble_scanner. And I ran it manually with success.