[petition] return ID of existing active commissioner if one exists

[wgtdkp]

This PR returns the commissioner ID when petition failed due to an already-active commissioner in the Thread Network.

[codecov-io]

Codecov Report

Merging #36 into master will decrease coverage by 0.24%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   59.92%   59.67%   -0.25%     
==========================================
  Files          54       53       -1     
  Lines        5397     5379      -18     
==========================================
- Hits         3234     3210      -24     
- Misses       2163     2169       +6

Impacted Files	Coverage Δ
src/app/commissioner_app.hpp	75% <ø> (ø)	⬆️
include/commissioner/commissioner.hpp	100% <ø> (ø)	⬆️
src/library/commissioner_impl.hpp	100% <100%> (ø)	⬆️
src/app/commissioner_app.cpp	29.76% <100%> (ø)	⬆️
src/library/commissioner_impl.cpp	61.9% <57.14%> (-0.16%)	⬇️
src/app/cli/interpreter.cpp	33.96% <75%> (+0.17%)	⬆️
src/library/commissioner_safe.cpp	63.77% <85.71%> (+0.17%)	⬆️
src/library/uri.hpp

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[bukepo]

LGTM:+1:

[jwhui]

@wgtdkp, thanks for the updates!

Would it be possible to implement a functional test on Travis for this feature?

[wgtdkp]

@wgtdkp, thanks for the updates!

Would it be possible to implement a functional test on Travis for this feature?

I has been thinking about this but find it to be not possible for now. We currently doing functional tests with OTBR and opethread simulation. But border agent on device side has only one DTLS server and a new connection will be rejected if the DTLS session is active. In this case, the connection between a new commissioner and border agent cannot be established and we got no chance to get the existing commissioner ID. So, with current openthread border agent implementation, the user actually can never see existing commissioner ID.

We can add one more DTLS session to border agent to allow a second commissioner to get connected to border agent but later rejected because of existing active commissioner. How about I create an issue for this and add the functional tests back after making the change to border agent?

[jwhui]

How about I create an issue for this and add the functional tests back after making the change to border agent?

I am fine to defer this as long as we track it. I see you already submitted #41.