Keep the list of free channels in sorted order

[softins]

Short description of changes

Maintain the list of free channel IDs in sorted order, so that the lowest available channel ID will be used for a new client.

The fix has two parts, the first of which is not really necessary, but makes the output of DumpChannels() nicer:

• Reverse the polarity of the comparison in the binary search, so that the list of active channel IDs has the lowest ID first (this should have been done originally).
• When moving the channel IDs downwards in the list while freeing a channel, instead of stopping at the end of the active channels, keep going until the correct ordered place is found to insert the freed channel ID.

Context: Fixes an issue?

Fixes #2144

Does this change need documentation? What needs to be documented and how?

No documentation - internal enhancement only.

Status of this Pull Request

Tested and ready to merge.

What is missing until this pull request can be merged?

Nothing

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[softins]

The operation of the list can be observed for testing by temporarily uncommenting the two calls to DumpChannels() in src/server.cpp.

[pljones]

This all now seems very cumbersome. If it's going to sit somewhere in vecChannelOrder, why not just leave it where it was?

[softins]

Because the binary search relies on all the active channels being listed before all the free ones. All we are changing is how many IDs we shunt down the list when freeing, so that the freed ID is placed in order instead of just at the start of the free list. Doesn't seem cumbersome to me at all :)

[pljones]

So you're saying it was (allocated | free):

1 2 3 4 5   ||
1 3 4 5 | 2 ||
1 3 4 | 5 2 ||
3 4 | 1 5 2 ||

and now it's

1 2 3 4 5   ||
1 3 4 5 | 2 ||
1 3 4 | 2 5 ||
3 4 | 1 2 5 ||

? I think the comment doesn't explain anything.

[softins]

Yes, exactly that. I'll change the comment to say "put deleted channel in the vacated position ready for re-use"

[ann0see]

Looks ok.