Scan can fail to add channels because new chanid collides with existing chanid

[Paul Milazzo <pgm0203@…>]

When adding a channel, ChannelUtil::CreateChanID synthesizes a new chanid from the source and channel number, then tests the resulting chanid to ensure it does not collide with an existing chanid. If it does, ChannelUtil::CreateChanID chooses a new value one larger than the largest chanid currently assigned to the source in question.

Unfortunately, this second new chanid can (and on my system, does) itself collide with an existing chanid assigned to a higher-numbered source. As a result, the attempt to add the new channel fails without notification in the UI. An error message is, fortunately, written to the console.

The attached patch simply checks for the second collision and chooses a third value one larger than any current chanid. Another solution that would result in chanid values closer to the original would be to resolve the collisions using linear probing, but doing so could be much slower.

[pgm0203@…]

Patch to avoid generating a new chanid that conflicts with an existing chanid

[danielk]

(In [9726]) Closes #1683. Adds another backup chanid creation method which should cope with collisions across sources.

This fixes a bug, but triggering this bug it is pretty difficult so I'm not backporting the fix to 0.19-fixes.