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b38dd1ae06
In suggestExitNodeLocked, if no exit node candidates have a home DERP or
valid location info, `bestCandidates` is an empty slice. This slice is
passed to `selectNode` (`randomNode` in prod):
```go func randomNode(nodes views.Slice[tailcfg.NodeView], …) tailcfg.NodeView {
…
return nodes.At(rand.IntN(nodes.Len()))
}
```
An empty slice becomes a call to `rand.IntN(0)`, which panics.
This patch changes the behaviour, so if we've filtered out all the
candidates before calling `selectNode`, reset the list and then pick
from any of the available candidates.
This patch also updates our tests to give us more coverage of `randomNode`,
so we can spot other potential issues.
Updates #17661
Change-Id: I63eb5e4494d45a1df5b1f4b1b5c6d5576322aa72
Signed-off-by: Alex Chan <alexc@tailscale.com>