internet-latency-collector: ping a fixed target address per measurement - #4316
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Cloud region probes do not answer ICMP, so a measurement between two regions has to ping a published address in the target region rather than the target's probe. The wanted measurement carries that address, the state file records it, and a stored address that no longer matches recreates the measurement. An empty stored address predates the field and is not a mismatch.
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Part of measuring latency between AWS regions with the internet latency collector.
Every RIPE Atlas measurement pings the target probe's own address, but the probes hosted inside AWS
regions do not answer ping, so measurements aimed at them would come back empty. This gives each
measurement its own target address: the target probe's address as before, or, when the collector
reads its regions from a node file, the ping target that file lists for the region.
Getting this wrong is hard to see. The collector drops a result that carries no reply, so pinging an
address that never answers writes zero rows and logs no error. There is no failed run and no
warning.
One behaviour change on the existing path: a measurement is recreated when the target address stored
with it differs from the wanted one. The stored field is new, so it is empty in every state file
written before this change and the comparison is skipped. It can only fire on state written after
it.
Nothing else changes in production. Without a node file the address is the one used today. A region
in a node file with no ping target is skipped with a warning.
Test:
go test ./controlplane/internet-latency-collector/internal/ripeatlas/.... No Docker needed.