In split service mode, and during periods of inactivity, the gRPC
connections to the databroker may fall idle. Some network firewalls may
eventually time out an idle TCP connection and even start dropping
subsequent packets once connection traffic resumes. Combined with Linux
default TCP retransmission settings, this could cause a broken
connection to persist for over 15 minutes.
In an attempt to avoid this scenario, enable TCP keepalive for outbound
gRPC connections, matching the Go standard library default settings for
time & interval: 15 seconds for both. (The probe count does not appear
to be set, so it will remain at the OS default.)
Add a test case exercising the BuildClusters() method with the default
configuration options, comparing the results with a reference "golden"
file in the testdata directory. Also add an '-update' flag to make it
easier to update the reference golden when needed:
go test ./config/envoyconfig -update
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