cmd/derper: fix TestLookupMetric to pass when run alone

TestLookupMetric was added in e8d140654 (2023-08-17) without
initializing the dnsCache and dnsCacheBytes globals. When run in
isolation, handleBootstrapDNS writes a nil body (from the
uninitialized dnsCacheBytes), causing getBootstrapDNS to fail
decoding an empty response with EOF.

Add a setDNSCache test helper that stores the dnsEntryMap, marshals
dnsCacheBytes, and registers a t.Cleanup to nil both out, so tests
that forget to call it will hit the dnsCache-nil fatal in
getBootstrapDNS rather than silently depending on prior test state.

Also add AssertNotParallel and a dnsCache-nil fatal check to
getBootstrapDNS, the central helper all bootstrap DNS tests flow
through, to prevent future tests from running in parallel (they
all mutate package-level DNS caches and metrics) and to give a
clear error if a test forgets to initialize the DNS caches.

Fixes #19388

Change-Id: I8ad454ec6026c71f13ecfa14d25925df5478b908
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Avery Pennarun <apenwarr@tailscale.com>

0 files changed, 0 insertions, 0 deletions