wgengine/magicsock: deflake TestTwoDevicePing compare-metrics-stats

The compare-metrics-stats subtest reset two independent counting
systems (physical connection counters and expvar.Int user metrics)
non-atomically. Background WireGuard keepalives arriving between the
resets could increment one system but not the other, causing
off-by-one packet/byte mismatches in either direction.

Replace the reset-then-compare pattern with snapshot-and-delta:
snapshot both systems before pings, snapshot again after, and compare
the deltas. This eliminates the non-atomic reset window entirely.
As a belt-and-suspenders safety net, tolerate a difference of exactly
one packet (and corresponding bytes) from a stray keepalive that
could still arrive in the narrow window between the two snapshots.

flakestress passes with ~5900 runs (~2800 without -race, ~3100 with
-race) but it also passed previously too. This is an annoying one to
repro.

Fixes #11762

Change-Id: I3447ad67e71c8146e85eed38b7a665033ef9e284
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

0 files changed, 0 insertions, 0 deletions