cmd/coordinator: exempt plan9-arm from repeatedCommunicationError

The coordinator has a special case for Plan 9 builders to treat
communication errors (when the builder stops responding) as
permanent instead of retrying the build. This isn't appropriate
for the plan9-arm builder (which is not on a virtual machine and
can suffer temporary network disconnects or unexpected reboots).
Exempting plan9-arm from this check will prevent spurious build
failures which need a manual retry.

Fixes golang/go#52677

Change-Id: I7059eec115f680b3f6cd98368c1c566b7310f86f
Reviewed-on: https://go-review.googlesource.com/c/build/+/470355
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: millerresearch

cmd/coordinator/buildstatus.go

@@ -2000,8 +2000,9 @@ func (st *buildStatus) repeatedCommunicationError(execErr error) error {
 	if execErr == nil {
 		return nil
 	}
-	// For now, only do this for plan9, which is flaky (Issue 31261)
-	if strings.HasPrefix(st.Name, "plan9-") && execErr == errBuildletsGone {
+	// For now, only do this for plan9, which is flaky (Issue 31261),
+	// but not for plan9-arm (Issue 52677)
+	if strings.HasPrefix(st.Name, "plan9-") && st.Name != "plan9-arm" && execErr == errBuildletsGone {
 		// TODO: give it two tries at least later (store state
 		// somewhere; global map?). But for now we're going to
 		// only give it one try.