Proxy: Query goroutine leak when store.response-timeout is set

time.AfterFunc() returns a time.Timer object whose C field is nil,
accroding to the documentation. A goroutine blocks forever on reading
from a `nil` channel, leading to a goroutine leak on random slow
queries.

Signed-off-by: Mikhail Nozdrachev <[email protected]>

pkg/store/proxy_merge.go

@@ -361,7 +361,9 @@ func newLazyRespSet(
 				var rerr error
 				// If timer is already stopped
 				if t != nil && !t.Stop() {
-					<-t.C // Drain the channel if it was already stopped.
+					if t.C != nil {
+						<-t.C // Drain the channel if it was already stopped.
+					}
 					rerr = errors.Wrapf(err, "failed to receive any data in %s from %s", l.frameTimeout, st)
 				} else {
 					rerr = errors.Wrapf(err, "receive series from %s", st)
@@ -614,7 +616,9 @@ func newEagerRespSet(
 				var rerr error
 				// If timer is already stopped
 				if t != nil && !t.Stop() {
-					<-t.C // Drain the channel if it was already stopped.
+					if t.C != nil {
+						<-t.C // Drain the channel if it was already stopped.
+					}
 					rerr = errors.Wrapf(err, "failed to receive any data in %s from %s", l.frameTimeout, storeName)
 				} else {
 					rerr = errors.Wrapf(err, "receive series from %s", storeName)