rpc: cleanup exit and blocking condition logic in blockingQuery

Remove some unnecessary comments around query_blocking metric. The only
line that needs any comments in the atomic decrement.

Cleanup the block and return comments and logic. The old comment about
AbandonCh may have been relevant before, but it is expected behaviour
now.

The logic was simplified by inverting the err condition.

agent/consul/rpc.go

@@ -942,13 +942,10 @@ func (s *Server) blockingQuery(queryOpts structs.QueryOptionsCompat, queryMeta s
 	ctx, cancel := context.WithTimeout(ctx, timeout)
 	defer cancel()
 
-	// instrument blockingQueries
-	// atomic inc our server's count of in-flight blockingQueries and store the new value
-	queriesBlocking := atomic.AddUint64(&s.queriesBlocking, 1)
-	// atomic dec when we return from blockingQuery()
+	count := atomic.AddUint64(&s.queriesBlocking, 1)
+	metrics.SetGauge([]string{"rpc", "queries_blocking"}, float32(count))
+	// decrement the count when the function returns.
 	defer atomic.AddUint64(&s.queriesBlocking, ^uint64(0))
-	// set the gauge directly to the new value of s.blockingQueries
-	metrics.SetGauge([]string{"rpc", "queries_blocking"}, float32(queriesBlocking))
 
 	for {
 		if queryOpts.GetRequireConsistent() {
@@ -977,24 +974,17 @@ func (s *Server) blockingQuery(queryOpts structs.QueryOptionsCompat, queryMeta s
 			return nil
 		}
 
-		// block up to the timeout if we don't see anything fresh.
-		if err := ws.WatchCtx(ctx); err == nil {
-			// a non-nil error only occurs when the context is cancelled
-
-			// If a restore may have woken us up then bail out from
-			// the query immediately. This is slightly race-ey since
-			// this might have been interrupted for other reasons,
-			// but it's OK to kick it back to the caller in either
-			// case.
-			select {
-			case <-state.AbandonCh():
-				return nil
-			default:
-			}
+		// block until something changes, or the timeout
+		if err := ws.WatchCtx(ctx); err != nil {
+			// exit if we've reached the timeout, or other cancellation
+			return nil
 		}
 
-		if ctx.Err() != nil {
+		// exit if the state store has been abandoned
+		select {
+		case <-state.AbandonCh():
 			return nil
+		default:
 		}
 	}
 }