storage: fix deadlock in consistency queue

When `CheckConsistency` returns an error, the queue checks whether the
store is draining to decide whether the error is worth logging.

Unfortunately this check was incorrect and would block until the store
actually started draining.

A toy example of this problem is below (this will deadlock). The dual
return form of chan receive isn't non-blocking -- the second parameter
indicates whether the received value corresponds to a closing of the
channel.

Switch to a `select` instead.

```
package main

import (
	"fmt"
)

func main() {
	ch := make(chan struct{})
	_, ok := <-ch
	fmt.Println(ok)
}
```

Touches cockroachdb#21824.

Release note (bug fix): Prevent the consistency checker from
deadlocking. This would previously manifest itself as a steady number of
replicas queued for consistency checking on one or more nodes and would
resolve by restarting the affected nodes.

pkg/storage/consistency_queue.go

@@ -102,19 +102,31 @@ func (q *consistencyQueue) process(
 	if q.interval() <= 0 {
 		return nil
 	}
+
+	// Update the last processed time for this queue. We do this before actually
+	// processing because this queue will do it even in the case of an error
+	// (since a failed processing run could be caused by down followers). Note
+	// that this decision may not have been well-motivated.
+	if err := repl.setQueueLastProcessed(ctx, q.name, repl.store.Clock().Now()); err != nil {
+		log.VErrEventf(ctx, 2, "failed to update last processed time: %v", err)
+	}
+
 	req := roachpb.CheckConsistencyRequest{}
 	if _, pErr := repl.CheckConsistency(ctx, req); pErr != nil {
-		_, shouldQuiesce := <-repl.store.Stopper().ShouldQuiesce()
+		var shouldQuiesce bool
+		select {
+		case <-repl.store.Stopper().ShouldQuiesce():
+			shouldQuiesce = true
+		default:
+		}
+
 		if !shouldQuiesce || !grpcutil.IsClosedConnection(pErr.GoError()) {
 			// Suppress noisy errors about closed GRPC connections when the
 			// server is quiescing.
 			log.Error(ctx, pErr.GoError())
+			return pErr.GoError()
 		}
 	}
-	// Update the last processed time for this queue.
-	if err := repl.setQueueLastProcessed(ctx, q.name, repl.store.Clock().Now()); err != nil {
-		log.VErrEventf(ctx, 2, "failed to update last processed time: %v", err)
-	}
 	return nil
 }
 

pkg/storage/replica_test.go

@@ -9267,6 +9267,36 @@ func TestReplicaRecomputeStats(t *testing.T) {
 	}
 }
 
+// TestConsistencyQueueErrorFromCheckConsistency exercises the case in which
+// the queue receives an error from CheckConsistency.
+func TestConsistenctQueueErrorFromCheckConsistency(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+
+	ctx := context.Background()
+	stopper := stop.NewStopper()
+	defer stopper.Stop(ctx)
+
+	cfg := TestStoreConfig(nil)
+	cfg.TestingKnobs = StoreTestingKnobs{
+		TestingRequestFilter: func(ba roachpb.BatchRequest) *roachpb.Error {
+			if _, ok := ba.GetArg(roachpb.ComputeChecksum); ok {
+				return roachpb.NewErrorf("boom")
+			}
+			return nil
+		},
+	}
+	tc := testContext{}
+	tc.StartWithStoreConfig(t, stopper, cfg)
+
+	for i := 0; i < 2; i++ {
+		// Do this twice because it used to deadlock. See #25456.
+		sysCfg, _ := tc.store.Gossip().GetSystemConfig()
+		if err := tc.store.consistencyQueue.process(ctx, tc.repl, sysCfg); !testutils.IsError(err, "boom") {
+			t.Fatal(err)
+		}
+	}
+}
+
 // TestReplicaLocalRetries verifies local retry logic for transactional
 // and non transactional batches. Verifies the timestamp cache is updated
 // to reflect the timestamp at which retried batches are executed.