storage: re-enqueue Raft groups on paginated application

Fixes cockroachdb#31330.

This change re-enqueues Raft groups for processing immediately if they
still have more to do after a Raft ready iteration. This comes up in
practice when a Range has sufficient load to force Raft application
pagination. See cockroachdb#31330 for a discussion on the symptoms this can
cause.

Release note (bug fix): Fix bug where Raft followers could fall behind
leaders will entry application, causing stalls during splits.

pkg/storage/replica.go

@@ -4444,6 +4444,14 @@ func (r *Replica) handleRaftReadyRaftMuLocked(
 	const expl = "during advance"
 	if err := r.withRaftGroup(true, func(raftGroup *raft.RawNode) (bool, error) {
 		raftGroup.Advance(rd)
+
+		// If the Raft group still has more to process then we immediately
+		// re-enqueue it for another round of processing. This is possible if
+		// the group's committed entries were paginated due to size limitations
+		// and we didn't apply all of them in this pass.
+		if raftGroup.HasReady() {
+			r.store.enqueueRaftUpdateCheck(r.RangeID)
+		}
 		return true, nil
 	}); err != nil {
 		return stats, expl, errors.Wrap(err, expl)

pkg/storage/replica_test.go

@@ -9581,6 +9581,98 @@ func TestProposeWithAsyncConsensus(t *testing.T) {
 	close(blockRaftApplication)
 }
 
+// TestApplyPaginatedCommittedEntries tests that a Raft group's committed
+// entries are quickly applied, even if their application is paginated due to
+// the RaftMaxSizePerMsg configuration. This is a regession test for #31330.
+func TestApplyPaginatedCommittedEntries(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	ctx := context.Background()
+	tc := testContext{}
+	tsc := TestStoreConfig(nil)
+
+	// Drop the RaftMaxSizePerMsg so that even small Raft entries have their
+	// application paginated.
+	// TODO(nvanbenschoten): Switch this to using the new MaxCommitedSizePerReady
+	// configuration once #31511 is addressed.
+	tsc.RaftMaxSizePerMsg = 128
+	// Slow down the tick interval dramatically so that Raft groups can't rely
+	// on ticks to trigger Raft ready iterations.
+	tsc.RaftTickInterval = 5 * time.Second
+
+	var filterActive int32
+	blockRaftApplication := make(chan struct{})
+	blockingRaftApplication := make(chan struct{}, 1)
+	tsc.TestingKnobs.TestingApplyFilter =
+		func(filterArgs storagebase.ApplyFilterArgs) *roachpb.Error {
+			if atomic.LoadInt32(&filterActive) == 1 {
+				select {
+				case blockingRaftApplication <- struct{}{}:
+				default:
+				}
+				<-blockRaftApplication
+			}
+			return nil
+		}
+
+	stopper := stop.NewStopper()
+	defer stopper.Stop(ctx)
+	tc.StartWithStoreConfig(t, stopper, tsc)
+	repl := tc.repl
+
+	// Block command application then propose a command to Raft.
+	var ba roachpb.BatchRequest
+	key := roachpb.Key("a")
+	put := putArgs(key, []byte("val"))
+	ba.Add(&put)
+	ba.Timestamp = tc.Clock().Now()
+
+	atomic.StoreInt32(&filterActive, 1)
+	exLease, _ := repl.GetLease()
+	_, _, _, pErr := repl.propose(ctx, exLease, ba, nil /* endCmds */, &allSpans)
+	if pErr != nil {
+		t.Fatal(pErr)
+	}
+
+	// Once that command is stuck applying, propose a number of large commands.
+	// This will allow them to all build up without any being applied so that
+	// their application will require pagination.
+	<-blockingRaftApplication
+	var ch chan proposalResult
+	for i := 0; i < 50; i++ {
+		var ba2 roachpb.BatchRequest
+		key := roachpb.Key("a")
+		put := putArgs(key, make([]byte, 2*tsc.RaftMaxSizePerMsg))
+		ba2.Add(&put)
+		ba2.Timestamp = tc.Clock().Now()
+
+		var pErr *roachpb.Error
+		ch, _, _, pErr = repl.propose(ctx, exLease, ba, nil /* endCmds */, &allSpans)
+		if pErr != nil {
+			t.Fatal(pErr)
+		}
+	}
+
+	// Stop blocking Raft application. All of the proposals should quickly
+	// commit and apply, even if their application is paginated due to the
+	// small RaftMaxSizePerMsg.
+	close(blockRaftApplication)
+	const maxWait = 5 * time.Second
+	select {
+	case propRes := <-ch:
+		if propRes.Err != nil {
+			t.Fatalf("unexpected proposal result error: %v", propRes.Err)
+		}
+		if propRes.Reply == nil || len(propRes.Reply.Responses) != 1 {
+			t.Fatalf("expected proposal result with 1 response, found: %v", propRes.Reply)
+		}
+	case <-time.After(maxWait):
+		// If we don't re-enqueue Raft groups for another round of processing
+		// when their committed entries are paginated and not all immediately
+		// applied, this test will take more than three minutes to finish.
+		t.Fatalf("stall detected, proposal did not finish within %s", maxWait)
+	}
+}
+
 func TestSplitMsgApps(t *testing.T) {
 	defer leaktest.AfterTest(t)()