raft: don't panic in Inflights.Add

In lazy replication mode, the inflights struct should not enforce the
in-flight limit. Instead, this policy is implemented at the higher level
(RACv2). The tracker nevertheless correctly tracks all the in-flight
state, so that it can correctly switch in/out of the lazy replication.

Epic: none
Release note: none

pkg/raft/tracker/inflights.go

@@ -64,13 +64,15 @@ func (in *Inflights) Clone() *Inflights {
 }
 
 // Add notifies the Inflights that a new message with the given index and byte
-// size is being dispatched. Full() must be called prior to Add() to verify that
-// there is room for one more message, and consecutive calls to Add() must
-// provide a monotonic sequence of indexes.
+// size is being dispatched. Consecutive calls to Add() must provide a monotonic
+// sequence of log indices.
+//
+// The caller should check that the tracker is not Full(), before calling Add().
+// If the tracker is full, the caller should hold off sending entries to the
+// peer. However, Add() is still valid and allowed, for cases when this pacing
+// is implemented at the higher app level. The tracker correctly tracks all the
+// in-flight entries.
 func (in *Inflights) Add(index, bytes uint64) {
-	if in.Full() {
-		panic("cannot add into a Full inflights")
-	}
 	next := in.start + in.count
 	size := in.size
 	if next >= size {
@@ -134,7 +136,7 @@ func (in *Inflights) FreeLE(to uint64) {
 
 // Full returns true if no more messages can be sent at the moment.
 func (in *Inflights) Full() bool {
-	return in.count == in.size || (in.maxBytes != 0 && in.bytes >= in.maxBytes)
+	return in.count >= in.size || (in.maxBytes != 0 && in.bytes >= in.maxBytes)
 }
 
 // Count returns the number of inflight messages.

pkg/raft/tracker/inflights_test.go

@@ -1,3 +1,6 @@
+// This code has been modified from its original form by The Cockroach Authors.
+// All modifications are Copyright 2024 The Cockroach Authors.
+//
 // Copyright 2019 The etcd Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -203,26 +206,18 @@ func TestInflightsFull(t *testing.T) {
 
 			addUntilFull := func(begin, end int) {
 				for i := begin; i < end; i++ {
-					if in.Full() {
-						t.Fatalf("full at %d, want %d", i, end)
-					}
+					require.False(t, in.Full(), "full at %d, want %d", i, end)
 					in.Add(uint64(i), uint64(100+i))
 				}
-				if !in.Full() {
-					t.Fatalf("not full at %d", end)
-				}
+				require.True(t, in.Full())
 			}
 
 			addUntilFull(0, tc.fullAt)
 			in.FreeLE(tc.freeLE)
 			addUntilFull(tc.fullAt, tc.againAt)
 
-			defer func() {
-				if r := recover(); r == nil {
-					t.Errorf("Add() did not panic")
-				}
-			}()
 			in.Add(100, 1024)
+			require.True(t, in.Full()) // the full tracker remains full
 		})
 	}
 }