raft: fix bug in unbounded log growth prevention mechanism

The previous code was using the proto-generated `Size()` method to
track the size of an incoming proposal at the leader. This includes
the Index and Term, which were mutated after the call to `Size()`
when appending to the log. Additionally, it was not taking into
account that an ignored configuration change would ignore the
original proposal and append an empty entry instead.

As a result, a fully committed Raft group could end up with a non-
zero tracked uncommitted Raft log counter that would eventually hit
the ceiling and drop all future proposals indiscriminately. It would
also immediately imply that proposals exceeding the threshold alone
would get refused (as the "first uncommitted proposal" gets special
treatment and is always allowed in).

Track only the size of the payload actually appended to the Raft log
instead.

For context, see:
cockroachdb/cockroach#31618 (comment)

raft/node_test.go

@@ -1006,7 +1006,7 @@ func TestNodeBoundedLogGrowthWithPartition(t *testing.T) {
 	const maxEntries = 16
 	data := []byte("testdata")
 	testEntry := raftpb.Entry{Data: data}
-	maxEntrySize := uint64(maxEntries * testEntry.Size())
+	maxEntrySize := uint64(maxEntries * PayloadSize(testEntry))
 
 	s := NewMemoryStorage()
 	cfg := newTestConfig(1, []uint64{1}, 10, 1, s)

raft/raft.go

@@ -970,10 +970,6 @@ func stepLeader(r *raft, m pb.Message) error {
 			r.logger.Debugf("%x [term %d] transfer leadership to %x is in progress; dropping proposal", r.id, r.Term, r.leadTransferee)
 			return ErrProposalDropped
 		}
-		if !r.increaseUncommittedSize(m.Entries) {
-			r.logger.Debugf("%x appending new entries to log would exceed uncommitted entry size limit; dropping proposal", r.id)
-			return ErrProposalDropped
-		}
 
 		for i, e := range m.Entries {
 			if e.Type == pb.EntryConfChange {
@@ -986,6 +982,12 @@ func stepLeader(r *raft, m pb.Message) error {
 				}
 			}
 		}
+		// Track the size of this uncommitted proposal. Note that the payload
+		// was potentially mutated above, so ordering matters.
+		if !r.increaseUncommittedSize(m.Entries) {
+			r.logger.Debugf("%x appending new entries to log would exceed uncommitted entry size limit; dropping proposal", r.id)
+			return ErrProposalDropped
+		}
 		r.appendEntry(m.Entries...)
 		r.bcastAppend()
 		return nil
@@ -1490,7 +1492,7 @@ func (r *raft) abortLeaderTransfer() {
 func (r *raft) increaseUncommittedSize(ents []pb.Entry) bool {
 	var s uint64
 	for _, e := range ents {
-		s += uint64(e.Size())
+		s += uint64(PayloadSize(e))
 	}
 
 	if r.uncommittedSize > 0 && r.uncommittedSize+s > r.maxUncommittedSize {
@@ -1513,7 +1515,7 @@ func (r *raft) reduceUncommittedSize(ents []pb.Entry) {
 
 	var s uint64
 	for _, e := range ents {
-		s += uint64(e.Size())
+		s += uint64(PayloadSize(e))
 	}
 	if s > r.uncommittedSize {
 		// uncommittedSize may underestimate the size of the uncommitted Raft

raft/raft_test.go

@@ -363,12 +363,19 @@ func TestProgressFlowControl(t *testing.T) {
 }
 
 func TestUncommittedEntryLimit(t *testing.T) {
-	const maxEntries = 16
+	// Use a relatively large number of entries here to prevent regression of a
+	// bug in which we'd use the Size() instead of the PayloadSize(). Size()
+	// changes when the Index and Term are assigned before putting the proposal
+	// in the log. This test would fail with the bug, either because we'd get
+	// dropped proposals earlier than we expect them, or because the final tally
+	// ends up nonzero. (At the time of writing, the former).
+	const maxEntries = 1024
 	testEntry := pb.Entry{Data: []byte("testdata")}
-	maxEntrySize := maxEntries * testEntry.Size()
+	maxEntrySize := maxEntries * PayloadSize(testEntry)
 
 	cfg := newTestConfig(1, []uint64{1, 2, 3}, 5, 1, NewMemoryStorage())
 	cfg.MaxUncommittedEntriesSize = uint64(maxEntrySize)
+	cfg.MaxInflightMsgs = 2*1024 // avoid interference
 	r := newRaft(cfg)
 	r.becomeCandidate()
 	r.becomeLeader()
@@ -425,6 +432,9 @@ func TestUncommittedEntryLimit(t *testing.T) {
 		t.Fatalf("expected %d messages, got %d", e, len(ms))
 	}
 	r.reduceUncommittedSize(propEnts)
+	if n := r.uncommittedSize; n != 0 {
+		t.Fatalf("expected zero uncommitted size, got %d", n)
+	}
 }
 
 func TestLeaderElection(t *testing.T) {

raft/rawnode_test.go

@@ -493,7 +493,7 @@ func TestRawNodeBoundedLogGrowthWithPartition(t *testing.T) {
 	const maxEntries = 16
 	data := []byte("testdata")
 	testEntry := raftpb.Entry{Data: data}
-	maxEntrySize := uint64(maxEntries * testEntry.Size())
+	maxEntrySize := uint64(maxEntries * PayloadSize(testEntry))
 
 	s := NewMemoryStorage()
 	cfg := newTestConfig(1, []uint64{1}, 10, 1, s)

raft/util.go

@@ -101,6 +101,12 @@ func DescribeMessage(m pb.Message, f EntryFormatter) string {
 	return buf.String()
 }
 
+// PayloadSize is the size of the payload of this Entry. Notably, it does not
+// depend on its Index or Term.
+func PayloadSize(e pb.Entry) int {
+	return len(e.Data)
+}
+
 // DescribeEntry returns a concise human-readable description of an
 // Entry for debugging.
 func DescribeEntry(e pb.Entry, f EntryFormatter) string {