raft: count storage reads in MsgAppResp test

pkg/raft/raft_test.go

@@ -2572,22 +2572,50 @@ func TestLeaderAppResp(t *testing.T) {
 		reject bool
 		// progress
 		wmatch uint64
-		wnext  uint64
-		// message
-		wmsgNum    int
-		windex     uint64
+		// log index of the next entry to send to this follower
+		wnext uint64
+		// number of messages the leader sends out
+		wmsgNum int
+		// prevLogIndex in MsgApp from leader to followers
+		windex uint64
+		// leader's commit index
 		wcommitted uint64
+		// storage access counts for getting term number
+		ctStgTerm int
 	}{
-		{2, true, 0, 4, 0, 0, 0}, // stale resp; no replies
-		{6, true, 0, 4, 0, 0, 0}, // stale resp; no replies
-		{3, true, 0, 3, 1, 2, 0}, // denied resp; leader does not commit; decrease next and send probing msg
-
-		{4, false, 4, 7, 1, 4, 4}, // accept resp; leader commits; broadcast with commit index
-		// Follower is StateProbing at 4, it sends MsgAppResp for 4, and is moved to
-		// StateReplicate and as many entries as possible are sent to it (5, 6).
+		// stale resp; no replies
+		{2, true, 0, 4, 0, 0, 0, 1},
+		// stale resp; no replies
+		{6, true, 0, 4, 0, 0, 0, 1},
+
+		// denied resp; leader does not commit; decrease next and send probing msg
+		// An additional term storage access is involved for an entry
+		// that's already persisted since we are probing backwards.
+		{3, true, 0, 3, 1, 2, 0, 2},
+
+		// Follower 2 responds to leader, indicating log index 2 is replicated.
+		// Leader tries to commit, but commit index doesn't advance since the index
+		// is from a previous term.
+		// We hit maybeCommit() and do term check by getting the term number from
+		// storage.
+		{2, false, 2, 7, 1, 2, 0, 3},
+
+		// NB: For the following tests, we are skipping the MsgAppResp for the first
+		// 3 entries, by directly processing MsgAppResp for later entries.
+		//
+		// Follower 2 is StateProbing at 4, it sends MsgAppResp for 4, and is moved
+		// to StateReplicate and as many entries as possible are sent to it (5, 6).
 		// Correspondingly the Next is then 7 (entry 7 does not exist, indicating
 		// the follower will be up to date should it process the emitted MsgApp).
-		{4, false, 4, 7, 1, 4, 4},
+		// accept resp; leader commits; respond with commit index
+		{4, false, 4, 7, 1, 4, 4, 1},
+
+		// Follower 2 says term2, index5 is already replicated.
+		// The leader responds with the updated commit index to follower 2.
+		{5, false, 5, 7, 1, 5, 5, 1},
+		// Follower 2 says term2, index6 is already replicated.
+		// The leader responds with the updated commit index to follower 2.
+		{6, false, 6, 7, 1, 6, 6, 1},
 	} {
 		t.Run("", func(t *testing.T) {
 			storage := newTestMemoryStorage(withPeers(1, 2, 3))
@@ -2600,9 +2628,10 @@ func TestLeaderAppResp(t *testing.T) {
 			sm.becomeLeader()
 			require.Equal(t, uint64(4), sm.raftLog.lastIndex()) // appended a dummy
 			sm.appendEntry(index(5).terms(2, 2)...)
+			require.Equal(t, uint64(0), sm.raftLog.committed)
 			sm.bcastAppend()
-
 			sm.readMessages()
+
 			require.NoError(t, sm.Step(pb.Message{
 				From:       2,
 				Type:       pb.MsgAppResp,
@@ -2622,6 +2651,8 @@ func TestLeaderAppResp(t *testing.T) {
 				require.Equal(t, tt.windex, msg.Index, "%v", DescribeMessage(msg, nil))
 				require.Equal(t, tt.wcommitted, msg.Commit, "%v", DescribeMessage(msg, nil))
 			}
+
+			assert.Equal(t, tt.ctStgTerm, storage.callStats.term)
 		})
 	}
 }