From 75e0936a2d70c118e7820261bea0eecdd739f1d1 Mon Sep 17 00:00:00 2001
From: Richard Walters <rwalters@digitalstirling.com>
Date: Mon, 8 Mar 2021 12:10:42 -0800
Subject: [PATCH] Fix issues with follower committing log entries

* Do not advance commit index if appending entries
  is not successful.
* Even if successful, only advance commit index as far
  as the last matching entry.
---
 src/server/inner.rs                      | 7 +++++--
 src/server/tests/replication_response.rs | 6 +++---
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/server/inner.rs b/src/server/inner.rs
index 07b3a59..569ccd9 100644
--- a/src/server/inner.rs
+++ b/src/server/inner.rs
@@ -802,8 +802,11 @@ impl<S, T> Inner<S, T> {
                 append_entries,
             )
         };
-        if leader_commit > self.commit_index {
-            self.commit_log(leader_commit);
+        if success {
+            let new_commit_index = leader_commit.min(next_log_index - 1);
+            if new_commit_index > self.commit_index {
+                self.commit_log(new_commit_index);
+            }
         }
         let message = Message {
             content: MessageContent::AppendEntriesResponse {
diff --git a/src/server/tests/replication_response.rs b/src/server/tests/replication_response.rs
index d377974..b866e1c 100644
--- a/src/server/tests/replication_response.rs
+++ b/src/server/tests/replication_response.rs
@@ -320,7 +320,7 @@ fn follower_match_appended_entries() {
                 Message {
                     content: MessageContent::AppendEntries(
                         AppendEntriesContent {
-                            leader_commit: 0,
+                            leader_commit: 3,
                             prev_log_index: 0,
                             prev_log_term: 0,
                             log: vec![
@@ -343,7 +343,7 @@ fn follower_match_appended_entries() {
             .await;
         fixture
             .expect_append_entries_response(&AwaitAppendEntriesResponseArgs {
-                commit_index: None,
+                commit_index: Some(2),
                 expect_state_change: false,
                 success: true,
                 next_log_index: 3,
@@ -483,7 +483,7 @@ fn follower_rejects_appended_entries_with_mismatched_previous_term() {
                 Message {
                     content: MessageContent::AppendEntries(
                         AppendEntriesContent {
-                            leader_commit: 0,
+                            leader_commit: 3,
                             prev_log_index: 2,
                             prev_log_term: 3,
                             log: vec![