[QUEUED scylla next] raft: server: fix bad_variant_access in `modify_config`

[Commit Bot]

From: Kamil Braun <kbr...@scylladb.com>
Committer: Kamil Braun <kbr...@scylladb.com>
Branch: next

raft: server: fix bad_variant_access in `modify_config`

`modify_config` would call `execute_modify_config` or
`_rpc->send_modify_config`, which returned a reply of type
`add_entry_reply`. This is a variant of 3 options: `entry_id`,
`not_a_leader`, or `commit_status_unknown`. The code would check
for the `entry_id` option and otherwise assume that it was `not_a_leader`.
During nemesis testing however, the reply was sometimes
`commit_status_unknown`, which caused a `bad_variant_access` exception
during `std::get` call. Fix this.

There is a similar piece of code in `add_entry`, but there it should be
impossible to obtain `commit_status_unknown` even though the types don't
enforce it. Make it more explicit with a comment and an assertion.

---
diff --git a/raft/server.cc b/raft/server.cc
--- a/raft/server.cc
+++ b/raft/server.cc
@@ -502,8 +502,15 @@ future<> server_impl::add_entry(command command, wait_type type, seastar::abort_
}();
if (std::holds_alternative<raft::entry_id>(reply)) {
co_return co_await wait_for_entry(std::get<raft::entry_id>(reply), type, as);
+ } else if (std::holds_alternative<raft::commit_status_unknown>(reply)) {
+ // It should be impossible to obtain `commit_status_unknown` here
+ // because neither `execute_add_entry` nor `send_add_entry` wait for the entry
+ // to be committed/applied.
+ on_internal_error(logger, "add_entry: `execute_add_entry` or `send_add_entry`"
+ " returned `commit_status_unknown`");
+ } else {
+ leader = std::get<raft::not_a_leader>(reply).leader;
}
- leader = std::get<raft::not_a_leader>(reply).leader;
}
}
}
@@ -596,7 +603,11 @@ future<> server_impl::modify_config(std::vector<server_address> add, std::vector
// See also #9981.
co_return;
}
- leader = std::get<raft::not_a_leader>(reply).leader;
+ if (auto nal = std::get_if<raft::not_a_leader>(&reply)) {
+ leader = nal->leader;
+ } else {
+ throw std::get<raft::commit_status_unknown>(reply);
+ }
}
}
}

[Commit Bot]

[Commit Bot]

From: Kamil Braun <kbr...@scylladb.com>
Committer: Kamil Braun <kbr...@scylladb.com>

Branch: master