raft: add fmt::formatter for raft tracker types

before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter.

in this change, we define formatters for

* raft::election_tracker
* raft::votes
* raft::vote_result

and drop their operator<<:s.

Refs scylladb#13245

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb#17670

raft/tracker.cc

@@ -256,36 +256,38 @@ vote_result votes::tally_votes() const {
     return _current.tally_votes();
 }
 
-std::ostream& operator<<(std::ostream& os, const election_tracker& v) {
-    os << "responded: " << v._responded.size() << ", ";
-    os << "granted: " << v._granted;
-    return os;
-}
+} // end of namespace raft
 
+auto fmt::formatter<raft::election_tracker>::format(const raft::election_tracker& v, fmt::format_context& ctx) const
+        -> decltype(ctx.out()) {
+    return fmt::format_to(ctx.out(), "responded: {}, granted: {}",
+                          v._responded.size(), v._granted);
+}
 
-std::ostream& operator<<(std::ostream& os, const votes& v) {
-    os << "current: " << v._current << std::endl;
+auto fmt::formatter<raft::votes>::format(const raft::votes& v, fmt::format_context& ctx) const
+        -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    out = fmt::format_to(out, "current: {}\n", v._current);
     if (v._previous) {
-        os << "previous: " << v._previous.value() << std::endl;
+        out = fmt::format_to(out, "previous: {}\n", v._previous.value());
     }
-    return os;
+    return out;
 }
 
-std::ostream& operator<<(std::ostream& os, const vote_result& v) {
-    static const char *n;
+auto fmt::formatter<raft::vote_result>::format(const raft::vote_result& v, fmt::format_context& ctx) const
+        -> decltype(ctx.out()) {
+    std::string_view name;
+    using enum raft::vote_result;
     switch (v) {
-    case vote_result::UNKNOWN:
-        n = "UNKNOWN";
+    case UNKNOWN:
+        name = "UNKNOWN";
         break;
-    case vote_result::WON:
-        n = "WON";
+    case WON:
+        name = "WON";
         break;
-    case vote_result::LOST:
-        n = "LOST";
+    case LOST:
+        name = "LOST";
         break;
     }
-    os << n;
-    return os;
+    return formatter<std::string_view>::format(name, ctx);
 }
-
-} // end of namespace raft

raft/tracker.hh

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <seastar/core/condition-variable.hh>
+#include <fmt/core.h>
 #include "raft.hh"
 
 namespace raft {
@@ -145,8 +146,6 @@ enum class vote_result {
     LOST,
 };
 
-std::ostream& operator<<(std::ostream& os, const vote_result& v);
-
 // State of election in a single quorum
 class election_tracker {
     // All eligible voters
@@ -182,7 +181,7 @@ public:
         auto unknown = _suffrage.size() - _responded.size();
         return _granted + unknown >= quorum ? vote_result::UNKNOWN : vote_result::LOST;
     }
-    friend std::ostream& operator<<(std::ostream& os, const election_tracker& v);
+    friend fmt::formatter<election_tracker>;
 };
 
 // Candidate's state specific to election
@@ -202,8 +201,19 @@ public:
     void register_vote(server_id from, bool granted);
     vote_result tally_votes() const;
 
-    friend std::ostream& operator<<(std::ostream& os, const votes& v);
+    friend fmt::formatter<votes>;
 };
 
 } // namespace raft
 
+template <> struct fmt::formatter<raft::election_tracker> : fmt::formatter<std::string_view> {
+    auto format(const raft::election_tracker& v, fmt::format_context& ctx) const  -> decltype(ctx.out());
+};
+
+template <> struct fmt::formatter<raft::votes> : fmt::formatter<std::string_view> {
+    auto format(const raft::votes& v, fmt::format_context& ctx) const -> decltype(ctx.out());
+};
+
+template <> struct fmt::formatter<raft::vote_result> : fmt::formatter<std::string_view> {
+    auto format(const raft::vote_result& v, fmt::format_context& ctx) const -> decltype(ctx.out());
+};

test/raft/fsm_test.cc

@@ -8,10 +8,20 @@
 
 #define BOOST_TEST_MODULE raft
 
+#include "raft/tracker.hh"
 #include "test/raft/helpers.hh"
 
 using namespace raft;
 
+namespace raft {
+
+std::ostream& boost_test_print_type(std::ostream& os, const vote_result& v) {
+    fmt::print(os, "{}", v);
+    return os;
+}
+
+}
+
 BOOST_AUTO_TEST_CASE(test_votes) {
     auto id1 = id();