gms: add formatter for gms::endpoint_state

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 a formatter for gms::endpoint_state, and
change update the callers of `operator<<` to use `fmt::print()`.
but we cannot drop `operator<<` yet, as we are still using the
templated operator<< and templated fmt::formatter to print containers
in scylla and in seastar -- they are still using `operator<<`
under the hood.

Refs #13245

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

Closes scylladb/scylladb#16705

gms/endpoint_state.cc

@@ -35,12 +35,7 @@ const versioned_value* endpoint_state::get_application_state_ptr(application_sta
 }
 
 std::ostream& operator<<(std::ostream& os, const endpoint_state& x) {
-    fmt::print(os, "HeartBeatState = {}, AppStateMap =", x._heart_beat_state);
-    for (auto&entry : x._application_state) {
-        const application_state& state = entry.first;
-        const versioned_value& value = entry.second;
-        os << " { " << state << " : " << value << " } ";
-    }
+    fmt::print(os, "{}", x);
     return os;
 }
 
@@ -67,3 +62,13 @@ future<> i_endpoint_state_change_subscriber::on_application_state_change(inet_ad
 }
 
 }
+
+auto fmt::formatter<gms::endpoint_state>::format(const gms::endpoint_state& x,
+                                                 fmt::format_context& ctx) const -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    out = fmt::format_to(out, "HeartBeatState = {}, AppStateMap =", x._heart_beat_state);
+    for (auto& [state, value] : x._application_state) {
+        out = fmt::format_to(out, " {{ {} : {} }} ", state, value);
+    }
+    return out;
+}

gms/endpoint_state.hh

@@ -149,9 +149,11 @@ public:
 
     bool is_cql_ready() const noexcept;
 
-    friend std::ostream& operator<<(std::ostream& os, const endpoint_state& x);
+    friend fmt::formatter<endpoint_state>;
 };
 
+std::ostream& operator<<(std::ostream& os, const endpoint_state& x);
+
 using endpoint_state_ptr = lw_shared_ptr<const endpoint_state>;
 
 inline endpoint_state_ptr make_endpoint_state_ptr(const endpoint_state& eps) {
@@ -174,3 +176,9 @@ using permit_id = utils::tagged_uuid<struct permit_id_tag>;
 constexpr permit_id null_permit_id = permit_id::create_null_id();
 
 } // gms
+
+template <>
+struct fmt::formatter<gms::endpoint_state> {
+    constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }
+    auto format(const gms::endpoint_state&, fmt::format_context& ctx) const -> decltype(ctx.out());
+};

gms/gossip_digest_ack.cc

@@ -21,7 +21,7 @@ std::ostream& operator<<(std::ostream& os, const gossip_digest_ack& ack) {
     os << "} ";
     os << "endpoint_state:{";
     for (auto& d : ack._map) {
-        os << "[" << d.first << "->" << d.second << "]";
+        fmt::print(os, "[{}->{}]", d.first, d.second);
     }
     return os << "}";
 }

gms/gossip_digest_ack2.cc

@@ -16,7 +16,7 @@ namespace gms {
 std::ostream& operator<<(std::ostream& os, const gossip_digest_ack2& ack2) {
     os << "endpoint_state:{";
     for (auto& d : ack2._map) {
-        os << "[" << d.first << "->" << d.second << "]";
+        fmt::print(os, "[{}->{}]", d.first, d.second);
     }
     return os << "}";
 }