raft: operator<<(ostream&, ...) implementation for server_address and configuration

Useful for debugging.

Had to make `configuration` constructor explicit. Otherwise the
`operator<<` implementation for `configuration` would implicitly convert
the `server_address` to `configuration` when trying to output it, causing
infinite recursion.

Removed implicit uses of the constructor.

raft/raft.cc

@@ -6,9 +6,21 @@
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
 #include "raft.hh"
+#include "to_string.hh"
 
 namespace raft {
 
 seastar::logger logger("raft");
 
+std::ostream& operator<<(std::ostream& os, const raft::server_address& addr) {
+    return os << addr.id;
+}
+
+std::ostream& operator<<(std::ostream& os, const raft::configuration& cfg) {
+    if (cfg.previous.empty()) {
+        return os << cfg.current;
+    }
+    return os << format("{}->{}", cfg.previous, cfg.current);
+}
+
 } // end of namespace raft

raft/raft.hh

@@ -63,6 +63,8 @@ struct server_address {
     bool operator<(const server_address& rhs) const {
         return id < rhs.id;
     }
+
+    friend std::ostream& operator<<(std::ostream&, const server_address&);
 };
 
 } // end of namespace raft
@@ -105,7 +107,7 @@ struct configuration {
         }
     }
 
-    configuration(server_address_set current_arg = {}, server_address_set previous_arg = {})
+    explicit configuration(server_address_set current_arg = {}, server_address_set previous_arg = {})
         : current(std::move(current_arg)), previous(std::move(previous_arg)) {
             if (current.count(server_address{server_id()}) || previous.count(server_address{server_id()})) {
                 throw std::invalid_argument("raft::configuration: id zero is not supported");
@@ -195,6 +197,8 @@ struct configuration {
         assert(is_joint());
         previous.clear();
     }
+
+    friend std::ostream& operator<<(std::ostream&, const configuration&);
 };
 
 struct log_entry {
@@ -270,7 +274,7 @@ struct snapshot_descriptor {
     index_t idx = index_t(0);
     term_t term = term_t(0);
     // The committed configuration in the snapshot
-    configuration config;
+    configuration config{};
     // Id of the snapshot.
     snapshot_id id;
 };

test/raft/randomized_nemesis_test.cc

@@ -2403,12 +2403,6 @@ std::ostream& operator<<(std::ostream& os, const AppendReg::ret& r) {
     return os << format("ret{{{}, {}}}", r.x, r.prev);
 }
 
-namespace raft {
-std::ostream& operator<<(std::ostream& os, const raft::server_address& a) {
-    return os << a.id;
-}
-}
-
 SEASTAR_TEST_CASE(basic_generator_test) {
     using op_type = operation::invocable<operation::either_of<
             raft_call<AppendReg>,

test/raft/replication_test.cc

@@ -375,7 +375,7 @@ RAFT_TEST_CASE(rpc_configuration_truncate_restore_from_snp, (test_case{
                 .log = { raft::log_entry{raft::term_t(1), raft::index_t(1),
                         config{.curr = {node_id{0},node_id{1},node_id{2},node_id{3}},
                                .prev = {node_id{0},node_id{1},node_id{2}}}}},
-                .snapshot = {.config  = address_set({node_id{0},node_id{1},node_id{2}})
+                .snapshot = {.config  = raft::configuration{address_set({node_id{0},node_id{1},node_id{2}})}
                 }
             }},
             // A should see {A, B, C, D} as RPC config since
@@ -472,7 +472,7 @@ RAFT_TEST_CASE(rpc_configuration_truncate_restore_from_log, (test_case{
                         },
                 },
                 // all nodes in snapshot config {A, B, C, D} (original)
-                .snapshot = {.config  = address_set({node_id{0},node_id{1},node_id{2},node_id{3}})
+                .snapshot = {.config  = raft::configuration{address_set({node_id{0},node_id{1},node_id{2},node_id{3}})}
                 }
             }},
 
@@ -522,7 +522,7 @@ RAFT_TEST_CASE(rpc_configuration_truncate_restore_from_log, (test_case{
                         },
                 },
                 // all nodes in snapshot config {A, B, C, D} (original)
-                .snapshot = {.config  = address_set({node_id{0},node_id{1},node_id{2},node_id{3}})
+                .snapshot = {.config  = raft::configuration{address_set({node_id{0},node_id{1},node_id{2},node_id{3}})}
                 }
             }},