SERVER-45082 Add ConfigVersionAndTerm structure for comparing replica set configs

2 files changed, 82 insertions, 0 deletions

diff --git a/src/mongo/db/repl/repl_set_config.h b/src/mongo/db/repl/repl_set_config.h
index d5f065d5f7b..b82f0ed5e22 100644
--- a/src/mongo/db/repl/repl_set_config.h
+++ b/src/mongo/db/repl/repl_set_config.h
@@ -48,6 +48,67 @@ class BSONObj;
 namespace repl {
 
 /**
+ * A structure that stores a ReplSetConfig (version, term) pair.
+ *
+ * This can be used to compare two ReplSetConfig objects to determine which is logically newer.
+ */
+class ConfigVersionAndTerm {
+public:
+    ConfigVersionAndTerm() : _version(0), _term(OpTime::kUninitializedTerm) {}
+    ConfigVersionAndTerm(int version, long long term) : _version(version), _term(term) {}
+
+    inline bool operator==(const ConfigVersionAndTerm& rhs) const {
+        // If term of either item is uninitialized (-1), then we ignore terms entirely and only
+        // compare versions.
+        if (_term == OpTime::kUninitializedTerm || rhs._term == OpTime::kUninitializedTerm) {
+            return _version == rhs._version;
+        }
+        // Compare term first, then the versions.
+        return std::tie(_term, _version) == std::tie(rhs._term, rhs._version);
+    }
+
+    inline bool operator<(const ConfigVersionAndTerm& rhs) const {
+        // If term of either item is uninitialized (-1), then we ignore terms entirely and only
+        // compare versions. This allows force reconfigs, which set the config term to -1, to
+        // override other configs by using a high config version.
+        if (_term == OpTime::kUninitializedTerm || rhs._term == OpTime::kUninitializedTerm) {
+            return _version < rhs._version;
+        }
+        // Compare term first, then the versions.
+        return std::tie(_term, _version) < std::tie(rhs._term, rhs._version);
+    }
+
+    inline bool operator!=(const ConfigVersionAndTerm& rhs) const {
+        return !(*this == rhs);
+    }
+
+    inline bool operator<=(const ConfigVersionAndTerm& rhs) const {
+        return *this < rhs || *this == rhs;
+    }
+
+    inline bool operator>(const ConfigVersionAndTerm& rhs) const {
+        return !(*this <= rhs);
+    }
+
+    inline bool operator>=(const ConfigVersionAndTerm& rhs) const {
+        return !(*this < rhs);
+    }
+
+    // TODO (SERVER-45082): Implement string conversion.
+    std::string toString() const {
+        return "";
+    };
+
+    friend std::ostream& operator<<(std::ostream& out, const ConfigVersionAndTerm& cvt) {
+        return out << cvt.toString();
+    }
+
+private:
+    long long _version;
+    long long _term;
+};
+
+/**
  * Representation of the configuration information about a particular replica set.
  */
 class ReplSetConfig {
diff --git a/src/mongo/db/repl/repl_set_config_test.cpp b/src/mongo/db/repl/repl_set_config_test.cpp
index ddebd5f25f9..bdb4770bfa2 100644
--- a/src/mongo/db/repl/repl_set_config_test.cpp
+++ b/src/mongo/db/repl/repl_set_config_test.cpp
@@ -1714,6 +1714,27 @@ TEST(ReplSetConfig, ReplSetId) {
                            "\"replicaSetId\" had the wrong type. Expected objectId, found int");
 }
 
+TEST(ReplSetConfig, ConfigVersionAndTermComparison) {
+    // Test equality.
+    ASSERT_EQ(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(1, 1));
+    ASSERT_EQ(ConfigVersionAndTerm(1, 2), ConfigVersionAndTerm(1, 2));
+    ASSERT_EQ(ConfigVersionAndTerm(2, 2), ConfigVersionAndTerm(2, 2));
+    ASSERT_EQ(ConfigVersionAndTerm(1, -1), ConfigVersionAndTerm(1, 1));
+    ASSERT_EQ(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(1, -1));
+    ASSERT_EQ(ConfigVersionAndTerm(1, -1), ConfigVersionAndTerm(1, -1));
+    // Test greater/less than or equal to.
+    ASSERT_GT(ConfigVersionAndTerm(2, 1), ConfigVersionAndTerm(1, 1));
+    ASSERT_GTE(ConfigVersionAndTerm(2, 1), ConfigVersionAndTerm(1, 1));
+    ASSERT_GT(ConfigVersionAndTerm(1, 2), ConfigVersionAndTerm(1, 1));
+    ASSERT_GTE(ConfigVersionAndTerm(1, 2), ConfigVersionAndTerm(1, 1));
+    ASSERT_LT(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(2, 1));
+    ASSERT_LTE(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(2, 1));
+    ASSERT_LT(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(1, 2));
+    ASSERT_LTE(ConfigVersionAndTerm(1, 1), ConfigVersionAndTerm(1, 2));
+    ASSERT_GT(ConfigVersionAndTerm(2, 1), ConfigVersionAndTerm(1, -1));
+    ASSERT_GT(ConfigVersionAndTerm(2, -1), ConfigVersionAndTerm(1, 1));
+    ASSERT_GT(ConfigVersionAndTerm(2, -1), ConfigVersionAndTerm(1, -1));
+}
 }  // namespace
 }  // namespace repl
 }  // namespace mongo