Add TCP keepalive warnings for issue #5212

- Add warnings in flush_mysql_variables___database_to_runtime() when mysql-use_tcp_keepalive=false
- Add warnings in flush_pgsql_variables___database_to_runtime() when pgsql-use_tcp_keepalive=false
- Include comprehensive Doxygen documentation explaining why disabling TCP keepalive is unsafe
- Warn users about potential connection drops when ProxySQL is deployed behind network load balancers

When TCP keepalive is disabled:
- Load balancers may drop idle connections from connection pools
- NAT devices may remove connection state
- Cloud load balancers may terminate connections during idle periods
- Results in sudden connection failures and "connection reset" errors

Resolves: #5212

lib/Admin_FlushVariables.cpp

@@ -549,6 +549,38 @@ void ProxySQL_Admin::flush_mysql_variables___database_to_runtime(SQLite3DB *db,
 			flush_GENERIC_variables__checksum__database_to_runtime("mysql", checksum, epoch);
 			pthread_mutex_unlock(&GloVars.checksum_mutex);
 		}
+		/**
+	 * @brief Check and warn if TCP keepalive is disabled for MySQL connections.
+	 *
+	 * This safety check warns users when mysql-use_tcp_keepalive is set to false,
+	 * which can cause connection instability in certain deployment scenarios.
+	 *
+	 * @warning Disabling TCP keepalive is unsafe when ProxySQL is deployed behind:
+	 *   - Network load balancers with idle connection timeouts
+	 *   - NAT firewalls with connection state timeout
+	 *   - Cloud environments with connection pooling
+	 *   - Any intermediate network device that drops idle connections
+	 *
+	 * @why_unsafe TCP keepalive sends periodic keep-alive packets on idle connections.
+	 * When disabled:
+	 *   - Load balancers may drop connections from their connection pools
+	 *   - NAT devices may remove connection state from their tables
+	 *   - Cloud load balancers (AWS ELB, GCP Load Balancer, etc.) may terminate
+	 *     connections during idle periods
+	 *   - Results in sudden connection failures and "connection reset" errors
+	 *   - Can cause application downtime and poor user experience
+	 *
+	 * @recommendation Always set mysql-use_tcp_keepalive=true when deploying
+	 * behind load balancers or in cloud environments.
+	 */
+	// Check for TCP keepalive setting and warn if disabled
+	char *mysql_use_tcp_keepalive = GloMTH->get_variable_string((char *)"use_tcp_keepalive");
+	if (mysql_use_tcp_keepalive && strcmp(mysql_use_tcp_keepalive, "false") == 0) {
+		proxy_warning("mysql-use_tcp_keepalive is set to false. This may cause connection drops when ProxySQL is behind a network load balancer. Consider setting this to true.\n");
+	}
+	if (mysql_use_tcp_keepalive) {
+		free(mysql_use_tcp_keepalive);
+	}
 	}
 	if (resultset) delete resultset;
 }
@@ -885,6 +917,42 @@ void ProxySQL_Admin::flush_pgsql_variables___database_to_runtime(SQLite3DB* db,
 			GloVars.checksums_values.mysql_variables.checksum, GloVars.checksums_values.mysql_variables.epoch
 		);
 		*/
+		/**
+	 * @brief Check and warn if TCP keepalive is disabled for PostgreSQL connections.
+	 *
+	 * This safety check warns users when pgsql-use_tcp_keepalive is set to false,
+	 * which can cause connection instability in certain deployment scenarios.
+	 *
+	 * @warning Disabling TCP keepalive is unsafe when ProxySQL is deployed behind:
+	 *   - Network load balancers with idle connection timeouts
+	 *   - NAT firewalls with connection state timeout
+	 *   - Cloud environments with connection pooling
+	 *   - Any intermediate network device that drops idle connections
+	 *
+	 * @why_unsafe TCP keepalive sends periodic keep-alive packets on idle connections.
+	 * When disabled for PostgreSQL:
+	 *   - Load balancers may drop connections from their connection pools
+	 *   - NAT devices may remove connection state from their tables
+	 *   - Cloud load balancers (AWS ELB, GCP Load Balancer, etc.) may terminate
+	 *     connections during idle periods
+	 *   - PostgreSQL connections may appear "stale" to the database server
+	 *   - Results in sudden connection failures and "connection reset" errors
+	 *   - Can cause application downtime and poor user experience
+	 *
+	 * @note PostgreSQL connections are often long-lived and benefit greatly from
+	 * TCP keepalive, especially in connection-pooled environments.
+	 *
+	 * @recommendation Always set pgsql-use_tcp_keepalive=true when deploying
+	 * behind load balancers or in cloud environments.
+	 */
+	// Check for TCP keepalive setting and warn if disabled
+	char *pgsql_use_tcp_keepalive = GloPTH->get_variable_string((char *)"use_tcp_keepalive");
+	if (pgsql_use_tcp_keepalive && strcmp(pgsql_use_tcp_keepalive, "false") == 0) {
+		proxy_warning("pgsql-use_tcp_keepalive is set to false. This may cause connection drops when ProxySQL is behind a network load balancer. Consider setting this to true.\n");
+	}
+	if (pgsql_use_tcp_keepalive) {
+		free(pgsql_use_tcp_keepalive);
+	}
 	}
 	if (resultset) delete resultset;
 }