Do not print "Updating metadata for stmt" for EXPLAIN

ProxySQL generates warnings when updating prepared statements metadatas.
This commit filters these warnings for EXPLAIN statements.

See #4478

It also include a TAP test for code coverage.

lib/MySQL_PreparedStatement.cpp

@@ -418,7 +418,11 @@ void MySQL_STMT_Global_info::update_metadata(MYSQL_STMT *stmt) {
 		}
 	}
 	if (need_refresh) {
-		proxy_warning("Updating metadata for stmt %lu , user %s, query %s\n", statement_id, username, query);
+		if (digest_text && strncasecmp(digest_text, "EXPLAIN", strlen("EXPLAIN"))==0) {
+			// do not print any message in case of EXPLAIN
+		} else {
+			proxy_warning("Updating metadata for stmt %lu , user %s, query %s\n", statement_id, username, query);
+		}
 // from here is copied from destructor
 		if (num_columns) {
 			uint16_t i;

test/tap/tests/stmt_explain-t.cpp

@@ -0,0 +1,73 @@
+/**
+ * @file stmt_explain-t.cpp
+ * @brief executes EXPLAIN using prepared statements.
+ * It doesn't perform any real test, only code coverage.
+ */
+
+
+#include <string>
+#include <stdio.h>
+#include <cstring>
+#include <unistd.h>
+
+#include "mysql.h"
+
+#include "tap.h"
+#include "command_line.h"
+#include "utils.h"
+
+using std::string;
+
+
+int main(int argc, char** argv) {
+	CommandLine cl;
+
+	// Checking for required environmental variables
+	if (cl.getEnv()) {
+		diag("Failed to get the required environmental variables.");
+		return -1;
+	}
+
+	plan(1); // Plan for testing purposes
+
+	MYSQL* mysql = mysql_init(NULL); ///< MySQL connection object
+	if (!mysql) {
+		fprintf(stderr, "File %s, line %d, Error: %s\n", __FILE__, __LINE__, mysql_error(mysql));
+		return exit_status();
+	}
+
+	// Connecting to ProxySQL
+	diag("Connecting to '%s@%s:%d'", cl.mysql_username, cl.mysql_host, cl.port);
+	if (!mysql_real_connect(mysql, cl.mysql_host, cl.mysql_username, cl.mysql_password, NULL, cl.port, NULL, 0)) {
+		fprintf(stderr, "File %s, line %d, Error: %s\n", __FILE__, __LINE__, mysql_error(mysql));
+		return exit_status();
+	}
+
+	// Initialize and prepare all the statements
+	MYSQL_STMT* stmt = mysql_stmt_init(mysql);
+		if (!stmt) {
+			fprintf(stderr, "mysql_stmt_init(), out of memory\n");
+			return exit_status();
+		}
+
+	std::string select_query = "EXPLAIN SELECT 1";
+	diag("select_query: %s", select_query.c_str());
+	if (mysql_stmt_prepare(stmt, select_query.c_str(), strlen(select_query.c_str()))) {
+		fprintf(stderr, "mysql_stmt_prepare at line %d failed: %s\n", __LINE__ , mysql_error(mysql));
+		mysql_close(mysql);
+		mysql_library_end();
+		return exit_status();
+	}
+
+	int rc  = mysql_stmt_execute(stmt);
+	ok (rc == 0 , "mysql_stmt_execute() succeeded");
+	if (rc) {
+		fprintf(stderr, "mysql_stmt_execute at line %d failed: %d , %s\n", __LINE__ , rc , mysql_stmt_error(stmt));
+		mysql_close(mysql);
+		mysql_library_end();
+		return exit_status();
+	}
+	mysql_close(mysql);
+
+	return exit_status();
+}