Fix Admin support for MySQL client '8.1.0' - Closes #4300

MySQL client check command for dollars quote support ('SELECT $$'), is
now intercepted in both MySQL and Admin interfaces. The error reported
to the client is dependent of 'mysql-server_version' variable value.

include/proxysql_admin.h

@@ -390,7 +390,7 @@ class ProxySQL_Admin {
 	void admin_shutdown();
 	bool is_command(std::string);
 	void send_MySQL_OK(MySQL_Protocol *myprot, char *msg, int rows=0);
-	void send_MySQL_ERR(MySQL_Protocol *myprot, char *msg);
+	void send_MySQL_ERR(MySQL_Protocol *myprot, char *msg, uint32_t code=1045);
 #ifdef DEBUG
 	// these two following functions used to just call and return one function each
 	// this approach was replaced when we introduced debug filters

include/proxysql_utils.h

@@ -210,4 +210,39 @@ std::string replace_str(const std::string& str, const std::string& match, const
 std::string generate_multi_rows_query(int rows, int params);
 
 void close_all_non_term_fd(std::vector<int> excludeFDs);
+
+/**
+ * @brief Suggested implementation of 'mismatch_' from ['cppreference'](https://en.cppreference.com/w/cpp/algorithm/mismatch).
+ *
+ * @param first1 begin of the first range of the elements.
+ * @param last1 end of the first range of the elements.
+ * @param first2 begin of the second range of the elements.
+ * @param last2 end of the second range of the elements.
+ * @param p binary predicate which returns `true` if the elements should be treated as equal.
+ *
+ * @return std::pair with iterators to the first two non-equal elements.
+ */
+template<class InputIt1, class InputIt2, class BinaryPredicate>
+std::pair<InputIt1, InputIt2> mismatch_(
+	InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, BinaryPredicate p
+) {
+    while (first1 != last1 && first2 != last2 && p(*first1, *first2)) {
+        ++first1, ++first2;
+    }
+    return std::make_pair(first1, first2);
+}
+
+/**
+ * @brief Returns a sorted copy in ascending order of the supplied version numbers.
+ * @details Expected version numbers formats are: ['N', 'N.N', 'N.N.N', ...]
+ */
+std::vector<std::string> sort_versions(std::vector<std::string> versions);
+
+/**
+ * @brief Returns the expected error for query 'SELECT $$'.
+ * @param version The 'server_version' for which the error should match.
+ * @return A pair of the shape '{err_code,err_msg}'.
+ */
+std::pair<int,const char*> get_dollar_quote_error(const char* version);
+
 #endif

lib/MySQL_Session.cpp

@@ -1365,6 +1365,22 @@ bool MySQL_Session::handler_special_queries(PtrSize_t *pkt) {
 		l_free(pkt->size,pkt->ptr);
 		return true;
 	}
+	// MySQL client check command for dollars quote support, starting at version '8.1.0'. See #4300.
+	if ((pkt->size == strlen("SELECT $$") + 5) && strncasecmp("SELECT $$", (char*)pkt->ptr + 5, pkt->size - 5) == 0) {
+		pair<int,const char*> err_info { get_dollar_quote_error(mysql_thread___server_version) };
+
+		client_myds->DSS=STATE_QUERY_SENT_NET;
+		client_myds->myprot.generate_pkt_ERR(true, NULL, NULL, 1, err_info.first, (char *)"HY000", err_info.second, true);
+		client_myds->DSS=STATE_SLEEP;
+		status=WAITING_CLIENT_DATA;
+
+		if (mirror==false) {
+			RequestEnd(NULL);
+		}
+		l_free(pkt->size,pkt->ptr);
+
+		return true;
+	}
 	if (locked_on_hostgroup >= 0 && (strncasecmp((char *)"SET ",(char *)pkt->ptr+5,4)==0)) {
 		// this is a circuit breaker, we will send everything to the backend
 		//

lib/ProxySQL_Admin.cpp

@@ -4561,6 +4561,14 @@ void admin_session_handler(MySQL_Session *sess, void *_pa, PtrSize_t *pkt) {
 		goto __run_query;
 	}
 
+	// MySQL client check command for dollars quote support, starting at version '8.1.0'. See #4300.
+	if (!strncasecmp("SELECT $$", query_no_space, strlen("SELECT $$"))) {
+		pair<int,const char*> err_info { get_dollar_quote_error(mysql_thread___server_version) };
+		SPA->send_MySQL_ERR(&sess->client_myds->myprot, const_cast<char*>(err_info.second), err_info.first);
+		run_query=false;
+		goto __run_query;
+	}
+
 	if (query_no_space_length==SELECT_VERSION_COMMENT_LEN) {
 		if (!strncasecmp(SELECT_VERSION_COMMENT, query_no_space, query_no_space_length)) {
 			l_free(query_length,query);
@@ -11166,14 +11174,14 @@ void ProxySQL_Admin::send_MySQL_OK(MySQL_Protocol *myprot, char *msg, int rows)
 	myds->DSS=STATE_SLEEP;
 }
 
-void ProxySQL_Admin::send_MySQL_ERR(MySQL_Protocol *myprot, char *msg) {
+void ProxySQL_Admin::send_MySQL_ERR(MySQL_Protocol *myprot, char *msg, uint32_t code) {
 	assert(myprot);
 	MySQL_Data_Stream *myds=myprot->get_myds();
 	myds->DSS=STATE_QUERY_SENT_DS;
 	char *a = (char *)"ProxySQL Admin Error: ";
 	char *new_msg = (char *)malloc(strlen(msg)+strlen(a)+1);
 	sprintf(new_msg, "%s%s", a, msg);
-	myprot->generate_pkt_ERR(true,NULL,NULL,1,1045,(char *)"28000",new_msg);
+	myprot->generate_pkt_ERR(true,NULL,NULL,1,code,(char *)"28000",new_msg);
 	free(new_msg);
 	myds->DSS=STATE_SLEEP;
 }

lib/proxysql_utils.cpp

@@ -1,4 +1,5 @@
 #include "proxysql_utils.h"
+#include "mysqld_error.h"
 
 #include <functional>
 #include <sstream>
@@ -431,3 +432,47 @@ void close_all_non_term_fd(std::vector<int> excludeFDs) {
 		}
 	}
 }
+
+vector<string> sort_versions(vector<string> versions) {
+	std::sort(
+		versions.begin(), versions.end(),
+		[](const string& v1, const string& v2) {
+			const auto result =
+				mismatch_(
+					v1.cbegin(), v1.cend(), v2.cbegin(), v2.cend(),
+					[](const unsigned char lhs, const unsigned char rhs) {
+						return tolower(lhs) == tolower(rhs);
+					}
+				);
+
+			const bool not_equal = result.second != v2.cend();
+			const bool fst_shorter = result.first == v1.cend();
+			const bool fst_lesser = std::tolower(*result.first) < std::tolower(*result.second);
+
+			return not_equal && (fst_shorter || fst_lesser);
+		}
+	);
+
+	return versions;
+}
+
+std::pair<int,const char*> get_dollar_quote_error(const char* version) {
+	const char* ER_PARSE_MSG {
+		"You have an error in your SQL syntax; check the manual that corresponds to your MySQL server"
+			" version for the right syntax to use near '$$' at line 1'"
+	};
+
+	if (strcasecmp(version,"8.1.0") == 0) {
+		return { ER_PARSE_ERROR, ER_PARSE_MSG };
+	} else {
+		const vector<string> sorted { sort_versions({"8.1.0", version}) };
+
+		if (sorted[0] == "8.1.0") {
+			// SQLSTATE: 42000
+			return { ER_PARSE_ERROR, ER_PARSE_MSG };
+		} else {
+			// SQLSTATE: 42S22
+			return { ER_BAD_FIELD_ERROR, "Unknown column '$$' in 'field list'" };
+		}
+	}
+}