Fix cache_empty_result=0 not caching non-empty resultsets (issue #5248)

The `cache_empty_result` field in query rules has three possible values:
• -1: Use global setting (`query_cache_stores_empty_result`)
•  0: Do NOT cache empty resultsets, but cache non-empty resultsets
•  1: Always cache resultsets (both empty and non-empty)

Previously, when `cache_empty_result` was set to 0, nothing was cached at all,
even for non-empty resultsets. This prevented users from disabling caching
for empty resultsets while still allowing caching of non-empty resultsets
on a per-rule basis.

Changes:
1. Modified caching logic in MySQL_Session.cpp and PgSQL_Session.cpp to
   add the condition `(qpo->cache_empty_result == 0 && MyRS->num_rows)`
   (MySQL) and `(qpo->cache_empty_result == 0 && num_rows)` (PgSQL)
   to allow caching when cache_empty_result=0 AND result has rows.

2. Added comprehensive Doxygen documentation in query_processor.h explaining
   the semantics of cache_empty_result values.

3. Updated Query_Processor.cpp with inline comments explaining the
   three possible values.

Now when cache_empty_result is set to 0:
- Empty resultsets (0 rows) are NOT cached
- Non-empty resultsets (>0 rows) ARE cached
- This matches the intended per-rule behavior described in issue #5248.

Fixes: https://github.com/sysown/proxysql/issues/5248

include/query_processor.h

@@ -104,6 +104,16 @@ typedef struct _Query_Processor_rule_t {
 	char *replace_pattern;
 	int destination_hostgroup;
 	int cache_ttl;
+	/** @brief Controls caching of empty resultsets for query rules
+	 *
+	 * Values:
+	 * - -1: Use global setting (query_cache_stores_empty_result)
+	 * - 0: Do NOT cache empty resultsets, but cache non-empty resultsets
+	 * - 1: Always cache resultsets (both empty and non-empty)
+	 *
+	 * Previously, setting cache_empty_result to 0 would prevent ALL caching,
+	 * even for non-empty resultsets. This was fixed in issue #5248.
+	 */
 	int cache_empty_result;
 	int cache_timeout;
 	int reconnect;
@@ -139,6 +149,16 @@ class Query_Processor_Output {
 	int mirror_flagOUT;
 	int next_query_flagIN;
 	int cache_ttl;
+	/** @brief Controls caching of empty resultsets for query rule output
+	 *
+	 * Values:
+	 * - -1: Use global setting (query_cache_stores_empty_result)
+	 * - 0: Do NOT cache empty resultsets, but cache non-empty resultsets
+	 * - 1: Always cache resultsets (both empty and non-empty)
+	 *
+	 * Previously, setting cache_empty_result to 0 would prevent ALL caching,
+	 * even for non-empty resultsets. This was fixed in issue #5248.
+	 */
 	int cache_empty_result;
 	int cache_timeout;
 	int reconnect;

lib/MySQL_Session.cpp

@@ -7490,10 +7490,25 @@ void MySQL_Session::MySQL_Result_to_MySQL_wire(MYSQL *mysql, MySQL_ResultSet *My
 		if (transfer_started==false) { // we have all the resultset when MySQL_Result_to_MySQL_wire was called
 			if (qpo && qpo->cache_ttl>0 && com_field_list==false) { // the resultset should be cached
 				if (mysql_errno(mysql)==0 &&
-					(mysql_warning_count(mysql)==0 || 
+					(mysql_warning_count(mysql)==0 ||
 					 mysql_thread___query_cache_handle_warnings==1)) { // no errors
+					/**
+					 * @brief Check if the query result should be cached based on cache_empty_result setting
+					 *
+					 * The cache_empty_result field in query rule has three possible values:
+					 * - 1: Always cache the result, regardless of whether it's empty or not
+					 * - 0: Cache only non-empty results (num_rows > 0). Empty resultsets are not cached.
+					 * - -1: Use global setting (thread->variables.query_cache_stores_empty_result)
+					 *       OR cache if result is non-empty (num_rows > 0)
+					 *
+					 * Previously, when cache_empty_result was set to 0, nothing was cached at all.
+					 * This fix adds support for caching non-empty results when cache_empty_result=0.
+					 *
+					 * @see Issue #5248: Setting cache_empty_result to "0" on individual mysql_query_rules doesn't work
+					 */
 					if (
 						(qpo->cache_empty_result==1)
+						|| (qpo->cache_empty_result == 0 && MyRS->num_rows)
 						|| (
 							(qpo->cache_empty_result == -1)
 							&&

lib/PgSQL_Session.cpp

@@ -4809,11 +4809,26 @@ void PgSQL_Session::PgSQL_Result_to_PgSQL_wire(PgSQL_Connection* _conn, PgSQL_Da
 			if (qpo && qpo->cache_ttl > 0 && is_tuple == true) { // the resultset should be cached
 				
 				if (_conn->is_error_present() == false &&
-					(/* check warnings count here*/ true || 
+					(/* check warnings count here*/ true ||
 						pgsql_thread___query_cache_handle_warnings == 1)) { // no errors
 
+					/**
+					 * @brief Check if the query result should be cached based on cache_empty_result setting
+					 *
+					 * The cache_empty_result field in query rule has three possible values:
+					 * - 1: Always cache the result, regardless of whether it's empty or not
+					 * - 0: Cache only non-empty results (num_rows > 0). Empty resultsets are not cached.
+					 * - -1: Use global setting (thread->variables.query_cache_stores_empty_result)
+					 *       OR cache if result is non-empty (num_rows > 0)
+					 *
+					 * Previously, when cache_empty_result was set to 0, nothing was cached at all.
+					 * This fix adds support for caching non-empty results when cache_empty_result=0.
+					 *
+					 * @see Issue #5248: Setting cache_empty_result to "0" on individual mysql_query_rules doesn't work
+					 */
 					if (
-						(qpo->cache_empty_result == 1) || 
+						(qpo->cache_empty_result == 1) ||
+						(qpo->cache_empty_result == 0 && num_rows) ||
 							(
 								(qpo->cache_empty_result == -1) &&
 								(thread->variables.query_cache_stores_empty_result || num_rows)

lib/Query_Processor.cpp

@@ -1559,6 +1559,10 @@ __internal_loop:
 		}
 		if (qr->cache_empty_result >= 0) {
 			// Note: negative value means this rule doesn't change
+			// cache_empty_result values:
+			// -1: Use global setting (query_cache_stores_empty_result)
+			//  0: Do NOT cache empty resultsets, but cache non-empty resultsets
+			//  1: Always cache resultsets (both empty and non-empty)
 			proxy_debug(PROXY_DEBUG_MYSQL_QUERY_PROCESSOR, 5, "query rule %d has set cache_empty_result: %d. Query with empty result will%s hit the cache\n", qr->rule_id, qr->cache_empty_result, (qr->cache_empty_result == 0 ? " NOT" : "" ));
 			ret->cache_empty_result=qr->cache_empty_result;
 		}