MDEV-26724 Endless loop in json_escape_to_string upon ... empty string

Correctly handle empty string when [un]escaping JSON

mysql-test/main/statistics_json.result

@@ -7853,3 +7853,19 @@ test.t1	analyze	status	OK
 select * from t1 where a = 'foo';
 a
 drop table t1;
+#
+# MDEV-26724 Endless loop in json_escape_to_string upon ... empty string 
+#
+CREATE TABLE t1 (f VARCHAR(8));
+INSERT INTO t1 VALUES ('a'),(''),('b');
+SET histogram_type=JSON_HB;
+ANALYZE TABLE t PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t	analyze	Error	Table 'test.t' doesn't exist
+test.t	analyze	status	Operation failed
+select * from t1;
+f
+a
+
+b
+drop table t1;

mysql-test/main/statistics_json.test

@@ -193,3 +193,13 @@ analyze table t1 persistent for all;
 select * from t1 where a = 'foo';
 drop table t1;
 
+--echo #
+--echo # MDEV-26724 Endless loop in json_escape_to_string upon ... empty string 
+--echo #
+CREATE TABLE t1 (f VARCHAR(8));
+INSERT INTO t1 VALUES ('a'),(''),('b');
+SET histogram_type=JSON_HB;
+ANALYZE TABLE t PERSISTENT FOR ALL;
+select * from t1;
+drop table t1;
+

sql/opt_histogram_json.cc

@@ -41,7 +41,7 @@ static bool json_unescape_to_string(const char *val, int val_len, String* out)
                            (const uchar*)val + val_len,
                            &my_charset_utf8mb4_bin,
                            buf, buf + out->length());
-    if (res > 0)
+    if (res >= 0)
     {
       out->length(res);
       return false; // Ok
@@ -74,7 +74,7 @@ static bool json_escape_to_string(const char *val, int val_len, String* out)
                          (const uchar*)val + val_len,
                          &my_charset_utf8mb4_bin,
                          buf, buf + out->length());
-    if (res > 0)
+    if (res >= 0)
     {
       out->length(res);
       return false; // Ok