Fix for Bug #39243 SELECT WHERE does not find row

Symptom was that records_in_range() found 0 matching keys which confused the optimizer to belive that there was no matching rows for the query

mysql-test/r/maria.result:
  New testcase
mysql-test/t/maria.test:
  New testcase
storage/maria/ma_search.c:
  Fix bug in skip_key for keys that starts with a CHAR/VARCHAR NULL key.

mysql-test/r/maria.result

@@ -2243,3 +2243,35 @@ lock table t1 write concurrent;
 delete from t1;
 ERROR 42000: The storage engine for the table doesn't support DELETE in WRITE CONCURRENT
 drop table t1;
+create table t1 (p int primary key, i int, a char(10), key k1(i), key k2(a))
+engine maria;
+insert into t1 values (1, 1, 'qqqq'), (2, 1, 'pppp'),
+(3, 1, 'yyyy'), (4, 3, 'zzzz');
+insert into t1 values (5, 3, 'yyyy'), (6, 3, 'yyyy'), (7, 0, NULL),
+(8, 0, NULL);
+select * from t1 where a='zzzz';
+p	i	a
+4	3	zzzz
+select * from t1 where a='yyyy';
+p	i	a
+3	1	yyyy
+5	3	yyyy
+6	3	yyyy
+select * from t1 where a is NULL;
+p	i	a
+7	0	NULL
+8	0	NULL
+select * from t1;
+p	i	a
+1	1	qqqq
+2	1	pppp
+3	1	yyyy
+4	3	zzzz
+5	3	yyyy
+6	3	yyyy
+7	0	NULL
+8	0	NULL
+check table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+drop table t1;

mysql-test/t/maria.test

@@ -1542,3 +1542,20 @@ eval set global storage_engine=$default_engine, maria_page_checksum=$default_che
 --enable_result_log
 --enable_query_log
 
+#
+# Bug#39243 SELECT WHERE does not find row
+# (Problem with skip_row)
+#
+
+create table t1 (p int primary key, i int, a char(10), key k1(i), key k2(a))
+engine maria;
+insert into t1 values (1, 1, 'qqqq'), (2, 1, 'pppp'),
+                      (3, 1, 'yyyy'), (4, 3, 'zzzz');
+insert into t1 values (5, 3, 'yyyy'), (6, 3, 'yyyy'), (7, 0, NULL),
+                      (8, 0, NULL);
+select * from t1 where a='zzzz';
+select * from t1 where a='yyyy';
+select * from t1 where a is NULL;
+select * from t1;
+check table t1;
+drop table t1;

storage/maria/ma_search.c

@@ -931,7 +931,7 @@ uint _ma_get_static_key(MARIA_KEY *key, uint page_flag, uint nod_flag,
 /**
    Skip over static length key from key-block
 
-  @fn _ma_skip_pack_key()
+  @fn _ma_skip_static_key()
   @param key       Keyinfo and buffer that can be used
   @param nod_flag  If nod: Length of node pointer, else zero.
   @param key       Points at key
@@ -1049,6 +1049,7 @@ uint _ma_get_pack_key(MARIA_KEY *int_key, uint page_flag,
       }
       else
       {
+        /* Key that is not packed against previous key */
         if (keyseg->flag & HA_NULL_PART)
         {
           if (!length--)                        /* Null part */
@@ -1121,6 +1122,9 @@ uint _ma_get_pack_key(MARIA_KEY *int_key, uint page_flag,
   @param nod_flag  If nod: Length of node pointer, else zero.
   @param key       Points at key
 
+  @note
+  This is in principle a simpler version of _ma_get_pack_key()
+
   @retval pointer to next key
 */
 
@@ -1150,6 +1154,14 @@ uchar *_ma_skip_pack_key(MARIA_KEY *key, uint page_flag,
 	page+= length;
 	continue;
       }
+      if ((keyseg->flag & HA_NULL_PART) && length)
+      {
+        /*
+          Keys that can have null use length+1 as the length for date as the
+          number 0 is reserved for keys that have a NULL value
+        */
+        length--;
+      }
       page+= length;
     }
     else
@@ -1846,11 +1858,14 @@ _ma_calc_var_key_length(const MARIA_KEY *key, uint nod_flag,
 
     prefix byte(s) The high bit is set if this is a prefix for the prev key
     length         Packed length if the previous was a prefix byte
-    [length]       data bytes ('length' bytes)
+    [data_length]  data bytes ('length' bytes)
     next-key-seg   Next key segments
 
     If the first segment can have NULL:
-    The length is 0 for NULLS and 1+length for not null columns.
+       If key was packed
+         data_length is length of rest of key
+       If key was not packed
+         The data_length is 0 for NULLS and 1+data_length for not null columns
 */
 
 int