MDEV-6878: Use of uninitialized saved_primary_key in Mrr_ordered_index_reader::resume_read()

(Backport to 5.3)
(variant #2, with fixed coding style)
- Make Mrr_ordered_index_reader::resume_read() restore index position 
  only if it was saved before with Mrr_ordered_index_reader::interrupt_read().

mysql-test/r/innodb_mrr_cpk.result

@@ -149,3 +149,41 @@ set @@join_cache_level= @save_join_cache_level;
 set storage_engine=@save_storage_engine;
 set optimizer_switch=@innodb_mrr_cpk_tmp;
 drop table t0;
+#
+# MDEV-6878: Use of uninitialized saved_primary_key in Mrr_ordered_index_reader::resume_read()
+#
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (
+pk varchar(32) character set utf8 primary key,
+kp1 char(32) not null,
+col1 varchar(32),
+key (kp1)
+) engine=innodb;
+insert into t1
+select
+concat('pk-', 1000 +A.a),
+concat('kp1-', 1000 +A.a),
+concat('val-', 1000 +A.a)
+from test.t0 A ;
+create table t2 as select kp1 as a from t1;
+set join_cache_level=8;
+set optimizer_switch='mrr=on,mrr_sort_keys=on';
+explain
+select * from t2 straight_join t1 force index(kp1) where t1.kp1=t2.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	10	
+1	SIMPLE	t1	ref	kp1	kp1	32	test.t2.a	1	Using join buffer (flat, BKAH join); Key-ordered Rowid-ordered scan
+select * from t2 straight_join t1 force index(kp1) where t1.kp1=t2.a;
+a	pk	kp1	col1
+kp1-1000	pk-1000	kp1-1000	val-1000
+kp1-1001	pk-1001	kp1-1001	val-1001
+kp1-1002	pk-1002	kp1-1002	val-1002
+kp1-1003	pk-1003	kp1-1003	val-1003
+kp1-1004	pk-1004	kp1-1004	val-1004
+kp1-1005	pk-1005	kp1-1005	val-1005
+kp1-1006	pk-1006	kp1-1006	val-1006
+kp1-1007	pk-1007	kp1-1007	val-1007
+kp1-1008	pk-1008	kp1-1008	val-1008
+kp1-1009	pk-1009	kp1-1009	val-1009
+drop table t0,t1,t2;

mysql-test/t/innodb_mrr_cpk.test

@@ -139,3 +139,32 @@ set storage_engine=@save_storage_engine;
 set optimizer_switch=@innodb_mrr_cpk_tmp;
 drop table t0;
 
+--echo #
+--echo # MDEV-6878: Use of uninitialized saved_primary_key in Mrr_ordered_index_reader::resume_read()
+--echo #
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (
+  pk varchar(32) character set utf8 primary key,
+  kp1 char(32) not null,
+  col1 varchar(32),
+  key (kp1)
+) engine=innodb;
+
+insert into t1
+select
+  concat('pk-', 1000 +A.a),
+  concat('kp1-', 1000 +A.a),
+  concat('val-', 1000 +A.a)
+from test.t0 A ;
+
+create table t2 as select kp1 as a from t1;
+
+set join_cache_level=8;
+set optimizer_switch='mrr=on,mrr_sort_keys=on';
+explain
+select * from t2 straight_join t1 force index(kp1) where t1.kp1=t2.a;
+select * from t2 straight_join t1 force index(kp1) where t1.kp1=t2.a;
+
+drop table t0,t1,t2;

sql/multi_range_read.cc

@@ -410,6 +410,7 @@ bool Mrr_ordered_index_reader::set_interruption_temp_buffer(uint rowid_length,
   *space_start += key_len;
 
   have_saved_rowid= FALSE;
+  read_was_interrupted= FALSE;
   return FALSE;
 }
 
@@ -418,6 +419,7 @@ void Mrr_ordered_index_reader::set_no_interruption_temp_buffer()
   support_scan_interruptions= FALSE;
   saved_key_tuple= saved_rowid= saved_primary_key= NULL; /* safety */
   have_saved_rowid= FALSE;
+  read_was_interrupted= FALSE;
 }
 
 void Mrr_ordered_index_reader::interrupt_read()
@@ -435,6 +437,7 @@ void Mrr_ordered_index_reader::interrupt_read()
              &table->key_info[table->s->primary_key],
              table->key_info[table->s->primary_key].key_length);
   }
+  read_was_interrupted= TRUE;
 
   /* Save the last rowid */
   memcpy(saved_rowid, file->ref, file->ref_length);
@@ -452,6 +455,10 @@ void Mrr_ordered_index_reader::position()
 void Mrr_ordered_index_reader::resume_read()
 {
   TABLE *table= file->get_table();
+
+  if (!read_was_interrupted)
+    return;
+
   KEY *used_index= &table->key_info[file->active_index];
   key_restore(table->record[0], saved_key_tuple, 
               used_index, used_index->key_length);
@@ -541,8 +548,7 @@ int Mrr_ordered_index_reader::init(handler *h_arg, RANGE_SEQ_IF *seq_funcs,
   is_mrr_assoc=    !test(mode & HA_MRR_NO_ASSOCIATION);
   mrr_funcs= *seq_funcs;
   source_exhausted= FALSE;
-  if (support_scan_interruptions)
-    bzero(saved_key_tuple, key_info->key_length);
+  read_was_interrupted= false;
   have_saved_rowid= FALSE;
   return 0;
 }

sql/multi_range_read.h

@@ -324,6 +324,12 @@ private:
   uchar *saved_key_tuple; /* Saved current key tuple */
   uchar *saved_primary_key; /* Saved current primary key tuple */
 
+  /*
+    TRUE<=> saved_key_tuple (and saved_primary_key when applicable) have
+    valid values.
+  */
+  bool read_was_interrupted;
+
   static int compare_keys(void* arg, uchar* key1, uchar* key2);
   static int compare_keys_reverse(void* arg, uchar* key1, uchar* key2);