MDEV-23318 Assertion `cache_empty(keycache)' failed in prepare_resize_simple_key_cache

The reason was that during MyISAM parallel repair two threads used the
same changed TABLE object to compute virtual columns

Fixed by adding a mutex in compute_vcols()

mysql-test/main/myisam_repair.result

+#
+# MDEV-23318 Assertion `cache_empty(keycache)' failed in
+# prepare_resize_simple_key_cache
+#
+SET @buffer_size.save= @@key_buffer_size;
+SET GLOBAL key_buffer_size= 134217728;
+SET myisam_repair_threads= 6;
+CREATE TABLE t1 (
+pk INT AUTO_INCREMENT,
+a INTEGER,
+b DATE,
+c VARCHAR(1),
+d BLOB,
+PRIMARY KEY (pk),
+KEY (a),
+KEY (b),
+KEY (c, a),
+UNIQUE (d)
+) ENGINE=MyISAM;
+INSERT INTO t1 (a,b,c) SELECT seq, '2020-12-12', 'x' FROM seq_1_to_20;
+ALTER TABLE t1 DISABLE KEYS;
+SET GLOBAL c.key_buffer_size= 13700864;
+Warnings:
+Warning	1292	Truncated incorrect key_buffer_size value: '13700864'
+INSERT INTO t1 SELECT 1;
+ERROR 21S01: Column count doesn't match value count at row 1
+SET  GLOBAL c.key_buffer_size= 0;
+DROP TABLE t1;
+SET GLOBAL key_buffer_size= 134217728;
+CREATE TABLE t1 (
+pk INT AUTO_INCREMENT,
+a INTEGER,
+b DATE,
+c VARCHAR(1),
+d VARBINARY(40982),
+PRIMARY KEY (pk),
+KEY (a),
+KEY (b),
+KEY (c, a),
+UNIQUE (d)
+) ENGINE=MyISAM;
+INSERT INTO t1 (a,b,c) SELECT seq, '2020-12-12', 'x' FROM seq_1_to_20;
+ALTER TABLE t1 DISABLE KEYS;
+SET GLOBAL c.key_buffer_size= 13700864;
+Warnings:
+Warning	1292	Truncated incorrect key_buffer_size value: '13700864'
+INSERT INTO t1 SELECT 1;
+ERROR 21S01: Column count doesn't match value count at row 1
+SET  GLOBAL c.key_buffer_size= 0;
+DROP TABLE t1;
+SET GLOBAL key_buffer_size= @buffer_size.save;

mysql-test/main/myisam_repair.test

+--source include/have_sequence.inc
+
+
+--echo #
+--echo # MDEV-23318 Assertion `cache_empty(keycache)' failed in
+--echo # prepare_resize_simple_key_cache
+--echo #
+
+SET @buffer_size.save= @@key_buffer_size;
+SET GLOBAL key_buffer_size= 134217728;
+SET myisam_repair_threads= 6;
+
+CREATE TABLE t1 (
+    pk INT AUTO_INCREMENT,
+    a INTEGER,
+    b DATE,
+    c VARCHAR(1),
+    d BLOB,
+    PRIMARY KEY (pk),
+    KEY (a),
+    KEY (b),
+    KEY (c, a),
+    UNIQUE (d)
+) ENGINE=MyISAM;
+ 
+INSERT INTO t1 (a,b,c) SELECT seq, '2020-12-12', 'x' FROM seq_1_to_20;
+
+ALTER TABLE t1 DISABLE KEYS;
+SET GLOBAL c.key_buffer_size= 13700864;
+--error ER_WRONG_VALUE_COUNT_ON_ROW
+INSERT INTO t1 SELECT 1;
+SET  GLOBAL c.key_buffer_size= 0;
+DROP TABLE t1;
+ 
+SET GLOBAL key_buffer_size= 134217728;
+
+CREATE TABLE t1 (
+    pk INT AUTO_INCREMENT,
+    a INTEGER,
+    b DATE,
+    c VARCHAR(1),
+    d VARBINARY(40982),
+    PRIMARY KEY (pk),
+    KEY (a),
+    KEY (b),
+    KEY (c, a),
+    UNIQUE (d)
+) ENGINE=MyISAM;
+INSERT INTO t1 (a,b,c) SELECT seq, '2020-12-12', 'x' FROM seq_1_to_20;
+ALTER TABLE t1 DISABLE KEYS;
+SET GLOBAL c.key_buffer_size= 13700864;
+--error ER_WRONG_VALUE_COUNT_ON_ROW
+INSERT INTO t1 SELECT 1;
+SET  GLOBAL c.key_buffer_size= 0;
+
+DROP TABLE t1;
+SET GLOBAL key_buffer_size= @buffer_size.save;

storage/myisam/ha_myisam.cc

@@ -688,6 +688,8 @@ my_bool mi_killed_in_mariadb(MI_INFO *info)
 
 static int compute_vcols(MI_INFO *info, uchar *record, int keynum)
 {
+  /* This mutex is needed for parallel repair */
+  mysql_mutex_lock(&info->s->intern_lock);
   TABLE *table= (TABLE*)(info->external_ref);
   table->move_fields(table->field, record, table->field[0]->record_ptr());
   if (keynum == -1) // update all vcols
@@ -695,6 +697,7 @@ static int compute_vcols(MI_INFO *info, uchar *record, int keynum)
     int error= table->update_virtual_fields(table->file, VCOL_UPDATE_FOR_READ);
     if (table->update_virtual_fields(table->file, VCOL_UPDATE_INDEXED))
       error= 1;
+    mysql_mutex_unlock(&info->s->intern_lock);
     return error;
   }
   // update only one key
@@ -703,9 +706,10 @@ static int compute_vcols(MI_INFO *info, uchar *record, int keynum)
   for (; kp < end; kp++)
   {
     Field *f= table->field[kp->fieldnr - 1];
-    if (f->vcol_info)
+    if (f->vcol_info && !f->vcol_info->stored_in_db)
       table->update_virtual_field(f);
   }
+  mysql_mutex_unlock(&info->s->intern_lock);
   return 0;
 }
 
@@ -956,7 +960,7 @@ void ha_myisam::setup_vcols_for_repair(HA_CHECK *param)
   if (!table->vfield)
     return;
 
-  if  (file->s->base.reclength == file->s->vreclength)
+  if (file->s->base.reclength == file->s->vreclength)
   {
     bool indexed_vcols= false;
     ulong new_vreclength= file->s->vreclength;
@@ -964,7 +968,8 @@ void ha_myisam::setup_vcols_for_repair(HA_CHECK *param)
     {
       if (!(*vf)->stored_in_db())
       {
-        uint vf_end= (*vf)->offset(table->record[0]) + (*vf)->pack_length_in_rec();
+        uint vf_end= ((*vf)->offset(table->record[0]) +
+                      (*vf)->pack_length_in_rec());
         set_if_bigger(new_vreclength, vf_end);
         indexed_vcols|= ((*vf)->flags & PART_KEY_FLAG) != 0;
       }
@@ -982,7 +987,8 @@ void ha_myisam::restore_vcos_after_repair()
 {
   if (file->s->base.reclength < file->s->vreclength)
   {
-    table->move_fields(table->field, table->record[0], table->field[0]->record_ptr());
+    table->move_fields(table->field, table->record[0],
+                       table->field[0]->record_ptr());
     table->default_column_bitmaps();
   }
 }