MDEV-15872 Crash in online ALTER TABLE...ADD PRIMARY KEY after instant ADD COLUMN...NULL

row_log_table_get_pk_col(): Replace a condition that was inadvertently
removed in MDEV-16365. PRIMARY KEY columns are never allowed to be NULL,
and failure to enforce the constraint caused a null pointer to be
dereferenced in mem_heap_dup().

mysql-test/suite/innodb/r/instant_alter_debug.result

@@ -205,12 +205,35 @@ INSERT INTO t1 SET a=1;
 INSERT INTO t1 SET a=2,b=3,c=4;
 SET DEBUG_SYNC = 'now SIGNAL logged';
 connection ddl;
-disconnect ddl;
 connection default;
 SET DEBUG_SYNC = RESET;
 SELECT * FROM t1;
 a	b	c
 1	2	NULL
 2	3	4
+#
+# MDEV-15872 Crash in online ALTER TABLE...ADD PRIMARY KEY
+# after instant ADD COLUMN ... NULL
+#
+ALTER TABLE t1 ADD COLUMN d INT, ALGORITHM=INSTANT;
+UPDATE t1 SET d=1;
+connection ddl;
+SET DEBUG_SYNC='row_log_table_apply1_before SIGNAL copied WAIT_FOR logged';
+ALTER IGNORE TABLE t1 DROP PRIMARY KEY, ADD PRIMARY KEY (a,d);
+connection default;
+SET DEBUG_SYNC = 'now WAIT_FOR copied';
+BEGIN;
+INSERT INTO t1 SET a=3;
+ROLLBACK;
+SET DEBUG_SYNC = 'now SIGNAL logged';
+connection ddl;
+ERROR 22004: Invalid use of NULL value
+disconnect ddl;
+connection default;
+SET DEBUG_SYNC = RESET;
+SELECT * FROM t1;
+a	b	c	d
+1	2	NULL	1
+2	3	4	1
 DROP TABLE t1;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;

mysql-test/suite/innodb/t/instant_alter_debug.test

@@ -234,6 +234,32 @@ SET DEBUG_SYNC = 'now SIGNAL logged';
 
 connection ddl;
 reap;
+
+connection default;
+SET DEBUG_SYNC = RESET;
+SELECT * FROM t1;
+
+--echo #
+--echo # MDEV-15872 Crash in online ALTER TABLE...ADD PRIMARY KEY
+--echo # after instant ADD COLUMN ... NULL
+--echo #
+ALTER TABLE t1 ADD COLUMN d INT, ALGORITHM=INSTANT;
+UPDATE t1 SET d=1;
+
+connection ddl;
+SET DEBUG_SYNC='row_log_table_apply1_before SIGNAL copied WAIT_FOR logged';
+send ALTER IGNORE TABLE t1 DROP PRIMARY KEY, ADD PRIMARY KEY (a,d);
+
+connection default;
+SET DEBUG_SYNC = 'now WAIT_FOR copied';
+BEGIN;
+INSERT INTO t1 SET a=3;
+ROLLBACK;
+SET DEBUG_SYNC = 'now SIGNAL logged';
+
+connection ddl;
+--error ER_INVALID_USE_OF_NULL
+reap;
 disconnect ddl;
 
 connection default;

storage/innobase/row/row0log.cc

@@ -1164,7 +1164,6 @@ row_log_table_get_pk_col(
 	field = rec_get_nth_field(rec, offsets, i, &len);
 
 	if (len == UNIV_SQL_NULL) {
-
 		if (!log->allow_not_null) {
 			return(DB_INVALID_NULL);
 		}
@@ -1173,6 +1172,9 @@ row_log_table_get_pk_col(
 
 		field = static_cast<const byte*>(
 			log->defaults->fields[n_default_cols].data);
+		if (!field) {
+			return(DB_INVALID_NULL);
+		}
 		len = log->defaults->fields[i - DATA_N_SYS_COLS].len;
 	}