MDEV-19575 Fixed assert in ma_pagecache

There was a bug in the page cache that didn't take into account that
another thread could be waiting for a page to be read by read_big_block().
Fixed by releasing all waiters in read_big_block()

mysql-test/suite/s3/alter2.result

+#
+# MDEV-19575 Assertion `page_st == 1' failed upon SELECT from S3
+# table which is being converted into Aria
+#
+CREATE TABLE t1 (f INT);
+insert into t1 values (1),(2);
+ALTER TABLE t1 ENGINE=S3;
+select * from t1;
+f
+1
+2
+connect  con1,localhost,root,,$database;
+ALTER TABLE t1 ENGINE=Aria;
+connection default;
+SELECT * FROM t1;
+f
+1
+2
+connection con1;
+disconnect con1;
+connection default;
+DROP TABLE t1;

mysql-test/suite/s3/alter2.test

+--source include/have_s3.inc
+--source create_database.inc
+
+--echo #
+--echo # MDEV-19575 Assertion `page_st == 1' failed upon SELECT from S3
+--echo # table which is being converted into Aria
+--echo #
+
+CREATE TABLE t1 (f INT);
+insert into t1 values (1),(2);
+
+ALTER TABLE t1 ENGINE=S3;
+select * from t1;
+--connect (con1,localhost,root,,$database)
+--send
+  ALTER TABLE t1 ENGINE=Aria;
+
+--connection default
+SELECT * FROM t1;
+
+# Cleanup
+
+--connection con1
+--reap
+--disconnect con1
+--connection default
+DROP TABLE t1;
+
+#
+# clean up
+#
+--source drop_database.inc

storage/maria/ma_pagecache.c

@@ -2867,6 +2867,9 @@ static my_bool read_big_block(PAGECACHE *pagecache,
       remove_reader(block_to_read);
       unreg_request(pagecache, block_to_read, 1);
     }
+    /* Signal that all pending requests for this page now can be processed */
+    if (block->wqueue[COND_FOR_REQUESTED].last_thread)
+      wqueue_release_queue(&block->wqueue[COND_FOR_REQUESTED]);
     DBUG_RETURN(FALSE); // no retry
   }
 
@@ -2940,6 +2943,8 @@ static my_bool read_big_block(PAGECACHE *pagecache,
   }
   if (block->wqueue[COND_FOR_BIG_BLOCK].last_thread)
     wqueue_release_queue(&block->wqueue[COND_FOR_BIG_BLOCK]);
+  if (block->wqueue[COND_FOR_REQUESTED].last_thread)
+    wqueue_release_queue(&block->wqueue[COND_FOR_REQUESTED]);
 
   DBUG_RETURN(FALSE);
 }
@@ -3684,7 +3689,7 @@ uchar *pagecache_read(PAGECACHE *pagecache,
     if (((block->status & PCBLOCK_ERROR) == 0) && (page_st != PAGE_READ))
     {
 #ifdef WITH_S3_STORAGE_ENGINE
-      if (!pagecache->big_block_read)
+      if (!pagecache->big_block_read || page_st == PAGE_WAIT_TO_BE_READ)
 #endif /* WITH_S3_STORAGE_ENGINE */
       {
         /* The requested page is to be read into the block buffer */