MDEV-31311: The test innodb.log_file_size_online occasionally hangs

log_t::write_checkpoint(), log_t::resize_start(): Invoke buf_flush_ahead()
with buf_pool.get_oldest_modification(0)+1 so that another checkpoint will
be invoked, to complete the log resizing.

Tested by: Oleg Smirnov (on Microsoft Windows, where this hung most often)

storage/innobase/buf/buf0flu.cc

@@ -1864,8 +1864,7 @@ inline void log_t::write_checkpoint(lsn_t end_lsn) noexcept
     ib::info() << "Resized log to " << ib::bytes_iec{resizing_completed}
       << "; start LSN=" << resizing;
   else
-    sql_print_error("InnoDB: Resize of log failed at " LSN_PF,
-                    get_flushed_lsn());
+    buf_flush_ahead(end_lsn + 1, false);
 }
 
 /** Initiate a log checkpoint, discarding the start of the log.

storage/innobase/log/log0log.cc

@@ -497,7 +497,14 @@ log_t::resize_start_status log_t::resize_start(os_offset_t size) noexcept
   log_resize_release();
 
   if (start_lsn)
+  {
+    mysql_mutex_lock(&buf_pool.flush_list_mutex);
+    lsn_t target_lsn= buf_pool.get_oldest_modification(0);
+    if (start_lsn < target_lsn)
+      start_lsn= target_lsn + 1;
+    mysql_mutex_unlock(&buf_pool.flush_list_mutex);
     buf_flush_ahead(start_lsn, false);
+  }
 
   return status;
 }