MDEV-38850 Dormant corruption in log_t::clear_mmap()

storage/innobase/handler/ha_innodb.cc

@@ -3697,7 +3697,7 @@ static MYSQL_SYSVAR_SIZE_T(buffer_pool_size_max, buf_pool.size_in_bytes_max,
 static MYSQL_SYSVAR_UINT(log_write_ahead_size, log_sys.write_size,
   PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
   "Redo log write size to avoid read-on-write; must be a power of two",
-  nullptr, nullptr, 512, 512, 4096, 1);
+  nullptr, nullptr, 512, 512, log_sys.WRITE_SIZE_MAX, 1);
 
 /****************************************************************//**
 Gives the file extension of an InnoDB single-table tablespace. */

storage/innobase/include/log0log.h

@@ -268,6 +268,8 @@ struct log_t
 public:
   /** current innodb_log_write_ahead_size */
   uint write_size;
+  /** maximum write_size */
+  static constexpr uint WRITE_SIZE_MAX{4096};
   /** format of the redo log: e.g., FORMAT_10_8 */
   uint32_t format;
   /** whether the memory-mapped interface is enabled for the log */

storage/innobase/log/log0log.cc

@@ -1247,22 +1247,31 @@ void log_t::clear_mmap() noexcept
   ut_ad(write_lsn == get_lsn());
   ut_ad(write_lsn == get_flushed_lsn(std::memory_order_relaxed));
 
-  if (buf) /* this may be invoked while creating a new database */
   {
-    alignas(16) byte log_block[4096];
-    const size_t bs{write_size};
+    if (buf) /* this may be invoked while creating a new database */
     {
-      const size_t bf=
-        size_t(write_lsn - base_lsn.load(std::memory_order_relaxed));
-      memcpy_aligned<16>(log_block, buf + (bf & ~(bs - 1)), bs);
-    }
+      alignas(16) byte log_block[log_t::WRITE_SIZE_MAX];
+      const size_t bs{write_size};
+      {
+        ut_ad(write_lsn >= first_lsn);
+        uint64_t bf{write_lsn - first_lsn};
+        if (bf > capacity())
+          bf%= capacity();
+        bf+= START_OFFSET;
+        const size_t bs_1{bs - 1};
+        write_lsn_offset= bf & bs_1;
+        base_lsn.store(write_lsn - write_lsn_offset,
+                       std::memory_order_relaxed);
+        memcpy_aligned<16>(log_block, buf + (bf & ~uint64_t{bs_1}), bs);
+      }
 
-    close_file(false);
-    log_mmap= false;
-    ut_a(attach(log, file_size));
-    ut_ad(!is_mmap());
+      close_file(false);
+      log_mmap= false;
+      ut_a(attach(log, file_size));
+      ut_ad(!is_mmap());
 
-    memcpy_aligned<16>(buf, log_block, bs);
+      memcpy_aligned<16>(buf, log_block, bs);
+    }
   }
   log_resize_release();
 }