MDEV-33966 preparation: Clean up recv_sys.pages bookkeeping

Instead of repurposing buf_page_t::access_time for state()==MEMORY
blocks that are part of recv_sys.pages, let us define an anonymous
union around buf_page_t::hash.  In this way, we will be able to
declare access_time private.

storage/innobase/include/buf0buf.h

@@ -509,8 +509,19 @@ class buf_page_t
   /** Page id. Protected by buf_pool.page_hash.lock_get() when
   the page is in buf_pool.page_hash. */
   page_id_t id_;
-  /** buf_pool.page_hash link; protected by buf_pool.page_hash.lock_get() */
-  buf_page_t *hash;
+  union {
+    /** for in_file(): buf_pool.page_hash link;
+    protected by buf_pool.page_hash.lock_get() */
+    buf_page_t *hash;
+    /** for state()==MEMORY that are part of recv_sys.pages and
+    protected by recv_sys.mutex */
+    struct {
+      /** number of recv_sys.pages entries stored in the block */
+      uint16_t used_records;
+      /** the offset of the next free record */
+      uint16_t free_offset;
+    };
+  };
 private:
   /** log sequence number of the START of the log entry written of the
   oldest modification to this block which has not yet been written
@@ -601,16 +612,7 @@ class buf_page_t
 	/* @} */
 	Atomic_counter<unsigned> access_time;	/*!< time of first access, or
 					0 if the block was never accessed
-					in the buffer pool.
-
-					For state() == MEMORY
-					blocks, this field can be repurposed
-					for something else.
-
-					When this field counts log records
-					and bytes allocated for recv_sys.pages,
-					the field is protected by
-					recv_sys_t::mutex. */
+					in the buffer pool. */
   buf_page_t() : id_{0}
   {
     static_assert(NOT_USED == 0, "compatibility");

storage/innobase/log/log0recv.cc

@@ -1497,6 +1497,7 @@ void recv_sys_t::clear()
   {
     buf_block_t *prev_block= UT_LIST_GET_PREV(unzip_LRU, block);
     ut_ad(block->page.state() == buf_page_t::MEMORY);
+    block->page.hash= nullptr;
     UT_LIST_REMOVE(blocks, block);
     MEM_MAKE_ADDRESSABLE(block->page.frame, srv_page_size);
     buf_block_free(block);
@@ -1546,14 +1547,11 @@ inline void recv_sys_t::free(const void *data)
     buf_block_t *block= &chunk->blocks[offs];
     ut_ad(block->page.frame == data);
     ut_ad(block->page.state() == buf_page_t::MEMORY);
-    ut_ad(static_cast<uint16_t>(block->page.access_time - 1) <
-          srv_page_size);
-    unsigned a= block->page.access_time;
-    ut_ad(a >= 1U << 16);
-    a-= 1U << 16;
-    block->page.access_time= a;
-    if (!(a >> 16))
+    ut_ad(uint16_t(block->page.free_offset - 1) < srv_page_size);
+    ut_ad(block->page.used_records);
+    if (!--block->page.used_records)
     {
+      block->page.hash= nullptr;
       UT_LIST_REMOVE(blocks, block);
       MEM_MAKE_ADDRESSABLE(block->page.frame, srv_page_size);
       buf_block_free(block);
@@ -2262,7 +2260,7 @@ bool recv_sys_t::add(map::iterator it, lsn_t start_lsn, lsn_t lsn,
     ut_ad(tail->lsn == lsn);
     block= UT_LIST_GET_LAST(blocks);
     ut_ad(block);
-    const size_t used= static_cast<uint16_t>(block->page.access_time - 1) + 1;
+    const size_t used= uint16_t(block->page.free_offset - 1) + 1;
     ut_ad(used >= ALIGNMENT);
     const byte *end= const_cast<const log_phys_t*>(tail)->end();
     if (!((reinterpret_cast<size_t>(end + len) ^
@@ -2283,7 +2281,7 @@ bool recv_sys_t::add(map::iterator it, lsn_t start_lsn, lsn_t lsn,
     ut_ad(new_used > used);
     if (new_used > srv_page_size)
       break;
-    block->page.access_time= (block->page.access_time & ~0U << 16) |
+    block->page.free_offset=
       ut_calc_align<uint16_t>(static_cast<uint16_t>(new_used), ALIGNMENT);
     goto append;
   }
@@ -2298,7 +2296,8 @@ bool recv_sys_t::add(map::iterator it, lsn_t start_lsn, lsn_t lsn,
     block= add_block();
     if (UNIV_UNLIKELY(!block))
       return true;
-    block->page.access_time= 1U << 16 |
+    block->page.used_records= 1;
+    block->page.free_offset=
       ut_calc_align<uint16_t>(static_cast<uint16_t>(size), ALIGNMENT);
     static_assert(ut_is_2pow(ALIGNMENT), "ALIGNMENT must be a power of 2");
     UT_LIST_ADD_FIRST(blocks, block);
@@ -2308,7 +2307,7 @@ bool recv_sys_t::add(map::iterator it, lsn_t start_lsn, lsn_t lsn,
   }
   else
   {
-    size_t free_offset= static_cast<uint16_t>(block->page.access_time);
+    size_t free_offset= block->page.free_offset;
     ut_ad(!ut_2pow_remainder(free_offset, ALIGNMENT));
     if (UNIV_UNLIKELY(!free_offset))
     {
@@ -2321,7 +2320,8 @@ bool recv_sys_t::add(map::iterator it, lsn_t start_lsn, lsn_t lsn,
     if (free_offset > srv_page_size)
       goto create_block;
 
-    block->page.access_time= ((block->page.access_time >> 16) + 1) << 16 |
+    block->page.used_records++;
+    block->page.free_offset=
       ut_calc_align<uint16_t>(static_cast<uint16_t>(free_offset), ALIGNMENT);
     MEM_MAKE_ADDRESSABLE(block->page.frame + free_offset - size, size);
     buf= block->page.frame + free_offset - size;