MDEV-22697 - InnoDB: remove trx->no

trx->no is duplicated by trx->rw_trx_hash_element->no for no good reason.
The latter is preferred for performance reasons: allows to avoid taking
trx->rw_trx_hash_element->mutex when snapshotting.

storage/innobase/include/trx0sys.h

@@ -363,6 +363,13 @@ struct rw_trx_hash_element_t
 
 
   trx_id_t id; /* lf_hash_init() relies on this to be first in the struct */
+
+  /**
+    Transaction serialization number.
+
+    Assigned shortly before the transaction is moved to COMMITTED_IN_MEMORY
+    state. Initially set to TRX_ID_MAX.
+  */
   Atomic_counter<trx_id_t> no;
   trx_t *trx;
   ib_mutex_t mutex;
@@ -930,8 +937,7 @@ class trx_sys_t
   */
   void assign_new_trx_no(trx_t *trx)
   {
-    trx->no= get_new_trx_id_no_refresh();
-    trx->rw_trx_hash_element->no= trx->no;
+    trx->rw_trx_hash_element->no= get_new_trx_id_no_refresh();
     refresh_rw_trx_hash_version();
   }
 
@@ -955,7 +961,7 @@ class trx_sys_t
     @param[in,out] caller_trx used to get access to rw_trx_hash_pins
     @param[out]    ids        array to store registered transaction identifiers
     @param[out]    max_trx_id variable to store m_max_trx_id value
-    @param[out]    mix_trx_no variable to store min(trx->no) value
+    @param[out]    mix_trx_no variable to store min(no) value
   */
 
   void snapshot_ids(trx_t *caller_trx, trx_ids_t *ids, trx_id_t *max_trx_id,

storage/innobase/include/trx0trx.h

@@ -734,15 +734,6 @@ struct trx_t {
 
 	trx_id_t	id;		/*!< transaction id */
 
-	trx_id_t	no;		/*!< transaction serialization number:
-					max trx id shortly before the
-					transaction is moved to
-					COMMITTED_IN_MEMORY state.
-					Accessed exclusively by trx owner
-					thread. Should be removed in favour of
-					trx->rw_trx_hash_element->no.
-					Initially set to TRX_ID_MAX. */
-
 	/** State of the trx from the point of view of concurrency control
 	and the valid state transitions.
 

storage/innobase/trx/trx0purge.cc

@@ -210,7 +210,7 @@ void
 trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 {
 	DBUG_PRINT("trx", ("commit(" TRX_ID_FMT "," TRX_ID_FMT ")",
-			   trx->id, trx->no));
+			   trx->id, trx->rw_trx_hash_element->no));
 	ut_ad(undo == trx->rsegs.m_redo.undo
 	      || undo == trx->rsegs.m_redo.old_insert);
 	trx_rseg_t*	rseg		= trx->rsegs.m_redo.rseg;
@@ -301,7 +301,8 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 					     + TRX_UNDO_HISTORY_NODE), mtr);
 
 	mtr->write<8,mtr_t::MAYBE_NOP>(*undo_page,
-				       undo_header + TRX_UNDO_TRX_NO, trx->no);
+				       undo_header + TRX_UNDO_TRX_NO,
+				       trx->rw_trx_hash_element->no);
 	/* This is needed for upgrading old undo log pages from
 	before MariaDB 10.3.1. */
 	if (UNIV_UNLIKELY(!mach_read_from_2(undo_header
@@ -313,7 +314,8 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 	if (rseg->last_page_no == FIL_NULL) {
 		rseg->last_page_no = undo->hdr_page_no;
 		rseg->last_offset = undo->hdr_offset;
-		rseg->set_last_trx_no(trx->no, undo == trx->rsegs.m_redo.undo);
+		rseg->set_last_trx_no(trx->rw_trx_hash_element->no,
+				      undo == trx->rsegs.m_redo.undo);
 		rseg->needs_purge = true;
 	}
 

storage/innobase/trx/trx0trx.cc

@@ -104,8 +104,6 @@ trx_init(
 /*=====*/
 	trx_t*	trx)
 {
-	trx->no = TRX_ID_MAX;
-
 	trx->state = TRX_STATE_NOT_STARTED;
 
 	trx->is_recovered = false;
@@ -677,7 +675,6 @@ static void trx_resurrect(trx_undo_t *undo, trx_rseg_t *rseg,
   trx->state= state;
   ut_d(trx->start_file= __FILE__);
   ut_d(trx->start_line= __LINE__);
-  ut_ad(trx->no == TRX_ID_MAX);
 
   if (is_old_insert)
     trx->rsegs.m_redo.old_insert= undo;
@@ -966,11 +963,6 @@ trx_start_low(
 	trx->xid->null();
 #endif /* WITH_WSREP */
 
-	/* The initial value for trx->no: TRX_ID_MAX is used in
-	read_view_open_now: */
-
-	trx->no = TRX_ID_MAX;
-
 	ut_a(ib_vector_is_empty(trx->autoinc_locks));
 	ut_a(trx->lock.table_locks.empty());
 
@@ -1046,7 +1038,8 @@ trx_serialise(trx_t* trx)
 	already in the rollback segment. User threads only
 	produce events when a rollback segment is empty. */
 	if (rseg->last_page_no == FIL_NULL) {
-		purge_sys.purge_queue.push(TrxUndoRsegs(trx->no, *rseg));
+		purge_sys.purge_queue.push(TrxUndoRsegs(trx->rw_trx_hash_element->no,
+							*rseg));
 		mutex_exit(&purge_sys.pq_mutex);
 	}
 }