MDEV-15326/MDEV-16136 dead code removal

Revert part of fa2a74e0.

trx_reference(): Remove, and merge the relevant part to the only caller
trx_rw_is_active(). If the statements trx = NULL; were ever executed,
the function would have dereferenced a NULL pointer and crashed in
trx_mutex_exit(trx). Hence, those statements must have been unreachable,
and they can be replaced with debug assertions.

trx_rw_is_active(): Avoid unnecessary acquisition and release of trx->mutex
when do_ref_count=false.

lock_trx_release_locks(): Do not reset trx->id=0. Had the statement been
necessary, we would have experienced crashes in trx_reference().

storage/innobase/include/trx0sys.ic

@@ -364,8 +364,13 @@ trx_rw_is_active(
 
 	trx_t* trx = trx_rw_is_active_low(trx_id, corrupt);
 
-	if (trx) {
-		trx = trx_reference(do_ref_count ? trx_id : 0, trx);
+	if (trx && do_ref_count) {
+		trx_mutex_enter(trx);
+		ut_ad(!trx_state_eq(trx, TRX_STATE_COMMITTED_IN_MEMORY));
+		ut_ad(trx->id == trx_id);
+		ut_ad(trx->n_ref >= 0);
+		++trx->n_ref;
+		trx_mutex_exit(trx);
 	}
 
 	trx_sys_mutex_exit();

storage/innobase/include/trx0trx.h

@@ -1270,32 +1270,6 @@ struct commit_node_t{
 	mutex_exit(&t->mutex);			\
 } while (0)
 
-/**
-Increase the reference count. If the transaction is in state
-TRX_STATE_COMMITTED_IN_MEMORY then the transaction is considered
-committed and the reference count is not incremented.
-@param id the transaction ID; 0 if not to increment the reference count
-@param trx Transaction that is being referenced
-@return trx
-@retval	NULL	if the transaction is no longer active */
-inline trx_t* trx_reference(trx_id_t id, trx_t* trx)
-{
-	trx_mutex_enter(trx);
-
-	if (trx_state_eq(trx, TRX_STATE_COMMITTED_IN_MEMORY)) {
-		trx = NULL;
-	} else if (!id) {
-	} else if (trx->id != id) {
-		trx = NULL;
-	} else {
-		ut_ad(trx->n_ref >= 0);
-		++trx->n_ref;
-	}
-
-	trx_mutex_exit(trx);
-	return(trx);
-}
-
 #include "trx0trx.ic"
 
 #endif

storage/innobase/lock/lock0lock.cc

@@ -6647,8 +6647,6 @@ lock_trx_release_locks(
 	the is_recovered flag. */
 
 	trx->is_recovered = false;
-	/* Ensure that trx_reference() will not find this transaction. */
-	trx->id = 0;
 
 	trx_mutex_exit(trx);
 

storage/innobase/trx/trx0trx.cc

@@ -557,6 +557,7 @@ trx_free_prepared(
 	transaction was never committed and therefore lock_trx_release()
 	was not called. */
 	trx->lock.table_locks.clear();
+	trx->id = 0;
 
 	trx_free(trx);
 }
@@ -1660,10 +1661,8 @@ trx_commit_in_memory(
 			trx_erase_lists(trx, serialised);
 		}
 
-		/* trx->id will be cleared in lock_trx_release_locks(trx). */
-		ut_ad(trx->read_only || !trx->rsegs.m_redo.rseg || trx->id);
 		lock_trx_release_locks(trx);
-		ut_ad(trx->id == 0);
+		ut_ad(trx->read_only || !trx->rsegs.m_redo.rseg || trx->id);
 
 		/* Remove the transaction from the list of active
 		transactions now that it no longer holds any user locks. */
@@ -1682,6 +1681,8 @@ trx_commit_in_memory(
 		} else {
 			MONITOR_INC(MONITOR_TRX_RW_COMMIT);
 		}
+
+		trx->id = 0;
 	}
 
 	ut_ad(!trx->rsegs.m_redo.update_undo);