MDEV-15326 after-merge fixes

trx_t::is_recovered: Revert most of the changes that were made by the
merge of MDEV-15326 from 10.2. The trx_sys.rw_trx_hash and the recovery
of transactions at startup is quite different in 10.3.

trx_free_at_shutdown(): Avoid excessive mutex protection. Reading fields
that can only be modified by the current thread (owning the transaction)
can be done outside mutex.

trx_t::commit_state(): Restore a tighter assertion.

trx_rollback_recovered(): Clarify why there is no potential race condition
with other transactions.

lock_trx_release_locks(): Merge with trx_t::release_locks(),
and avoid holding lock_sys.mutex unnecessarily long.

rw_trx_hash_t::find(): Remove redundant code, and avoid starving the
committer by checking trx_t::state before trx_t::reference().

storage/innobase/include/lock0lock.h

@@ -485,7 +485,7 @@ lock_rec_unlock(
 
 /** Release the explicit locks of a committing transaction,
 and release possible other transactions waiting because of these locks. */
-void lock_trx_release_locks(trx_t* trx);
+void lock_release(trx_t* trx);
 
 /*********************************************************************//**
 Calculates the fold value of a page file address: used in inserting or

storage/innobase/include/trx0sys.h

@@ -628,12 +628,7 @@ public:
       The caller should already have handled trx_id==0 specially.
     */
     ut_ad(trx_id);
-    if (caller_trx && caller_trx->id == trx_id)
-    {
-      if (do_ref_count)
-        caller_trx->reference();
-      return caller_trx;
-    }
+    ut_ad(!caller_trx || caller_trx->id != trx_id || !do_ref_count);
 
     trx_t *trx= 0;
     LF_PINS *pins= caller_trx ? get_pins(caller_trx) : lf_hash_get_pins(&hash);
@@ -648,9 +643,25 @@ public:
       lf_hash_search_unpin(pins);
       if ((trx= element->trx)) {
         DBUG_ASSERT(trx_id == trx->id);
-        if (do_ref_count)
-          trx->reference();
         ut_d(validate_element(trx));
+        if (do_ref_count)
+        {
+          /*
+            We have an early state check here to avoid committer
+            starvation in a wait loop for transaction references,
+            when there's a stream of trx_sys.find() calls from other
+            threads. The trx->state may change to COMMITTED after
+            trx->mutex is released, and it will have to be rechecked
+            by the caller after reacquiring the mutex.
+          */
+          trx_mutex_enter(trx);
+          const trx_state_t state= trx->state;
+          trx_mutex_exit(trx);
+          if (state == TRX_STATE_COMMITTED_IN_MEMORY)
+            trx= NULL;
+          else
+            trx->reference();
+        }
       }
       mutex_exit(&element->mutex);
     }

storage/innobase/include/trx0trx.h

@@ -699,10 +699,9 @@ Normally, only the thread that is currently associated with a running
 transaction may access (read and modify) the trx object, and it may do
 so without holding any mutex. The following are exceptions to this:
 
-* trx_rollback_resurrected() may access resurrected (connectionless)
-transactions while the system is already processing new user
-transactions. The trx_sys.mutex and trx->is_recovered prevent
-a race condition between it and trx_commit().
+* trx_rollback_recovered() may access resurrected (connectionless)
+transactions (state == TRX_STATE_ACTIVE && is_recovered)
+while the system is already processing new user transactions (!is_recovered).
 
 * trx_print_low() may access transactions not associated with the current
 thread. The caller must be holding lock_sys.mutex.
@@ -839,10 +838,6 @@ public:
 
 	Transitions to COMMITTED are protected by trx_t::mutex. */
 	trx_state_t	state;
-	/** whether this is a recovered transaction that should be
-	rolled back by trx_rollback_or_clean_recovered().
-	Protected by trx_t::mutex for transactions that are in trx_sys. */
-	bool		is_recovered;
 
 	ReadView	read_view;	/*!< consistent read view used in the
 					transaction, or NULL if not yet set */
@@ -852,6 +847,15 @@ public:
 					by trx_t::mutex). */
 
 	/* These fields are not protected by any mutex. */
+
+	/** false=normal transaction, true=recovered (must be rolled back)
+	or disconnected transaction in XA PREPARE STATE.
+
+	This field is accessed by the thread that owns the transaction,
+	without holding any mutex.
+	There is only one foreign-thread access in trx_print_low()
+	and a possible race condition with trx_disconnect_prepared(). */
+	bool		is_recovered;
 	const char*	op_info;	/*!< English text describing the
 					current operation, or an empty
 					string */

storage/innobase/lock/lock0lock.cc

@@ -4274,23 +4274,17 @@ lock_check_dict_lock(
 }
 #endif /* UNIV_DEBUG */
 
-/*********************************************************************//**
-Releases transaction locks, and releases possible other transactions waiting
-because of these locks. */
-static
-void
-lock_release(
-/*=========*/
-	trx_t*	trx)	/*!< in/out: transaction */
+/** Release the explicit locks of a committing transaction,
+and release possible other transactions waiting because of these locks. */
+void lock_release(trx_t* trx)
 {
-	lock_t*		lock;
 	ulint		count = 0;
 	trx_id_t	max_trx_id = trx_sys.get_max_trx_id();
 
-	ut_ad(lock_mutex_own());
+	lock_mutex_enter();
 	ut_ad(!trx_mutex_own(trx));
 
-	for (lock = UT_LIST_GET_LAST(trx->lock.trx_locks);
+	for (lock_t* lock = UT_LIST_GET_LAST(trx->lock.trx_locks);
 	     lock != NULL;
 	     lock = UT_LIST_GET_LAST(trx->lock.trx_locks)) {
 
@@ -4330,6 +4324,8 @@ lock_release(
 
 		++count;
 	}
+
+	lock_mutex_exit();
 }
 
 /* True if a lock mode is S or X */
@@ -4784,7 +4780,7 @@ lock_table_queue_validate(
 	     lock = UT_LIST_GET_NEXT(un_member.tab_lock.locks, lock)) {
 
 		/* lock->trx->state cannot change from or to NOT_STARTED
-		while we are holding the trx_sys.mutex. It may change
+		while we are holding the lock_sys.mutex. It may change
 		from ACTIVE or PREPARED to PREPARED or COMMITTED. */
 		trx_mutex_enter(lock->trx);
 		check_trx_state(lock->trx);
@@ -5390,13 +5386,13 @@ lock_rec_convert_impl_to_expl_for_trx(
 	trx_t*			trx,	/*!< in/out: active transaction */
 	ulint			heap_no)/*!< in: rec heap number to lock */
 {
+	ut_ad(trx->is_referenced());
 	ut_ad(page_rec_is_leaf(rec));
 	ut_ad(!rec_is_metadata(rec, index));
 
 	DEBUG_SYNC_C("before_lock_rec_convert_impl_to_expl_for_trx");
 	lock_mutex_enter();
 	trx_mutex_enter(trx);
-	ut_ad(trx->is_referenced());
 	ut_ad(!trx_state_eq(trx, TRX_STATE_NOT_STARTED));
 
 	if (!trx_state_eq(trx, TRX_STATE_COMMITTED_IN_MEMORY)
@@ -6235,27 +6231,6 @@ lock_unlock_table_autoinc(
 	}
 }
 
-/** Release the explicit locks of a committing transaction,
-and release possible other transactions waiting because of these locks. */
-void lock_trx_release_locks(trx_t* trx)
-{
-	ut_ad(UT_LIST_GET_LEN(trx->lock.trx_locks));
-
-	lock_mutex_enter();
-	lock_release(trx);
-	trx->lock.n_rec_locks = 0;
-	/* We don't remove the locks one by one from the vector for
-	efficiency reasons. We simply reset it because we would have
-	released all the locks anyway. */
-
-	trx->lock.table_locks.clear();
-
-	ut_ad(UT_LIST_GET_LEN(trx->lock.trx_locks) == 0);
-	ut_ad(ib_vector_is_empty(trx->autoinc_locks));
-	lock_mutex_exit();
-	mem_heap_empty(trx->lock.lock_heap);
-}
-
 static inline dberr_t lock_trx_handle_wait_low(trx_t* trx)
 {
 	ut_ad(lock_mutex_own());

storage/innobase/trx/trx0purge.cc

@@ -252,7 +252,7 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 	/* After the purge thread has been given permission to exit,
 	we may roll back transactions (trx->undo_no==0)
 	in THD::cleanup() invoked from unlink_thd() in fast shutdown,
-	or in trx_rollback_resurrected() in slow shutdown.
+	or in trx_rollback_recovered() in slow shutdown.
 
 	Before any transaction-generating background threads or the
 	purge have been started, recv_recovery_rollback_active() can

storage/innobase/trx/trx0roll.cc

@@ -764,12 +764,8 @@ static my_bool trx_rollback_recovered_callback(rw_trx_hash_element_t *element,
   mutex_enter(&element->mutex);
   if (trx_t *trx= element->trx)
   {
-    /* The trx->is_recovered flag and trx->state are set
-    atomically under the protection of the trx->mutex in
-    trx_t::commit_state(). We do not want to accidentally clean up
-    a non-recovered transaction here. */
     mutex_enter(&trx->mutex);
-    if (trx->is_recovered && trx_state_eq(trx, TRX_STATE_ACTIVE))
+    if (trx_state_eq(trx, TRX_STATE_ACTIVE) && trx->is_recovered)
       trx_list->push_back(trx);
     mutex_exit(&trx->mutex);
   }
@@ -815,7 +811,8 @@ void trx_rollback_recovered(bool all)
 
     ut_ad(trx);
     ut_d(trx_mutex_enter(trx));
-    ut_ad(trx->is_recovered && trx_state_eq(trx, TRX_STATE_ACTIVE));
+    ut_ad(trx->is_recovered);
+    ut_ad(trx_state_eq(trx, TRX_STATE_ACTIVE));
     ut_d(trx_mutex_exit(trx));
 
     if (!srv_is_being_started && !srv_undo_sources && srv_fast_shutdown)
@@ -831,6 +828,22 @@ void trx_rollback_recovered(bool all)
         ut_ad(!srv_undo_sources);
         ut_ad(srv_fast_shutdown);
 discard:
+        /* Note: before kill_server() invoked innobase_end() via
+        unireg_end(), it invoked close_connections(), which should initiate
+        the rollback of any user transactions via THD::cleanup() in the
+        connection threads, and wait for all THD::cleanup() to complete.
+        So, no active user transactions should exist at this point.
+
+        srv_undo_sources=false was cleared early in innobase_end().
+
+        Generally, the server guarantees that all connections using
+        InnoDB must be disconnected by the time we are reaching this code,
+        be it during shutdown or UNINSTALL PLUGIN.
+
+        Because there is no possible race condition with any
+        concurrent user transaction, we do not have to invoke
+        trx->commit_state() or wait for !trx->is_referenced()
+        before trx_sys.deregister_rw(trx). */
         trx_sys.deregister_rw(trx);
         trx_free_at_shutdown(trx);
       }

storage/innobase/trx/trx0trx.cc

@@ -463,6 +463,9 @@ void trx_free(trx_t*& trx)
 /** Transition to committed state, to release implicit locks. */
 inline void trx_t::commit_state()
 {
+  ut_ad(state == TRX_STATE_PREPARED
+	|| state == TRX_STATE_PREPARED_RECOVERED
+	|| state == TRX_STATE_ACTIVE);
   /* This makes the transaction committed in memory and makes its
   changes to data visible to other transactions. NOTE that there is a
   small discrepancy from the strict formal visibility rules here: a
@@ -475,23 +478,9 @@ inline void trx_t::commit_state()
   makes modifications to the database, will get an lsn larger than the
   committing transaction T. In the case where the log flush fails, and
   T never gets committed, also T2 will never get committed. */
-  ut_ad(trx_mutex_own(this));
-  ut_ad(state != TRX_STATE_NOT_STARTED);
-  ut_ad(state != TRX_STATE_COMMITTED_IN_MEMORY
-	|| (is_recovered && !UT_LIST_GET_LEN(lock.trx_locks)));
+  trx_mutex_enter(this);
   state= TRX_STATE_COMMITTED_IN_MEMORY;
-
-  /* If the background thread trx_rollback_or_clean_recovered()
-  is still active then there is a chance that the rollback
-  thread may see this trx as COMMITTED_IN_MEMORY and goes ahead
-  to clean it up calling trx_cleanup_at_db_startup(). This can
-  happen in the case we are committing a trx here that is left
-  in PREPARED state during the crash. Note that commit of the
-  rollback of a PREPARED trx happens in the recovery thread
-  while the rollback of other transactions happen in the
-  background thread. To avoid this race we unconditionally unset
-  the is_recovered flag. */
-  is_recovered= false;
+  trx_mutex_exit(this);
   ut_ad(id || !is_referenced());
 }
 
@@ -499,18 +488,24 @@ inline void trx_t::commit_state()
 inline void trx_t::release_locks()
 {
   DBUG_ASSERT(state == TRX_STATE_COMMITTED_IN_MEMORY);
+  DBUG_ASSERT(!is_referenced());
 
   if (UT_LIST_GET_LEN(lock.trx_locks))
-    lock_trx_release_locks(this);
-  else
-    lock.table_locks.clear(); // Work around a bug
+  {
+    lock_release(this);
+    lock.n_rec_locks = 0;
+    ut_ad(UT_LIST_GET_LEN(lock.trx_locks) == 0);
+    ut_ad(ib_vector_is_empty(autoinc_locks));
+    mem_heap_empty(lock.lock_heap);
+  }
+
+  lock.table_locks.clear(); /* outside "if" to work around MDEV-20483 */
 }
 
 /** At shutdown, frees a transaction object. */
 void
 trx_free_at_shutdown(trx_t *trx)
 {
-	trx_mutex_enter(trx);
 	ut_ad(trx->is_recovered);
 	ut_a(trx_state_eq(trx, TRX_STATE_PREPARED)
 	     || trx_state_eq(trx, TRX_STATE_PREPARED_RECOVERED)
@@ -525,7 +520,6 @@ trx_free_at_shutdown(trx_t *trx)
 	ut_a(trx->magic_n == TRX_MAGIC_N);
 
 	trx->commit_state();
-	trx_mutex_exit(trx);
 	trx->release_locks();
 	trx_undo_free_at_shutdown(trx);
 
@@ -1369,9 +1363,7 @@ trx_commit_in_memory(
 		DBUG_LOG("trx", "Autocommit in memory: " << trx);
 		trx->state = TRX_STATE_NOT_STARTED;
 	} else {
-		trx_mutex_enter(trx);
 		trx->commit_state();
-		trx_mutex_exit(trx);
 
 		if (trx->id) {
 			trx_sys.deregister_rw(trx);
@@ -1397,6 +1389,7 @@ trx_commit_in_memory(
 		} else {
 			trx_update_mod_tables_timestamp(trx);
 			MONITOR_INC(MONITOR_TRX_RW_COMMIT);
+			trx->is_recovered = false;
 		}
 	}