Re: [Tarantool-patches] [PATCH] txn: convert flags to explicit bitfield

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

On Thu, Feb 18, 2021 at 12:45:56AM +0300, Cyrill Gorcunov wrote:
> On Wed, Feb 17, 2021 at 10:15:30PM +0100, Vladislav Shpilevoy wrote:
> > Hi! Thanks for the patch!
> > 
> > > @@ -535,7 +535,7 @@ txn_complete_fail(struct txn *txn)
> > >  		txn_rollback_one_stmt(txn, stmt);
> > >  	if (txn->engine != NULL)
> > >  		engine_rollback(txn->engine, txn);
> > > -	if (txn_has_flag(txn, TXN_HAS_TRIGGERS))
> > > +	if (txn->flags & TXN_HAS_TRIGGERS)
> > 
> > 1. Did you do a self-review before sending this? Can you
> > tell yourself what is wrong here?
> 
> Yes I did, and I don't get what's wrong with this line? Previously
> we have `if (txn_has_flag(txn, TXN_HAS_TRIGGERS))` where txn_has_flag
> get expanded to
> 
> 	return (txn->flags & TXN_HAS_TRIGGERS) != 0;
> 
> so where is an error?
> 
> > 2. What was wrong with the idea of having the helpers
> > but operating on bitfields? And having has_flags() instead
> > of has_flag() to check presense of all specified flags +
> > clear_flags() to remove all specified flags.
> > 
> > I don't understand. I think we discussed it already 2 or 3
> > times and all seemed to agree?
> 
> The initial idea was to operate with flags directly, you pointed
> that lines of code as if (!(txn->flags & mask)) violates our
> code style. Then we thought of renaming the helpers to txn_x_flags
> to operate with multiple flags, which lead us to a vague moment --
> it is unclear how exactly txn_has_flag should operate: exact
> matching or any matching?
> 
> Finlly I simply moved back to direct access to the fields
> with explicit `!= 0` or `== 0` form.

Anyway, here is an another approach, sitting in
gorcunov/gh-5128-txn-flags-5 branch
---
From 0e4484213d53f58e34db16e3cb96c63805bdd5e7 Mon Sep 17 00:00:00 2001
From: Cyrill Gorcunov <gorcunov@gmail.com>
Date: Thu, 4 Feb 2021 14:19:12 +0300
Subject: [PATCH] txn: convert flags to explicit bitfield

Instead of shifting flags inside txn_x_flag() helpers lets
define precompiled value. This allows us to operate with
several flags at onces and shrink code a bit.

Closes #5128

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
---
 src/box/applier.cc             |  4 ++--
 src/box/memtx_engine.c         |  2 +-
 src/box/txn.c                  | 15 +++++++--------
 src/box/txn.h                  | 27 ++++++++++++++-------------
 src/box/txn_limbo.c            |  9 +++------
 test/unit/snap_quorum_delay.cc |  3 +--
 6 files changed, 28 insertions(+), 32 deletions(-)

diff --git a/src/box/applier.cc b/src/box/applier.cc
index 553db76fc..8d7ce5d99 100644
--- a/src/box/applier.cc
+++ b/src/box/applier.cc
@@ -237,7 +237,7 @@ apply_snapshot_row(struct xrow_header *row)
 	 * Do not wait for confirmation when fetching a snapshot.
 	 * Master only sends confirmed rows during join.
 	 */
-	txn_set_flag(txn, TXN_FORCE_ASYNC);
+	txn_set_flags(txn, TXN_FORCE_ASYNC);
 	if (txn_begin_stmt(txn, space) != 0)
 		goto rollback;
 	/* no access checks here - applier always works with admin privs */
@@ -308,7 +308,7 @@ apply_final_join_row(struct xrow_header *row)
 	 * Do not wait for confirmation while processing final
 	 * join rows. See apply_snapshot_row().
 	 */
-	txn_set_flag(txn, TXN_FORCE_ASYNC);
+	txn_set_flags(txn, TXN_FORCE_ASYNC);
 	if (apply_row(row) != 0) {
 		txn_rollback(txn);
 		fiber_gc();
diff --git a/src/box/memtx_engine.c b/src/box/memtx_engine.c
index 1174234d2..a4cd671f5 100644
--- a/src/box/memtx_engine.c
+++ b/src/box/memtx_engine.c
@@ -265,7 +265,7 @@ memtx_engine_recover_snapshot_row(struct memtx_engine *memtx,
 	 * Snapshot rows are confirmed by definition. They don't need to go to
 	 * the synchronous transactions limbo.
 	 */
-	txn_set_flag(txn, TXN_FORCE_ASYNC);
+	txn_set_flags(txn, TXN_FORCE_ASYNC);
 	rc = txn_commit(txn);
 	/*
 	 * Don't let gc pool grow too much. Yet to
diff --git a/src/box/txn.c b/src/box/txn.c
index a5edbfc60..71c89ce5f 100644
--- a/src/box/txn.c
+++ b/src/box/txn.c
@@ -282,7 +282,7 @@ txn_begin(void)
 	 * It's a responsibility of an engine to disable yields
 	 * if they are not supported.
 	 */
-	txn_set_flag(txn, TXN_CAN_YIELD);
+	txn_set_flags(txn, TXN_CAN_YIELD);
 	return txn;
 }
 
@@ -516,7 +516,7 @@ txn_free_or_wakeup(struct txn *txn)
 	if (txn->fiber == NULL)
 		txn_free(txn);
 	else {
-		txn_set_flag(txn, TXN_IS_DONE);
+		txn_set_flags(txn, TXN_IS_DONE);
 		if (txn->fiber != fiber())
 			/* Wake a waiting fiber up. */
 			fiber_wakeup(txn->fiber);
@@ -642,8 +642,7 @@ txn_journal_entry_new(struct txn *txn)
 	 */
 	if (!txn_has_flag(txn, TXN_FORCE_ASYNC)) {
 		if (is_sync) {
-			txn_set_flag(txn, TXN_WAIT_SYNC);
-			txn_set_flag(txn, TXN_WAIT_ACK);
+			txn_set_flags(txn, TXN_WAIT_SYNC | TXN_WAIT_ACK);
 		} else if (!txn_limbo_is_empty(&txn_limbo)) {
 			/*
 			 * There some sync entries on the
@@ -652,7 +651,7 @@ txn_journal_entry_new(struct txn *txn)
 			 * doesn't touch sync space (each sync txn
 			 * should be considered as a barrier).
 			 */
-			txn_set_flag(txn, TXN_WAIT_SYNC);
+			txn_set_flags(txn, TXN_WAIT_SYNC);
 		}
 	}
 
@@ -994,10 +993,10 @@ txn_can_yield(struct txn *txn, bool set)
 	assert(txn == in_txn());
 	bool could = txn_has_flag(txn, TXN_CAN_YIELD);
 	if (set && !could) {
-		txn_set_flag(txn, TXN_CAN_YIELD);
+		txn_set_flags(txn, TXN_CAN_YIELD);
 		trigger_clear(&txn->fiber_on_yield);
 	} else if (!set && could) {
-		txn_clear_flag(txn, TXN_CAN_YIELD);
+		txn_clear_flags(txn, TXN_CAN_YIELD);
 		trigger_create(&txn->fiber_on_yield, txn_on_yield, NULL, NULL);
 		trigger_add(&fiber()->on_yield, &txn->fiber_on_yield);
 	}
@@ -1229,6 +1228,6 @@ txn_on_yield(struct trigger *trigger, void *event)
 	assert(txn != NULL);
 	assert(!txn_has_flag(txn, TXN_CAN_YIELD));
 	txn_rollback_to_svp(txn, NULL);
-	txn_set_flag(txn, TXN_IS_ABORTED_BY_YIELD);
+	txn_set_flags(txn, TXN_IS_ABORTED_BY_YIELD);
 	return 0;
 }
diff --git a/src/box/txn.h b/src/box/txn.h
index fca9bc1d0..29fe6d5ce 100644
--- a/src/box/txn.h
+++ b/src/box/txn.h
@@ -56,25 +56,25 @@ struct Vdbe;
 
 enum txn_flag {
 	/** Transaction has been processed. */
-	TXN_IS_DONE,
+	TXN_IS_DONE = 0x1,
 	/**
 	 * Transaction has been aborted by fiber yield so
 	 * should be rolled back at commit.
 	 */
-	TXN_IS_ABORTED_BY_YIELD,
+	TXN_IS_ABORTED_BY_YIELD = 0x2,
 	/**
 	 * fiber_yield() is allowed inside the transaction.
 	 * See txn_can_yield() for more details.
 	 */
-	TXN_CAN_YIELD,
+	TXN_CAN_YIELD = 0x4,
 	/** on_commit and/or on_rollback list is not empty. */
-	TXN_HAS_TRIGGERS,
+	TXN_HAS_TRIGGERS = 0x8,
 	/**
 	 * Synchronous transaction touched sync spaces, or an
 	 * asynchronous transaction blocked by a sync one until it
 	 * is confirmed.
 	 */
-	TXN_WAIT_SYNC,
+	TXN_WAIT_SYNC = 0x10,
 	/**
 	 * Synchronous transaction 'waiting for ACKs' state before
 	 * commit. In this state it waits until it is replicated
@@ -82,14 +82,14 @@ enum txn_flag {
 	 * commit and returns success to a user.
 	 * TXN_WAIT_SYNC is always set, if TXN_WAIT_ACK is set.
 	 */
-	TXN_WAIT_ACK,
+	TXN_WAIT_ACK = 0x20,
 	/**
 	 * A transaction may be forced to be asynchronous, not
 	 * wait for any ACKs, and not depend on prepending sync
 	 * transactions. This happens in a few special cases. For
 	 * example, when applier receives snapshot from master.
 	 */
-	TXN_FORCE_ASYNC,
+	TXN_FORCE_ASYNC = 0x40,
 };
 
 enum {
@@ -396,19 +396,20 @@ struct txn {
 static inline bool
 txn_has_flag(struct txn *txn, enum txn_flag flag)
 {
-	return (txn->flags & (1 << flag)) != 0;
+	assert((flag & (flag - 1)) == 0);
+	return (txn->flags & flag) != 0;
 }
 
 static inline void
-txn_set_flag(struct txn *txn, enum txn_flag flag)
+txn_set_flags(struct txn *txn, unsigned int flags)
 {
-	txn->flags |= 1 << flag;
+	txn->flags |= flags;
 }
 
 static inline void
-txn_clear_flag(struct txn *txn, enum txn_flag flag)
+txn_clear_flags(struct txn *txn, unsigned int flags)
 {
-	txn->flags &= ~(1 << flag);
+	txn->flags &= ~flags;
 }
 
 /* Pointer to the current transaction (if any) */
@@ -491,7 +492,7 @@ txn_init_triggers(struct txn *txn)
 		rlist_create(&txn->on_commit);
 		rlist_create(&txn->on_rollback);
 		rlist_create(&txn->on_wal_write);
-		txn_set_flag(txn, TXN_HAS_TRIGGERS);
+		txn_set_flags(txn, TXN_HAS_TRIGGERS);
 	}
 }
 
diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
index 9c4c3cdf1..cf0ad9350 100644
--- a/src/box/txn_limbo.c
+++ b/src/box/txn_limbo.c
@@ -266,8 +266,7 @@ txn_limbo_wait_complete(struct txn_limbo *limbo, struct txn_limbo_entry *entry)
 					 in_queue, tmp) {
 		e->txn->signature = TXN_SIGNATURE_QUORUM_TIMEOUT;
 		txn_limbo_abort(limbo, e);
-		txn_clear_flag(e->txn, TXN_WAIT_SYNC);
-		txn_clear_flag(e->txn, TXN_WAIT_ACK);
+		txn_clear_flags(e->txn, TXN_WAIT_SYNC | TXN_WAIT_ACK);
 		txn_complete_fail(e->txn);
 		if (e == entry)
 			break;
@@ -399,8 +398,7 @@ txn_limbo_read_confirm(struct txn_limbo *limbo, int64_t lsn)
 		}
 		e->is_commit = true;
 		txn_limbo_remove(limbo, e);
-		txn_clear_flag(e->txn, TXN_WAIT_SYNC);
-		txn_clear_flag(e->txn, TXN_WAIT_ACK);
+		txn_clear_flags(e->txn, TXN_WAIT_SYNC | TXN_WAIT_ACK);
 		/*
 		 * If already written to WAL by now, finish tx processing.
 		 * Otherwise just clear the sync flags. Tx procesing will finish
@@ -456,8 +454,7 @@ txn_limbo_read_rollback(struct txn_limbo *limbo, int64_t lsn)
 		return;
 	rlist_foreach_entry_safe_reverse(e, &limbo->queue, in_queue, tmp) {
 		txn_limbo_abort(limbo, e);
-		txn_clear_flag(e->txn, TXN_WAIT_SYNC);
-		txn_clear_flag(e->txn, TXN_WAIT_ACK);
+		txn_clear_flags(e->txn, TXN_WAIT_SYNC | TXN_WAIT_ACK);
 		if (e->txn->signature >= 0) {
 			/* Rollback the transaction. */
 			e->txn->signature = TXN_SIGNATURE_SYNC_ROLLBACK;
diff --git a/test/unit/snap_quorum_delay.cc b/test/unit/snap_quorum_delay.cc
index b9d4cc6c4..803bbbea8 100644
--- a/test/unit/snap_quorum_delay.cc
+++ b/test/unit/snap_quorum_delay.cc
@@ -98,8 +98,7 @@ txn_process_func(va_list ap)
 	struct txn *txn = txn_begin();
 	txn->fiber = fiber();
 	/* Simulate a sync transaction. */
-	txn_set_flag(txn, TXN_WAIT_SYNC);
-	txn_set_flag(txn, TXN_WAIT_ACK);
+	txn_set_flags(txn, TXN_WAIT_SYNC | TXN_WAIT_ACK);
 	/*
 	 * The true way to push the transaction to limbo is to call
 	 * txn_commit() for sync transaction. But, if txn_commit()
-- 
2.29.2