[Tarantool-patches] [PATCH v2 10/16] raft: make worker non-cancellable during WAL write

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

This is a general practice throughout the code. If a fiber is not
cancellable, it always means a system fiber which can't be woken
up or canceled until its work done. It is used in gc (about
xlogs), recovery, vinyl, WAL, at least.

Before raft used flag raft.is_write_in_progress. But it won't
work soon, because the worker fiber will move to box/raft.c,
where it would be incorrect to rely on deeply internal parts of
struct raft, such as is_write_in_progress.

Hence, this patch makes raft use a more traditional way of
spurious wakeup avoidance.

Part of #5303
---
 src/box/raft.c    |  9 ++++++++-
 src/box/raftlib.c | 10 ++++++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/box/raft.c b/src/box/raft.c
index 2c9ed11b6..8a034687b 100644
--- a/src/box/raft.c
+++ b/src/box/raft.c
@@ -213,7 +213,14 @@ box_raft_write(struct raft *raft, const struct raft_msg *msg)
 	journal_entry_create(entry, 1, xrow_approx_len(&row), box_raft_write_cb,
 			     fiber());
 
-	if (journal_write(entry) != 0 || entry->res < 0) {
+	/*
+	 * A non-cancelable fiber is considered non-wake-able, generally. Raft
+	 * follows this pattern of 'protection'.
+	 */
+	bool cancellable = fiber_set_cancellable(false);
+	bool ok = (journal_write(entry) == 0 && entry->res >= 0);
+	fiber_set_cancellable(cancellable);
+	if (!ok) {
 		diag_set(ClientError, ER_WAL_IO);
 		diag_log();
 		goto fail;
diff --git a/src/box/raftlib.c b/src/box/raftlib.c
index d28e51871..a1fca34cd 100644
--- a/src/box/raftlib.c
+++ b/src/box/raftlib.c
@@ -986,11 +986,13 @@ raft_worker_wakeup(struct raft *raft)
 		fiber_set_joinable(raft->worker, true);
 	}
 	/*
-	 * Don't wake the fiber if it writes something. Otherwise it would be a
-	 * spurious wakeup breaking the WAL write not adapted to this. Also
-	 * don't wakeup the current fiber - it leads to undefined behaviour.
+	 * Don't wake the fiber if it writes something (not cancellable).
+	 * Otherwise it would be a spurious wakeup breaking the WAL write not
+	 * adapted to this. Also don't wakeup the current fiber - it leads to
+	 * undefined behaviour.
 	 */
-	if (!raft->is_write_in_progress && fiber() != raft->worker)
+	if ((raft->worker->flags & FIBER_IS_CANCELLABLE) != 0 &&
+	    fiber() != raft->worker)
 		fiber_wakeup(raft->worker);
 }
 
-- 
2.24.3 (Apple Git-128)