Hi! Thanks for changes. See two comments below.

From: Sergey Ostanevich <sergos@tarantool.org>

Hi!

Thanks to Oleg Babin's comment I found there's no need to update any lua
interfaces, since the reason was in C implementation. Also, there is one
place the change is played, so after I fixed it I got complete testing 
pass. 
Force-pushed branch, v2 patch attached.



Before this patch fiber.cond():wait() just returns for cancelled
fiber. In contrast fiber.channel():get() threw "fiber is
canceled" error.
This patch unify behaviour of channels and condvars and also fixes
related net.box module problem - it was impossible to interrupt
net.box call with fiber.cancel because it used fiber.cond under
the hood. Test cases for both bugs are added.

Closes #4834
Closes #5013

Co-authored-by: Oleg Babin <olegrok@tarantool.org>

@TarantoolBot document
Title: fiber.cond():wait() throws if fiber is cancelled

Currently fiber.cond():wait() throws an error if waiting fiber is
cancelled like in case with fiber.channel():get().
---

Github: https://gitlab.com/tarantool/tarantool/-/commits/sergos/gh-5013-fiber-cond                                                                                                                          
Issue: https://github.com/tarantool/tarantool/issues/5013                                                                                                                                                   

 src/box/box.cc                                |  6 +-
 src/lib/core/fiber_cond.c                     |  1 +
 test/app-tap/gh-5013-fiber-cancel.test.lua    | 23 +++++++
 test/box/net.box_fiber_cancel_gh-4834.result  | 65 +++++++++++++++++++
 .../box/net.box_fiber_cancel_gh-4834.test.lua | 29 +++++++++
 5 files changed, 120 insertions(+), 4 deletions(-)
 create mode 100755 test/app-tap/gh-5013-fiber-cancel.test.lua
 create mode 100644 test/box/net.box_fiber_cancel_gh-4834.result
 create mode 100644 test/box/net.box_fiber_cancel_gh-4834.test.lua

diff --git a/src/box/box.cc b/src/box/box.cc
index 18568df3b..bfa1051f9 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -305,10 +305,8 @@ box_wait_ro(bool ro, double timeout)
 {
 	double deadline = ev_monotonic_now(loop()) + timeout;
 	while (is_box_configured == false || box_is_ro() != ro) {
-		if (fiber_cond_wait_deadline(&ro_cond, deadline) != 0)
-			return -1;
-		if (fiber_is_cancelled()) {
-			diag_set(FiberIsCancelled);
+		if (fiber_cond_wait_deadline(&ro_cond, deadline) != 0) {
+                        if (fiber_is_cancelled()) diag_set(FiberIsCancelled);

Here you use spaces instead of tabs.

 			return -1;
 		}
 	}
diff --git a/src/lib/core/fiber_cond.c b/src/lib/core/fiber_cond.c
index 904a350d9..b0645069e 100644
--- a/src/lib/core/fiber_cond.c
+++ b/src/lib/core/fiber_cond.c
@@ -108,6 +108,7 @@ fiber_cond_wait_timeout(struct fiber_cond *c, double timeout)
 		diag_set(TimedOut);
 		return -1;
 	}
+	if (fiber_is_cancelled()) return -1;

It's qute strange to return -1 here but don't set a reason to diagnostic area. Look how it is done for channels

(https://github.com/tarantool/tarantool/blob/42c64d06d5d1a3ec937b3c596af083a672a68ad8/src/lib/core/fiber_channel.c#L180).

There is some inconsistency without it.

I've looked a bit deeper at the failure I reported before. Seems the problem is in "cbus_unpair" function.

The problem appears only if FiberIsCancelled is setted to diag area in "fiber_cond_wait" function.

This is where my expertise ends, as I'm not familiar with "cbus". However I have some minds how it could be eliminated.

Let's declare cbus_unpair fiber as is not cancellable and stop report is_cancellable flag for non-cancellable fibers. See some PoC below:

diff --git a/src/lib/core/cbus.c b/src/lib/core/cbus.c
index 5d91fb948..4167c756a 100644
--- a/src/lib/core/cbus.c
+++ b/src/lib/core/cbus.c
@@ -630,6 +630,7 @@ cbus_unpair(struct cpipe *dest_pipe, struct cpipe *src_pipe,
     msg.unpair_arg = unpair_arg;
     msg.src_pipe = src_pipe;
     msg.complete = false;
+    fiber_set_cancellable(false);
     fiber_cond_create(&msg.cond);
 
     cpipe_push(dest_pipe, &msg.cmsg);
@@ -643,6 +644,7 @@ cbus_unpair(struct cpipe *dest_pipe, struct cpipe *src_pipe,
         fiber_cond_wait(&msg.cond);
     }
 
+    fiber_set_cancellable(true);
     cpipe_destroy(dest_pipe);
 }
 
diff --git a/src/lib/core/fiber.c b/src/lib/core/fiber.c
index 483ae3ce1..8100c9da6 100644
--- a/src/lib/core/fiber.c
+++ b/src/lib/core/fiber.c
@@ -553,6 +553,9 @@ fiber_set_cancellable(bool yesno)
 bool
 fiber_is_cancelled(void)
 {
+    if ((fiber()->flags & FIBER_IS_CANCELLABLE) == 0) {
+        return false;
+    }
     return fiber()->flags & FIBER_IS_CANCELLED;
 }

To be honest I've not checked such change carefully and also have segfault at replication/gc.test.lua for "memtx" engine.

Finally, feel free to ignore this comment I hope Vlad or Sasha can give you more accurate and correct advices.