fifottl/limfifottl: stop() blocks forever while RW, drop() leaks the TTL fiber

[maksimuimin]

Environment: queue master (07fd732), Tarantool 3.8.0 (also the same code in 1.5.0). memtx.

Summary

fifottl (and therefore limfifottl) creates its stop channel as an unbuffered fiber.channel() (fifottl.lua#L200), but the TTL fiber only reads it in the box.info.ro == true branch (fifottl.lua#L171-L177). So:

1. tube.raw:stop() called while the instance is RW blocks the caller forever.
2. fifottl has no method.drop, so tube:drop() never calls stop() at all: the space is dropped and the TTL fiber leaks, one per dropped tube. After the instance becomes RO the leaked fibers sit in the sync_chan:get(0.1) poll loop forever.
3. utubettl uses fiber.channel(1) so its stop() returns, but the fiber keeps running in RW; utubettl's drop() then makes it crash on the dropped space (attempt to index field 'watch' (a nil value)).

Repro

local fiber = require('fiber')
box.cfg{}
local queue = require('queue')

-- 1. stop() blocks while RW
local tube = queue.create_tube('t', 'fifottl')
local done = false
fiber.create(function() tube.raw:stop(); done = true end)
fiber.sleep(1)
print('fifottl raw:stop() returned within 1s:', done)   -- false (utubettl: true)

-- 2. drop() leaks the fiber
local function ttl_fibers()
    local n = 0
    for _, f in pairs(fiber.info()) do if f.name == 'fifottl' then n = n + 1 end end
    return n
end
local before = ttl_fibers()
for i = 1, 3 do queue.create_tube('leak_' .. i, 'fifottl'):drop() end
fiber.sleep(0.5)
print('fifottl fibers leaked by 3 drops:', ttl_fibers() - before)   -- 3

Output on master:

fifottl raw:stop() returned within 1s:  false
fifottl fibers leaked by 3 drops:       3

Expected

stop() returns in both RW and RO mode and the fiber actually terminates; drop() stops the fiber before dropping the space (as utubettl tries to do), and does not leave the fiber running against a dropped space.

Suggested fix

Check a stop flag (or the channel with a zero timeout) in the RW branch of the loop as well, make stop() wait for the fiber to finish before clearing self.fiber, and add method.drop to fifottl that calls stop() first. fifottl should also use a buffered channel like utubettl so stop() cannot block the state machine fiber.