lua/fiber: add compat option for default slice

The new compat option 'fiber_slice_default' is added to control
the default value of the max fiber slice. The old default is no limit
(both warning and error slice equals TIMEOUT_INFINITY). The new default
is {warn = 0.5, err = 1.0}.

Follow-up #6085

NO_DOC=tarantool/doc#3057
NO_CHANGELOG=unreleased

changelogs/unreleased/gh-6085-log-and-limit-iteration.md

 ## feature/core
 
-* **[Breaking change]** Introduced the mechanism for catching fibers running
-  without yielding for too long. Now box operations, such as `select` and
-  `replace`, will throw an error if the fiber execution time since yield
-  exceeds the limit. The limit can also be checked from the application code
-  with `fiber.check_slice()`. The default limit is one second; you can
-  overwrite it with `fiber.set_slice()` (gh-6085).
-
-----
-
-Breaking change: long-running non-yielding fibers can be interrupted because of
-the fiber execution time slice limit.
+* Introduced the mechanism for catching fibers running without yielding for too
+  long. Now box operations, such as `select` and `replace`, will throw an error
+  if the fiber execution time since yield exceeds the limit. The limit can also
+  be checked from the application code with `fiber.check_slice()`. The default
+  limit is controlled by the new `compat` option `fiber_slice_default`. The old
+  default is no limit. The new default is one second. You can overwrite it with
+  `fiber.set_slice()` (gh-6085).

src/lua/fiber.lua

 -- fiber.lua (internal file)
 
+local compat = require('compat')
 local fiber = require('fiber')
 local ffi = require('ffi')
 ffi.cdef[[
@@ -14,6 +15,34 @@ fiber_clock64(void);
 ]]
 local C = ffi.C
 
+local TIMEOUT_INFINITY = 100 * 365 * 86400
+
+local FIBER_SLICE_DEFAULT_BRIEF = [[
+Sets the default value for the max fiber slice. The old value is infinity
+(no warnings or errors). The new value is {warn = 0.5, err = 1.0}.
+
+https://tarantool.io/compat/fiber_slice_default
+]]
+
+compat.add_option({
+    name = 'fiber_slice_default',
+    default = 'old',
+    obsolete = nil,
+    brief = FIBER_SLICE_DEFAULT_BRIEF,
+    action = function(is_new)
+        local slice = {}
+        if is_new then
+            slice.warn = 0.5
+            slice.err = 1.0
+        else
+            slice.warn = TIMEOUT_INFINITY
+            slice.err = TIMEOUT_INFINITY
+        end
+        fiber.set_max_slice(slice)
+    end,
+    run_action_now = true,
+})
+
 local function fiber_time()
     return tonumber(C.fiber_time())
 end

test/app-luatest/fiber_slice_test.lua

@@ -171,3 +171,26 @@ g.test_info = function()
         check_default(slice, info)
     end
 end
+
+g.before_test('test_compat', function()
+    g.server = server:new({alias = 'default'})
+    g.server:start()
+end)
+
+g.after_test('test_compat', function()
+    g.server:drop()
+end)
+
+g.test_compat = function()
+    g.server:exec(function()
+        local compat = require('compat')
+        local fiber = require('fiber')
+        t.assert_equals(compat.fiber_slice_default.current, 'default')
+        t.assert_equals(compat.fiber_slice_default.default, 'old')
+        t.assert_equals(fiber.self():info().max_slice, nil)
+        compat.fiber_slice_default = 'new'
+        t.assert_equals(fiber.self():info().max_slice, {warn = 0.5, err = 1.0})
+        compat.fiber_slice_default = 'old'
+        t.assert_equals(fiber.self():info().max_slice, nil)
+    end)
+end