log_ratelimit: create flush fiber during log

In the scope of #606 issue we're going to allow
disabling the log ratelimiter by setting `LOG_RATELIMIT_INTERVAL`
constant to 0.

Users, who don't use ratelimiter, should not pay for flush fiber, so for
that its creation is moved to the log function instead of creation of
the limiter.

Closes #606

NO_DOC=internal

Author: Serpentian

test/unit-luatest/log_ratelimit_test.lua

@@ -1,4 +1,5 @@
 local t = require('luatest')
+local fiber = require('fiber')
 local server = require('test.luatest_helpers.server')
 local verror = require('vshard.error')
 local vconsts = require('vshard.consts')
@@ -98,3 +99,36 @@ test_group.test_disable_ratelimit = function()
 
     vconsts.LOG_RATELIMIT_INTERVAL = old_interval
 end
+
+test_group.test_fiber_not_started_when_disabled = function()
+    -- Kill flush fiber from other tests before starting the test.
+    local function find_fiber_by_name(name)
+        local fibs = fiber.info()
+        for id, f in pairs(fibs) do
+            if f.name == name then
+                return fiber.find(id)
+            end
+        end
+    end
+
+    local flush_fiber_name = 'vshard.ratelimit_flush'
+    local flush_fiber = find_fiber_by_name(flush_fiber_name)
+    -- If all other tests are skipped, then it may happen, that the flush fiber
+    -- doesn't exist at all.
+    if flush_fiber then
+        flush_fiber:cancel()
+        t.helpers.retrying({}, function()
+            t.assert_not(find_fiber_by_name(flush_fiber_name))
+        end)
+    end
+    vratelimit.internal.flush_fiber = nil
+
+    -- Test, that the fiber is not started, if limiter is disabled.
+    local old_interval = vconsts.LOG_RATELIMIT_INTERVAL
+    vconsts.LOG_RATELIMIT_INTERVAL = 0
+    local limiter = vratelimit.create({name = 'test_disable_ratelimit'})
+    local err = verror.box(box.error.new(box.error.NO_CONNECTION))
+    limiter:log_error(err)
+    t.assert_not(find_fiber_by_name(flush_fiber_name))
+    vconsts.LOG_RATELIMIT_INTERVAL = old_interval
+end

vshard/log_ratelimit.lua

@@ -124,6 +124,10 @@ local function ratelimit_log_template(log_lvl)
                 -- entries are not printed until it's flushed.
                 ratelimit_add_entry(limiter, signed_entry)
                 level = log_lvl
+                if M.flush_fiber == nil then
+                    M.flush_fiber = util.reloadable_fiber_new(
+                        'vshard.ratelimit_flush', M, 'ratelimit_flush_f')
+                end
             else
                 -- The entry should be suppressed, it's in the limiter already.
                 ratelimit_suppress_entry(limiter, signed_entry)
@@ -172,10 +176,6 @@ local function ratelimit_create(cfg)
     }
     setmetatable(ratelimit, ratelimit_mt)
     M.limiters[ratelimit.name] = ratelimit
-    if M.flush_fiber == nil then
-        M.flush_fiber = util.reloadable_fiber_new(
-            'vshard.ratelimit_flush', M, 'ratelimit_flush_f')
-    end
     return ratelimit
 end