Add the "lazy start" technique to initialization stage

Previously, if an instance was started in read_only mode,
it could throw an error when initializing the queue module,
like this: "Can't modify data because this instance is in
read-only mode".
To avoid such behavior the "lazy start" technique has been added:
The queue module is loaded immediately if the instance was
configured with read_only = false. Otherwise, a start is delayed
until the instance will be configured with read_only = false.

Closes #122

Author: LeonidVas

queue/init.lua

@@ -26,27 +26,65 @@ queue = setmetatable({
     register_driver = register_driver,
 }, { __index = function() print(debug.traceback()) error("Please run box.cfg{} first") end })
 
-local function queue_init()
-    if rawget(box, 'space') == nil then
-        local orig_cfg = box.cfg
-        box.cfg = function(...)
-            local result = { orig_cfg(...) }
-
-            local abstract = require 'queue.abstract'
-            for name, val in pairs(abstract) do
-                rawset(queue, name, val)
-            end
-            abstract.driver = queue.driver
-            setmetatable(queue, getmetatable(abstract))
-            queue.start()
-
-            return unpack(result)
-        end
+-- Used to store the original methods
+local orig_cfg = nil
+local orig_call = nil
+
+local wrapper_impl
+
+local function cfg_wrapper(...)
+    box.cfg = orig_cfg
+    return wrapper_impl(...)
+end
+
+local function cfg_call_wrapper(cfg, ...)
+    local cfg_mt = getmetatable(box.cfg)
+    cfg_mt.__call = orig_call
+    return wrapper_impl(...)
+end
+
+local function wrap_cfg_call()
+    local cfg_mt = getmetatable(box.cfg)
+    orig_call = cfg_mt.__call
+    cfg_mt.__call = cfg_call_wrapper
+end
+
+function wrapper_impl(...)
+    local result = { box.cfg(...) }
+    if box.cfg.read_only then
+        -- Delay a start until the box will be configured
+        -- with read_only = false
+        wrap_cfg_call()
     else
-        queue = require 'queue.abstract'
+        local abstract = require 'queue.abstract'
+        for name, val in pairs(abstract) do
+            rawset(queue, name, val)
+        end
+        abstract.driver = queue.driver
+        setmetatable(queue, getmetatable(abstract))
+        queue.start()
+    end
+    return unpack(result)
+end
+
+--- Implementation of the “lazy start” procedure.
+-- The queue module is loaded immediately if the instance was
+-- configured with read_only = false. Otherwise, a start is
+-- delayed until the instance will be configured with read_only = false.
+local function queue_init()
+    if rawget(box, 'space') ~= nil and not box.cfg.read_only then
+        -- The box was configured with read_only = false
+        queue = require('queue.abstract')
         queue.register_driver = register_driver
         queue.driver = core_drivers
         queue.start()
+    elseif rawget(box, 'space') == nil then
+        -- The box is not configured yet
+        orig_cfg = box.cfg
+        box.cfg = cfg_wrapper
+    else
+        -- The box was configured with read_only = true
+        wrap_cfg_call()
     end
 end
 

t/150-lazy-start.t

@@ -0,0 +1,34 @@
+#!/usr/bin/env tarantool
+local tap = require('tap')
+local tnt = require('t.tnt')
+
+local test = tap.test('test driver register')
+test:plan(3)
+
+local function check_lazy_start()
+    -- Needed for bootstrap
+    tnt.cfg{}
+
+    tnt.cfg{read_only = true}
+    local queue = require('queue')
+
+    local res, err = pcall(function() queue.stats() end)
+    local check = not res and
+        string.match(err, 'Please run box.cfg{} first') ~= nil
+    test:ok(check, 'check queue delayed start')
+
+    tnt.cfg({read_only = true})
+    res, err = pcall(function() queue.stats() end)
+    check = not res and
+        string.match(err, 'Please run box.cfg{} first') ~= nil
+    test:ok(check, 'check box reconfiguration with read_only = true')
+
+    tnt.cfg({read_only = false})
+    res = pcall(function() queue.stats() end)
+    test:ok(res, 'queue has been started')
+end
+
+check_lazy_start()
+
+tnt.finish()
+os.exit(test:check() and 0 or 1)