perf(sync): reduce memory usage

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

sync.lua

@@ -7,8 +7,7 @@ local cond_methods = {}
 
 -- waiting to be awakened
 function cond_methods:wait(timeout)
-    local mt = getmetatable(self)
-    local watchers = mt.watchers
+    local watchers = self.watchers
 
     local w = eco.watcher(eco.ASYNC)
 
@@ -19,8 +18,7 @@ end
 
 -- wakes one coroutine waiting on the cond, if there is any.
 function cond_methods:signal()
-    local mt = getmetatable(self)
-    local watchers = mt.watchers
+    local watchers = self.watchers
 
     if #watchers > 0 then
         watchers[1]:send()
@@ -30,66 +28,63 @@ end
 
 -- wakes all coroutines waiting on the cond
 function cond_methods:broadcast()
-    local mt = getmetatable(self)
-
-    for _, w in ipairs(mt.watchers) do
+    for _, w in ipairs(self.watchers) do
         w:send()
     end
 
-    mt.watchers = {}
+    self.watchers = {}
 end
 
+local cond_mt = { __index = cond_methods }
+
 -- implements a condition variable, a rendezvous point for coroutines waiting for or announcing the occurrence of an event.
 function M.cond()
-    return setmetatable({}, {
-        watchers = {},
-        __index = cond_methods
-    })
+    return setmetatable({ watchers = {} }, cond_mt)
 end
 
 local waitgroup_methods = {}
 
 function waitgroup_methods:add(delta)
-   local mt = getmetatable(self)
-   mt.counter = mt.counter + delta
+   self.counter = self.counter + delta
 end
 
 function waitgroup_methods:done()
-    local mt = getmetatable(self)
+    local counter = self.counter
 
-    mt.counter = mt.counter - 1
+    counter = counter - 1
 
-    if mt.counter < 0 then
+    if counter < 0 then
         error('negative wait group counter')
     end
 
-    if mt.counter == 0 then
-        mt.cond:broadcast()
+    self.counter = counter
+
+    if counter == 0 then
+        self.cond:broadcast()
     end
 end
 
 function waitgroup_methods:wait(timeout)
-    local mt = getmetatable(self)
-
-    if mt.counter == 0 then
+    if self.counter == 0 then
         return true
     end
 
-    return mt.cond:wait(timeout)
+    return self.cond:wait(timeout)
 end
 
+local waitgroup_mt = { __index = waitgroup_methods }
+
 --[[
     A waitgroup waits for a collection of coroutines to finish.
     One coroutine calls add to set the number of coroutines to wait for.
     Then each of the coroutines runs and calls done when finished.
     At the same time, wait can be used to block until all coroutines have finished.
 --]]
 function M.waitgroup()
-    return setmetatable({}, {
+    return setmetatable({
         counter = 0,
-        cond = M.cond(),
-        __index = waitgroup_methods
-    })
+        cond = M.cond()
+    }, waitgroup_mt)
 end
 
 return M