Programming in Lua(Thrid Edition)笔记---9 Coroutines

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9 Coroutines

  • 协程和线程的相同点：
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    It is a line of execution, with its own stack, its own local variables, and its own instruction pointer; but it shares global variables and mostly anything else with other coroutines.

不同点：线程是并行的，而协程是合作式的，任何时候只会有一个协程运行，并且只有在其明确要求挂起时才会挂起

  • 协程可用yield()暂时挂起，等待后续resume
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    co = coroutine.create(function ()
    	for i = 1, 10 do
    		print("co", i)
    		coroutine.yield()
    	end
    end)
    coroutine.resume(co) --> co	1
    coroutine.resume(co) --> co	2
    ...
    coroutine.resume(co) --> co	10
    coroutine.resume(co) -- print nothig
    print(coroutine.resume(co)) --> false cannot resume dead coroutine

协程挂起期间的活动相当于发生在调用yield()之中，resume后yield()返回并继续运行

  • 协程运行在保护模式下，如果有错误发生Lua不会展示错误信息，而是返回给resume()

  • 协程A中resume协程B，则协程A处于normal状态

  • resume-yeild对可以交换数据，resume()返回传给yield()的任何参数，yield()返回传递给resume()的任何额外参数，协程结束时任何主函数的返回值将返回给resume()

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    co = coroutine.create(function (a, b)
    	coroutine.yield(a + b, a - b)
    end)
    print(coroutine.resume(co, 20, 10)) --> true	30	10
    co = coroutine.create(function (x)
    	print("co1", x)
    	print("co2", coroutine.yield())
    end)
    coroutine.resume(co, "hi") --> co1	hi
    coroutine.resume(co, 4, 5) --> co2	4	5
    co = coroutine.create(function ()
    	return 6, 7
    end)
    print(coroutine.resume(co)) --> true	6	7

  • pipes and filters

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    function receive(prod)
    	local status, value = coroutine.resume(prod)
    	return value
    end
    function send(x)
    	coroutine.yield(x)
    end
    function producer()
    	return coroutine.create(function ()
    		while true do
    			local x = io.read()   -- produce new value
    			send(x)
    		end
    	end)
    end
    function filter(prod)
    	return coroutine.create(function ()
    		for line = 1, math.huge do
    			local x =receive(prod) -- get new value
    			x = string.format("%5d %s", line, x)
    			send(x)                -- send it to consumer
    		end
    	end)
    end
    function consumer(prod)
    	while true do
    		local x = receive(prod)    -- get new value
    		io.write(x, "\n")          -- consume new value
    	end
    end
    consumer(filter(producer()))

该例基于consumer-driven

  • 协程用作迭代器
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    function permgen(a, n)
    	n = n or #a    -- default for 'n' is size of 'a'
    	if n <= 1 then -- nothing to change?
    		coroutine.yield(a)
    	else
    		for i = 1, n do
    			-- put i-th element as the last one
    			a[n], a[i] = a[i], a[n]
    			-- generate all permutations of the other elements
    			permgen(a, n - 1)
    			-- restore i-th element
    			a[n], a[i] = a[i], a[n]
    		end
    	end
    end
    function printResult(a)
    	for i = 1, #a do
    		io.write(a[i], " ")
    	end
    	io.write("\n")
    end
    function permutations(a)
    	local co = coroutine.create(function () permgen(a) end)
    	return function ()  -- iterator
    		local code, res = coroutine.resume(co)
    		return res
    	end
    end
    for p in permutations{"a", "b", "c"} do
    	printResult(p)
    end

permutations()的模式在Lua中十分常用，所以用coroutine.wrap()简化，其返回一个可以resume协程的函数，遇错误时提出，不返回错误码，但是不能检查协程的状态和运行时错误

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function permutations(a)
	return coroutine.wrap(function () permgen(a) end)
end

  • 用协程实现非阻塞
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    local socket = require "socket"
    function download(host, file)
    	local c = assert(socket.connect(host, 80))
    	local count = 0 -- counts number of bytes read
    	c:send("GET " .. file .. " HTTP/1.0\r\n\r\n")
    	while true do
    		local s, status = receive(c)
    		count = count + #s
    		if status == "closed" then break end
    	end
    	c:close()
    	print(file, count)
    end
    function receive(connection)
    	connection:settimeout(0) -- do not block
    	local s, status, partial = connection:receive(2 ^ 10)
    	if status == "timeout" then
    		coroutine.yield(connection)
    	end
    	return s or partial, status
    end
    function get(host, file)
    	-- create coroutine
    	local co = coroutine.create(function ()
    		download(host, file)
    	end)
    	-- insert it in the list
    	table.insert(threads, co)
    end
    function dispatch()
    	local i = 1
    	while true do
    		if threads[i] == nil then -- no more threads?
    			if threads[1] == nil then break end -- list is empty?
    			i = 1 -- restart the loop
    		end
    		local status, res = coroutine.resume(threads[i])
    		if not res then -- thread finished its task?
    			table.remove(threads, i)
    		else
    			i = i + 1 -- go to next thread
    		end
    	end
    end
    host = "www.w3.org"
    file1 = "/TR/REC-html32.html"
    file2 = "/TR/html401/html40.txt"
    file3 = "/TR/2002/REC-xhtml-20020801/xhtml1.pdf"
    file4 = "/TR/2000/REC-DOM-Level-2-Core-20001113/DOM2-Core.txt"
    threads = {} -- list of all live threads
    get(host, file1)
    get(host, file2)
    get(host, file3)
    get(host, file4)
    dispatch()

将各协程存储在table中，调度程序管理协程的执行，如果一个协程发生了阻塞则调用下一个协程，循环下去直到各协程任务都完成（table中没有协程）。但是当每个协程都阻塞时，CPU仍在不停轮询，可用socket.select()优化

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function dispatch()
	local i = 1
	local timedout = {}
	while true do
		if threads[i] == nil then -- no more threads
			if threads[1] == nil then break end
			i = 1 -- restart the loop
			timedout = {}
		end
		local status, res = coroutine.resume(threads[i])
		if not res then -- thread finished its task?
			table.remove(threads, i)
		else -- time out
			i = i + 1
			timedout[#timedout + 1] = res
			if #timedout == #threads then -- all threads blocked?
				socket.select(timedout)
			end
		end
	end
end

timedout是一个table，存储各阻塞的协程，如果其大小与threads大小相等，则说明所有协程均阻塞，这时即可将timedout传给socket.select()来监视各协程的变化，若有一个协程不再阻塞，则可继续任务的执行