sync: move pool related code to sync.pool, cleanup, add a README.md

vlib/sync/pool/README.md

@@ -0,0 +1,36 @@
+
+The `sync.pool` module provides a convenient way to run identical tasks over
+an array of items *in parallel*, without worrying about thread synchronization,
+waitgroups, mutexes etc.., you just need to supply a callback function, that
+will be called once per each item in your input array.
+
+After all the work is done in parallel by the worker threads in the pool,
+pool.work_on_items will return. You can then call pool.get_results<Result>()
+to retrieve a list of all the results, that the worker callbacks returned
+for each input item. Example:
+
+```v
+import sync.pool
+
+struct SResult {
+	s string
+}
+
+fn sprocess(pp &pool.PoolProcessor, idx int, wid int) &SResult {
+	item := pp.get_item<string>(idx)
+	println('idx: $idx, wid: $wid, item: ' + item)
+	return &SResult{item.reverse()}
+}
+
+fn main() {
+	mut pp := pool.new_pool_processor(callback: sprocess)
+	pp.work_on_items(['1abc', '2abc', '3abc', '4abc', '5abc', '6abc', '7abc'])
+	// optionally, you can iterate over the results too:
+	for x in pp.get_results<SResult>() {
+		println('result: $x.s')
+	}
+}
+```
+
+See https://github.com/vlang/v/blob/master/vlib/sync/pool/pool_test.v for a
+more detailed usage example.

vlib/sync/pool/pool.v

@@ -0,0 +1,172 @@
+module pool
+
+import sync
+
+import runtime
+
+pub const (
+	no_result = voidptr(0)
+)
+
+pub struct PoolProcessor {
+	thread_cb       voidptr
+mut:
+	njobs           int
+	items           []voidptr
+	results         []voidptr
+	ntask           int // writing to this should be locked by ntask_mtx.
+	ntask_mtx       &sync.Mutex
+	waitgroup       &sync.WaitGroup
+	shared_context  voidptr
+	thread_contexts []voidptr
+}
+
+pub type ThreadCB = fn (p &PoolProcessor, idx int, task_id int) voidptr
+
+pub struct PoolProcessorConfig {
+	maxjobs  int
+	callback ThreadCB
+}
+
+// new_pool_processor returns a new PoolProcessor instance.
+// The parameters of new_pool_processor are:
+//    context.maxjobs: when 0 (the default), the PoolProcessor will use a
+//      number of threads, that is optimal for your system to process your items.
+//    context.callback: this should be a callback function, that each worker
+//      thread in the pool will run for each item.
+//      The callback function will receive as parameters:
+//      1) the PoolProcessor instance, so it can call
+//            p.get_item<int>(idx) to get the actual item at index idx
+//      2) idx - the index of the currently processed item
+//      3) task_id - the index of the worker thread in which the callback
+//            function is running.
+pub fn new_pool_processor(context PoolProcessorConfig) &PoolProcessor {
+	if isnil(context.callback) {
+		panic('You need to pass a valid callback to new_pool_processor.')
+	}
+	pool := &PoolProcessor {
+		items: []
+		results: []
+		shared_context: voidptr(0)
+		thread_contexts: []
+		njobs: context.maxjobs
+		ntask: 0
+		ntask_mtx: sync.new_mutex()
+		waitgroup: sync.new_waitgroup()
+		thread_cb: voidptr(context.callback)
+	}
+	return pool
+}
+
+// set_max_jobs gives you the ability to override the number
+// of jobs *after* the PoolProcessor had been created already.
+pub fn (mut pool PoolProcessor) set_max_jobs(njobs int) {
+	pool.njobs = njobs
+}
+
+// work_on_items receives a list of items of type T,
+// then starts a work pool of pool.njobs threads, each running
+// pool.thread_cb in a loop, untill all items in the list,
+// are processed.
+// When pool.njobs is 0, the number of jobs is determined
+// by the number of available cores on the system.
+// work_on_items returns *after* all threads finish.
+// You can optionally call get_results after that.
+pub fn (mut pool PoolProcessor) work_on_items<T>(items []T) {
+	pool.work_on_pointers( items.pointers() )
+}
+
+pub fn (mut pool PoolProcessor) work_on_pointers(items []voidptr) {
+	mut njobs := runtime.nr_jobs()
+	if pool.njobs > 0 {
+		njobs = pool.njobs
+	}
+	pool.items = []
+	pool.results = []
+	pool.thread_contexts = []
+	pool.items << items
+	pool.results = []voidptr{len:(pool.items.len)}
+	pool.thread_contexts << []voidptr{len:(pool.items.len)}
+	pool.waitgroup.add(njobs)
+	for i := 0; i < njobs; i++ {
+		if njobs > 1 {
+			go process_in_thread(mut pool,i)
+		} else {
+			// do not run concurrently, just use the same thread:
+			process_in_thread(mut pool,i)
+		}
+	}
+	pool.waitgroup.wait()
+}
+
+// process_in_thread does the actual work of worker thread.
+// It is a workaround for the current inability to pass a
+// method in a callback.
+fn process_in_thread(mut pool PoolProcessor, task_id int) {
+	cb := ThreadCB(pool.thread_cb)
+	mut idx := 0
+	ilen := pool.items.len
+	for {
+		if pool.ntask >= ilen {
+			break
+		}
+		pool.ntask_mtx.@lock()
+		idx = pool.ntask
+		pool.ntask++
+		pool.ntask_mtx.unlock()
+		if idx >= ilen {
+			break
+		}
+		pool.results[idx] = cb(pool, idx, task_id)
+	}
+	pool.waitgroup.done()
+}
+
+// get_item - called by the worker callback.
+// Retrieves a type safe instance of the currently processed item
+pub fn (pool &PoolProcessor) get_item<T>(idx int) T {
+	return *(&T(pool.items[idx]))
+}
+
+// get_result - called by the main thread to get a specific result.
+// Retrieves a type safe instance of the produced result.
+pub fn (pool &PoolProcessor) get_result<T>(idx int) T {
+	return *(&T(pool.results[idx]))
+}
+
+// get_results - get a list of type safe results in the main thread.
+pub fn (pool &PoolProcessor) get_results<T>() []T {
+	mut res := []T{}
+	for i in 0 .. pool.results.len {
+		res << *(&T(pool.results[i]))
+	}
+	return res
+}
+
+// set_shared_context - can be called during the setup so that you can
+// provide a context that is shared between all worker threads, like
+// common options/settings.
+pub fn (mut pool PoolProcessor) set_shared_context(context voidptr) {
+	pool.shared_context = context
+}
+
+// get_shared_context - can be called in each worker callback, to get
+// the context set by pool.set_shared_context
+pub fn (pool &PoolProcessor) get_shared_context() voidptr {
+	return pool.shared_context
+}
+
+// set_thread_context - can be called during the setup at the start of
+// each worker callback, so that the worker callback can have some thread
+// local storage area where it can write/read information that is private
+// to the given thread, without worrying that it will get overwritten by
+// another thread
+pub fn (mut pool PoolProcessor) set_thread_context(idx int, context voidptr) {
+	pool.thread_contexts[idx] = context
+}
+
+// get_thread_context - returns a pointer, that was set with
+// pool.set_thread_context . This pointer is private to each thread.
+pub fn (pool &PoolProcessor) get_thread_context(idx int) voidptr {
+	return pool.thread_contexts[idx]
+}

vlib/sync/pool/pool_test.v

@@ -0,0 +1,52 @@
+import time
+import sync.pool
+
+pub struct SResult {
+	s string
+}
+
+pub struct IResult {
+	i int
+}
+
+fn worker_s(p &pool.PoolProcessor, idx int, worker_id int) &SResult {
+	item := p.get_item<string>(idx)
+	println('worker_s worker_id: $worker_id | idx: $idx | item: $item')
+	time.sleep_ms(3)
+	return &SResult{'$item $item'}
+}
+
+fn worker_i(p &pool.PoolProcessor, idx int, worker_id int) &IResult {
+	item := p.get_item<int>(idx)
+	println('worker_i worker_id: $worker_id | idx: $idx | item: $item')
+	time.sleep_ms(5)
+	return &IResult{item * 1000}
+}
+
+fn test_work_on_strings() {
+	mut pool_s := pool.new_pool_processor(
+		callback: worker_s
+		maxjobs: 8
+	)
+
+	pool_s.work_on_items(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'])
+	for x in pool_s.get_results<SResult>() {
+		println(x.s)
+		assert x.s.len > 1
+	}
+}
+
+fn test_work_on_ints() {
+	// NB: since maxjobs is left empty here,
+	// the pool processor will use njobs = runtime.nr_jobs so that
+	// it will work optimally without overloading the system
+	mut pool_i := pool.new_pool_processor(
+		callback: worker_i
+	)
+
+	pool_i.work_on_items([1, 2, 3, 4, 5, 6, 7, 8])
+	for x in pool_i.get_results<IResult>() {
+		println(x.i)
+		assert x.i > 100
+	}
+}