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FutureIterator.php
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FutureIterator.php
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<?php
/**
* FutureIterator aggregates @{class:Future}s and allows you to respond to them
* in the order they resolve. This is useful because it minimizes the amount of
* time your program spends waiting on parallel processes.
*
* $futures = array(
* 'a.txt' => new ExecFuture('wc -c a.txt'),
* 'b.txt' => new ExecFuture('wc -c b.txt'),
* 'c.txt' => new ExecFuture('wc -c c.txt'),
* );
*
* foreach (new FutureIterator($futures) as $key => $future) {
* // IMPORTANT: keys are preserved but the order of elements is not. This
* // construct iterates over the futures in the order they resolve, so the
* // fastest future is the one you'll get first. This allows you to start
* // doing followup processing as soon as possible.
*
* list($err, $stdout) = $future->resolve();
* do_some_processing($stdout);
* }
*
* For a general overview of futures, see @{article:Using Futures}.
*
* @task basics Basics
* @task config Configuring Iteration
* @task iterator Iterator Interface
* @task internal Internals
*/
final class FutureIterator
extends Phobject
implements Iterator {
private $hold = array();
private $wait = array();
private $work = array();
private $futures = array();
private $key;
private $limit;
private $timeout;
private $isTimeout = false;
private $hasRewound = false;
/* -( Basics )------------------------------------------------------------- */
/**
* Create a new iterator over a list of futures.
*
* @param list List of @{class:Future}s to resolve.
* @task basics
*/
public function __construct(array $futures) {
assert_instances_of($futures, 'Future');
foreach ($futures as $map_key => $future) {
$future->setFutureKey($map_key);
$this->addFuture($future);
}
}
/**
* Block until all futures resolve.
*
* @return void
* @task basics
*/
public function resolveAll() {
// If a caller breaks out of a "foreach" and then calls "resolveAll()",
// interpret it to mean that we should iterate over whatever futures
// remain.
if ($this->hasRewound) {
while ($this->valid()) {
$this->next();
}
} else {
iterator_to_array($this);
}
}
/**
* Add another future to the set of futures. This is useful if you have a
* set of futures to run mostly in parallel, but some futures depend on
* others.
*
* @param Future @{class:Future} to add to iterator
* @task basics
*/
public function addFuture(Future $future) {
$key = $future->getFutureKey();
if (isset($this->futures[$key])) {
throw new Exception(
pht(
'This future graph already has a future with key "%s". Each '.
'future must have a unique key.',
$key));
}
$this->futures[$key] = $future;
$this->hold[$key] = $key;
return $this;
}
/* -( Configuring Iteration )---------------------------------------------- */
/**
* Set a maximum amount of time you want to wait before the iterator will
* yield a result. If no future has resolved yet, the iterator will yield
* null for key and value. Among other potential uses, you can use this to
* show some busy indicator:
*
* $futures = id(new FutureIterator($futures))
* ->setUpdateInterval(1);
* foreach ($futures as $future) {
* if ($future === null) {
* echo "Still working...\n";
* } else {
* // ...
* }
* }
*
* This will echo "Still working..." once per second as long as futures are
* resolving. By default, FutureIterator never yields null.
*
* @param float Maximum number of seconds to block waiting on futures before
* yielding null.
* @return this
*
* @task config
*/
public function setUpdateInterval($interval) {
$this->timeout = $interval;
return $this;
}
/**
* Limit the number of simultaneously executing futures.
*
* $futures = id(new FutureIterator($futures))
* ->limit(4);
* foreach ($futures as $future) {
* // Run no more than 4 futures simultaneously.
* }
*
* @param int Maximum number of simultaneous jobs allowed.
* @return this
*
* @task config
*/
public function limit($max) {
$this->limit = $max;
return $this;
}
public function setMaximumWorkingSetSize($limit) {
$this->limit = $limit;
return $this;
}
public function getMaximumWorkingSetSize() {
return $this->limit;
}
/* -( Iterator Interface )------------------------------------------------- */
/**
* @task iterator
*/
public function rewind() {
if ($this->hasRewound) {
throw new Exception(
pht('Future graphs can not be rewound.'));
}
$this->hasRewound = true;
$this->next();
}
/**
* @task iterator
*/
public function next() {
$this->key = null;
$this->updateWorkingSet();
if (!$this->work) {
return;
}
$start = microtime(true);
$timeout = $this->timeout;
$this->isTimeout = false;
$working_set = array_select_keys($this->futures, $this->work);
while (true) {
// Update every future first. This is a no-op on futures which have
// already resolved or failed, but we want to give futures an
// opportunity to make progress even if we can resolve something.
foreach ($working_set as $future_key => $future) {
$future->updateFuture();
}
// Check if any future has resolved or failed. If we have any such
// futures, we'll return the first one from the iterator.
$resolve_key = null;
foreach ($working_set as $future_key => $future) {
if ($future->hasException()) {
$resolve_key = $future_key;
break;
}
if ($future->hasResult()) {
$resolve_key = $future_key;
break;
}
}
// We've found a future to resolve, so we're done here for now.
if ($resolve_key !== null) {
$this->moveFutureToDone($resolve_key);
return;
}
// We don't have any futures to resolve yet. Check if we're reached
// an update interval.
$wait_time = 1;
if ($timeout !== null) {
$elapsed = microtime(true) - $start;
if ($elapsed > $timeout) {
$this->isTimeout = true;
return;
}
$wait_time = min($wait_time, $timeout - $elapsed);
}
// We're going to wait. If possible, we'd like to wait with sockets.
// If we can't, we'll just sleep.
$read_sockets = array();
$write_sockets = array();
foreach ($working_set as $future_key => $future) {
$sockets = $future->getReadSockets();
foreach ($sockets as $socket) {
$read_sockets[] = $socket;
}
$sockets = $future->getWriteSockets();
foreach ($sockets as $socket) {
$write_sockets[] = $socket;
}
}
$use_sockets = ($read_sockets || $write_sockets);
if ($use_sockets) {
foreach ($working_set as $future) {
$wait_time = min($wait_time, $future->getDefaultWait());
}
$this->waitForSockets($read_sockets, $write_sockets, $wait_time);
} else {
usleep(1000);
}
}
}
/**
* @task iterator
*/
public function current() {
if ($this->isTimeout) {
return null;
}
return $this->futures[$this->key];
}
/**
* @task iterator
*/
public function key() {
if ($this->isTimeout) {
return null;
}
return $this->key;
}
/**
* @task iterator
*/
public function valid() {
if ($this->isTimeout) {
return true;
}
return ($this->key !== null);
}
/* -( Internals )---------------------------------------------------------- */
/**
* @task internal
*/
protected function updateWorkingSet() {
$limit = $this->getMaximumWorkingSetSize();
$work_count = count($this->work);
// If we're already working on the maximum number of futures, we just have
// to wait for something to resolve. There's no benefit to updating the
// queue since we can never make any meaningful progress.
if ($limit) {
if ($work_count >= $limit) {
return;
}
}
// If any futures that are currently held are no longer blocked by
// dependencies, move them from "hold" to "wait".
foreach ($this->hold as $future_key) {
if (!$this->canMoveFutureToWait($future_key)) {
continue;
}
$this->moveFutureToWait($future_key);
}
$wait_count = count($this->wait);
$hold_count = count($this->hold);
if (!$work_count && !$wait_count && $hold_count) {
throw new Exception(
pht(
'Future graph is stalled: some futures are held, but no futures '.
'are waiting or working. The graph can never resolve.'));
}
// Figure out how many futures we can start. If we don't have a limit,
// we can start every waiting future. If we do have a limit, we can only
// start as many futures as we have slots for.
if ($limit) {
$work_limit = min($limit, $wait_count);
} else {
$work_limit = $wait_count;
}
// If we're ready to start futures, start them now.
if ($work_limit) {
foreach ($this->wait as $future_key) {
$this->moveFutureToWork($future_key);
$work_limit--;
if (!$work_limit) {
return;
}
}
}
}
private function canMoveFutureToWait($future_key) {
return true;
}
private function moveFutureToWait($future_key) {
unset($this->hold[$future_key]);
$this->wait[$future_key] = $future_key;
}
private function moveFutureToWork($future_key) {
unset($this->wait[$future_key]);
$this->work[$future_key] = $future_key;
$this->futures[$future_key]->startFuture();
}
private function moveFutureToDone($future_key) {
$this->key = $future_key;
unset($this->work[$future_key]);
// Before we return, do another working set update so we start any
// futures that are ready to go as soon as we can.
$this->updateWorkingSet();
$this->futures[$future_key]->endFuture();
}
/**
* Wait for activity on one of several sockets.
*
* @param list List of sockets expected to become readable.
* @param list List of sockets expected to become writable.
* @param float Timeout, in seconds.
* @return void
*/
private function waitForSockets(
array $read_list,
array $write_list,
$timeout = 1.0) {
static $handler_installed = false;
if (!$handler_installed) {
// If we're spawning child processes, we need to install a signal handler
// here to catch cases like execing '(sleep 60 &) &' where the child
// exits but a socket is kept open. But we don't actually need to do
// anything because the SIGCHLD will interrupt the stream_select(), as
// long as we have a handler registered.
if (function_exists('pcntl_signal')) {
if (!pcntl_signal(SIGCHLD, array(__CLASS__, 'handleSIGCHLD'))) {
throw new Exception(pht('Failed to install signal handler!'));
}
}
$handler_installed = true;
}
$timeout_sec = (int)$timeout;
$timeout_usec = (int)(1000000 * ($timeout - $timeout_sec));
$exceptfds = array();
$ok = @stream_select(
$read_list,
$write_list,
$exceptfds,
$timeout_sec,
$timeout_usec);
if ($ok === false) {
// Hopefully, means we received a SIGCHLD. In the worst case, we degrade
// to a busy wait.
}
}
public static function handleSIGCHLD($signo) {
// This function is a dummy, we just need to have some handler registered
// so that PHP will get interrupted during "stream_select()". If we don't
// register a handler, "stream_select()" won't fail.
}
}

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Content licensed under Creative Commons Attribution-ShareAlike 4.0 (CC-BY-SA) unless otherwise noted; code licensed under Apache 2.0 or other open source licenses. · CC BY-SA 4.0 · Apache 2.0