From 8e0b2e383d3d00ce12521a4845cb1c83f0d77d2e Mon Sep 17 00:00:00 2001
From: Bowen Ding <dbw9580@live.com>
Date: Thu, 28 Jan 2021 21:55:15 +0800
Subject: [PATCH] Limit-number-of-concurrent-preview-generations

Signed-off-by: Bowen Ding <dbw9580@live.com>
---
 lib/private/Preview/Generator.php | 100 ++++++++++++++++++++++++++++++
 lib/private/PreviewManager.php    |   9 ++-
 2 files changed, 108 insertions(+), 1 deletion(-)

diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index 2be08d1b2c..075aeb9e19 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -44,6 +44,8 @@ use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\EventDispatcher\GenericEvent;
 
 class Generator {
+	public const SEMAPHORE_ID_ALL = 0x0a11;
+	public const SEMAPHORE_ID_NEW = 0x07ea;
 
 	/** @var IPreview */
 	private $previewManager;
@@ -204,6 +206,95 @@ class Generator {
 		return $preview;
 	}
 
+	/**
+	 * @param int $semId
+	 * @param int $concurrency
+	 * @return bool|resource the semaphore on success or false on failure
+	 */
+	public static function guardWithSemaphore(int $semId, int $concurrency) {
+		if (extension_loaded('sysvsem')) {
+			$sem = sem_get($semId, $concurrency);
+			if ($sem === false) {
+				return false;
+			}
+			if (!sem_acquire($sem)) {
+				return false;
+			}
+			return $sem;
+		}
+		return false;
+	}
+
+	/**
+	 * @param resource $sem
+	 * @return bool
+	 */
+	public static function unguardWithSemaphore($sem) {
+		if (!is_resource($sem)) {
+			return false;
+		}
+		if (extension_loaded('sysvsem')) {
+			return sem_release($sem);
+		}
+		return false;
+	}
+
+	/**
+	 * Get the number of concurrent threads supported by the host
+	 *
+	 * @return int number of concurrent threads, or 0 if it cannot be determined
+	 */
+	public static function getHardwareConcurrency() {
+		static $width;
+		if (!isset($width)) {
+			if (is_file("/proc/cpuinfo")) {
+				$width = substr_count(file_get_contents("/proc/cpuinfo"), "processor");
+			} else {
+				$width = 0;
+			}
+		}
+		return $width;
+	}
+
+	/**
+	 * Get number of concurrent preview generations from system config
+	 *
+	 * Two config entries, `preview_concurrency_new` and `preview_concurrency_all`,
+	 * are available. If not set, the default values are determined with the hardware concurrency
+	 * of the host. In case the hardware concurrency cannot be determined, or the user sets an
+	 * invalid value, fallback values are:
+	 * For new images whose previews do not exist and need to be generated, 4;
+	 * For all preview generation requests, 8.
+	 * Value of `preview_concurrency_all` should be greater than or equal to that of
+	 * `preview_concurrency_new`, otherwise, the latter is returned.
+	 *
+	 * @param string $type either `preview_concurrency_new` or `preview_concurrency_all`
+	 * @return int number of concurrent preview generations, or -1 if $type is invalid
+	 */
+	public function getNumConcurrentPreviews($type) {
+		static $cached = array();
+		if (array_key_exists($type, $cached)) {
+			return $cached[$type];
+		}
+
+		$hardwareConcurrency = self::getHardwareConcurrency();
+		switch ($type) {
+			case "preview_concurrency_all":
+				$fallback = $hardwareConcurrency > 0 ? $hardwareConcurrency * 2 : 8;
+				$concurrency_all = $this->config->getSystemValueInt($type, $fallback);
+				$concurrency_new = $this->getNumConcurrentPreviews("preview_concurrency_new");
+				$cached[$type] = max($concurrency_all, $concurrency_new);
+				break;
+			case "preview_concurrency_new":
+				$fallback = $hardwareConcurrency > 0 ? $hardwareConcurrency : 4;
+				$cached[$type] = $this->config->getSystemValueInt($type, $fallback);
+				break;
+			default:
+				return -1;
+		}
+		return $cached[$type];
+	}
+
 	/**
 	 * @param ISimpleFolder $previewFolder
 	 * @param File $file
@@ -241,8 +332,13 @@ class Generator {
 				$maxWidth = (int)$this->config->getSystemValue('preview_max_x', 4096);
 				$maxHeight = (int)$this->config->getSystemValue('preview_max_y', 4096);
 
+				$previewConcurrency = $this->getNumConcurrentPreviews('preview_concurrency_new');
+				$sem = self::guardWithSemaphore(self::SEMAPHORE_ID_NEW, $previewConcurrency);
+
 				$preview = $this->helper->getThumbnail($provider, $file, $maxWidth, $maxHeight);
 
+				self::unguardWithSemaphore($sem);
+
 				if (!($preview instanceof IImage)) {
 					continue;
 				}
@@ -407,6 +503,9 @@ class Generator {
 			throw new \InvalidArgumentException('Failed to generate preview, failed to load image');
 		}
 
+		$previewConcurrency = $this->getNumConcurrentPreviews('preview_concurrency_new');
+		$sem = self::guardWithSemaphore(self::SEMAPHORE_ID_NEW, $previewConcurrency);
+
 		if ($crop) {
 			if ($height !== $preview->height() && $width !== $preview->width()) {
 				//Resize
@@ -429,6 +528,7 @@ class Generator {
 			$preview = $maxPreview->resizeCopy(max($width, $height));
 		}
 
+		self::unguardWithSemaphore($sem);
 
 		$path = $this->generatePath($width, $height, $crop, $preview->dataMimeType(), $prefix);
 		try {
diff --git a/lib/private/PreviewManager.php b/lib/private/PreviewManager.php
index d6dc700e4a..0307b91763 100644
--- a/lib/private/PreviewManager.php
+++ b/lib/private/PreviewManager.php
@@ -187,7 +187,14 @@ class PreviewManager implements IPreview {
 	 * @since 11.0.0 - \InvalidArgumentException was added in 12.0.0
 	 */
 	public function getPreview(File $file, $width = -1, $height = -1, $crop = false, $mode = IPreview::MODE_FILL, $mimeType = null) {
-		return $this->getGenerator()->getPreview($file, $width, $height, $crop, $mode, $mimeType);
+		$previewConcurrency = $this->getGenerator()->getNumConcurrentPreviews('preview_concurrency_all');
+ 		$sem = Generator::guardWithSemaphore(Generator::SEMAPHORE_ID_ALL, $previewConcurrency);
+
+ 		$preview = $this->getGenerator()->getPreview($file, $width, $height, $crop, $mode, $mimeType);
+
+ 		Generator::unguardWithSemaphore($sem);
+
+ 		return $preview;
 	}
 
 	/**