Limit the concurrency when fetching pages

For a production test case, BookProvider::getPages() was opening 3492
concurrent connections to the Wikimedia servers. That exceeds the
default maxmimum file descriptor limit, and there is a risk that the
tool will be blocked by WMF sysadmins.

So, limit the concurrency to 10 connections.

src/BookProvider.php

@@ -193,16 +193,15 @@ protected function domDocumentFromHtml( $html ) {
 	 * @return Page[]
 	 */
 	protected function getPages( $pages ) {
-		$promises = [];
-
+		$pageTitles = [];
 		foreach ( $pages as $id => $page ) {
-			$promises[$id] = $this->api->getPageAsync( $page->title );
+			$pageTitles[$id] = $page->title;
 		}
-
+		$contents = $this->api->getPageBatch( $pageTitles );
 		foreach ( $pages as $id => $page ) {
-			$page->content = $this->domDocumentFromHtml( $promises[$id]->wait() );
+			$page->content = isset( $contents[$id] )
+				? $this->domDocumentFromHtml( $contents[$id] ) : null;
 		}
-
 		return $pages;
 	}
 

src/Util/Api.php

@@ -9,6 +9,7 @@
 use GuzzleHttp\Client;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Promise\Each;
 use GuzzleHttp\Promise\PromiseInterface;
 use Kevinrob\GuzzleCache\CacheMiddleware;
 use Kevinrob\GuzzleCache\Storage\Psr6CacheStorage;
@@ -28,6 +29,7 @@ class Api {
 	private const USER_AGENT = 'Wikisource Export/0.1';
 	private const CONNECT_TIMEOUT = 10; // in seconds
 	private const REQUEST_TIMEOUT = 60; // in seconds
+	private const MAX_CONNECTIONS = 10;
 
 	/** @var string */
 	private $lang = '';
@@ -252,6 +254,8 @@ public function completeQuery( $params ) {
 	}
 
 	/**
+	 * Get a page asynchronously, with unlimited concurrency
+	 *
 	 * @param string $title the title of the page
 	 * @return PromiseInterface promise with the content of a page
 	 */
@@ -269,6 +273,31 @@ function ( $reason ) use ( $title ) {
 				);
 	}
 
+	/**
+	 * Get a batch of pages, with a concurrency limit
+	 *
+	 * @param string[] $titles
+	 * @return string[] The contents of the pages
+	 */
+	public function getPageBatch( $titles ) {
+		$requests = function () use ( $titles ) {
+			foreach ( $titles as $id => $title ) {
+				yield $id => $this->getPageAsync( $title );
+			}
+		};
+		$this->logger->debug( "Sending request for " . count( $titles ) . " titles" );
+		$texts = [];
+		Each::ofLimit(
+			$requests(),
+			self::MAX_CONNECTIONS,
+			function ( $text, $id ) use ( &$texts ) {
+				$texts[$id] = $text;
+			}
+		)->wait();
+		$this->logger->debug( "Got responses for " . count( $texts ) . " pages" );
+		return $texts;
+	}
+
 	/**
 	 * @param string $url the url
 	 * @return string the file content