Read ahead over SFTP: take answers phpseclib has read already instead of waiting up to 10 s on the socket

tests/Unit/Backend/SftpRangeFetcherTest.php

@@ -87,6 +87,25 @@ class SftpRangeFetcherTest extends TestCase {
 		$this->assertSame(SftpRangeFetcher::DEPTH, $connections[1]->maxInFlight);
 	}
 
+	public function testAnAnswerPhpseclibHasReadAlreadyIsTakenWithoutWaitingOnTheSocket(): void {
+		// The fake's answers are all "read already": its socket never becomes readable, and
+		// phpseclib's packet buffer holds data. Waiting on the socket would take a second a step.
+		$connections = $this->connections(2);
+		$sockets = [];
+		foreach ($connections as $c) {
+			$pair = stream_socket_pair(STREAM_PF_UNIX, STREAM_SOCK_STREAM, STREAM_IPPROTO_IP);
+			$sockets[] = $pair;
+			$c->fsock = $pair[0];
+			$c->packet_buffer = 'more';
+		}
+		$ranges = array_slice($this->ranges(), 0, 3);
+		$start = microtime(true);
+
+		$this->assertSame($this->expected($ranges), self::collect(SftpRangeFetcher::fetch(self::opened($connections), '/home/', $ranges, $this->tempDir())));
+
+		$this->assertLessThan(1.0, microtime(true) - $start);
+	}
+
 	public function testAServerThatCapsAReadStillGivesEveryByte(): void {
 		$connections = $this->connections(3, array_fill(0, 3, ['cap' => 100000]));
 		$ranges = $this->ranges();

tests/Unit/Backend/SftpReadStreamTest.php

@@ -468,6 +468,10 @@ final class FakeSftpConnection {
 	public array $openHandles = [];
 	/** @var list<string> paths of every OPEN request */
 	public array $opened = [];
+	/** @var resource|null like phpseclib's socket (SftpRangeFetcher waits on it) */
+	public $fsock = null;
+	/** like phpseclib's: what came in with the last packet and is not read yet */
+	public string $packet_buffer = '';
 	/** Whether the SFTP channel is open: phpseclib 2 opens it on the first SFTP call. */
 	public bool $initialised = false;
 	/** Packets sent */