Support seeking also from the end of file on S3 storage

The PR https://github.com/nextcloud/server/pull/20033 added support
for `fseek` for  the S3 storage backend. However, the seek mode SEEK_END
was left out that time. This PR fills this gap.

Signed-off-by: Pauli Järvinen <pauli.jarvinen@gmail.com>

lib/private/Files/Stream/SeekableHttpStream.php

@@ -76,6 +76,8 @@ class SeekableHttpStream implements File {
 	private $current;
 	/** @var int */
 	private $offset = 0;
+	/** @var int */
+	private $length = 0;
 
 	private function reconnect(int $start) {
 		$range = $start . '-';
@@ -101,12 +103,14 @@ class SeekableHttpStream implements File {
 		$content = trim(explode(':', $contentRange)[1]);
 		$range = trim(explode(' ', $content)[1]);
 		$begin = intval(explode('-', $range)[0]);
+		$length = intval(explode('/', $range)[1]);
 
 		if ($begin !== $start) {
 			return false;
 		}
 
 		$this->offset = $begin;
+		$this->length = $length;
 
 		return true;
 	}
@@ -140,7 +144,12 @@ class SeekableHttpStream implements File {
 				}
 				return $this->reconnect($this->offset + $offset);
 			case SEEK_END:
-				return false;
+				if ($this->length === 0) {
+					return false;
+				} elseif ($this->length + $offset === $this->offset) {
+					return true;
+				}
+				return $this->reconnect($this->length + $offset);
 		}
 		return false;
 	}