Do not check all chunks of a chunked upload if we do not need to

Fixes #22601

Before we did a full test on all chunks to verify if a chunked upload
was completed. This is unneeded since if we are missing one chunk we can
already fail.

Also we look from back to front since it is much more likely that we
find a missing chunk thus can error out early.

lib/private/filechunking.php

@@ -75,14 +75,16 @@ public function store($index, $data) {
 
 	public function isComplete() {
 		$prefix = $this->getPrefix();
-		$parts = 0;
 		$cache = $this->getCache();
-		for($i=0; $i < $this->info['chunkcount']; $i++) {
-			if ($cache->hasKey($prefix.$i)) {
-				$parts ++;
+		$chunkcount = (int)$this->info['chunkcount'];
+
+		for($i=($chunkcount-1); $i >= 0; $i--) {
+			if (!$cache->hasKey($prefix.$i)) {
+				return false;
 			}
 		}
-		return $parts == $this->info['chunkcount'];
+
+		return true;
 	}
 
 	/**