APCng storage - avoid worst-case memory usage in buildPermutationTree

Author: Princemachiavelli

src/Prometheus/Storage/APCng.php

@@ -414,31 +414,22 @@ private function metaData(array $data): array
      *  [15] => ['/metrics', 'get', 'fail'],    [16] => ['/metrics', 'post', 'success'], [17] => ['/metrics', 'post', 'fail']
      * @param array<string> $labelNames
      * @param array<array> $labelValues
-     * @return array<array>
+     * @return Generator<array>
      */
-    private function buildPermutationTree(array $labelNames, array $labelValues): array /** @phpstan-ignore-line */
-    {
-        $treeRowCount = count(array_keys($labelNames));
-        $numElements = 1;
-        $treeInfo = [];
-        for ($i = $treeRowCount - 1; $i >= 0; $i--) {
-            $treeInfo[$i]['numInRow'] = count($labelValues[$i]);
-            $numElements *= $treeInfo[$i]['numInRow'];
-            $treeInfo[$i]['numInTree'] = $numElements;
-        }
-
-        $map = array_fill(0, $numElements, []);
-        for ($row = 0; $row < $treeRowCount; $row++) {
-            $col = $i = 0;
-            while ($i < $numElements) {
-                $val = $labelValues[$row][$col];
-                $map[$i] = array_merge($map[$i], array($val));
-                if (++$i % ($treeInfo[$row]['numInTree'] / $treeInfo[$row]['numInRow']) == 0) {
-                    $col = ++$col % $treeInfo[$row]['numInRow'];
-                }
+    private function buildPermutationTree(array $labelValues): \Generator /** @phpstan-ignore-line */
+    {
+        if($labelValues){
+          $lastIndex = array_key_last($labelValues);
+          if($currentValue = array_pop($labelValues)){
+            foreach($this->buildPermutationTree($labelValues) as $prefix){
+              foreach($currentValue as $value){
+                yield $prefix + [$lastIndex => $value];
+              }
             }
+          }
+        } else {
+          yield [];
         }
-        return $map;
     }
 
     /**
@@ -539,10 +530,9 @@ private function getValues(string $type, array $metaData): array /** @phpstan-ig
         if (isset($metaData['buckets'])) {
             $metaData['buckets'][] = 'sum';
             $labels[] = $metaData['buckets'];
-            $metaData['labelNames'][] = '__histogram_buckets';
         }
 
-        $labelValuesList = $this->buildPermutationTree($metaData['labelNames'], $labels);
+        $labelValuesList = $this->buildPermutationTree($labels);
         unset($labels);
         $histogramBucket = '';
         foreach ($labelValuesList as $labelValues) {