PHPLIB-608 Fix wrong validity check in CachingIterator (#803)

* PHPLIB-608 Fix wrong validity check in CachingIterator

* Apply review feedback

Author: alcaeus

src/Model/CachingIterator.php

@@ -153,12 +153,10 @@ private function exhaustIterator()
      */
     private function storeCurrentItem()
     {
-        $key = $this->iterator->key();
-
-        if ($key === null) {
+        if (! $this->iterator->valid()) {
             return;
         }
 
-        $this->items[$key] = $this->iterator->current();
+        $this->items[$this->iterator->key()] = $this->iterator->current();
     }
 }

tests/Model/CachingIteratorTest.php

@@ -3,6 +3,7 @@
 namespace MongoDB\Tests\Model;
 
 use Exception;
+use Iterator;
 use MongoDB\Model\CachingIterator;
 use MongoDB\Tests\TestCase;
 use Throwable;
@@ -110,6 +111,46 @@ public function testCountWithEmptySet()
         $this->assertCount(0, $iterator);
     }
 
+    /**
+     * This protects against iterators that return valid keys on invalid
+     * positions, which was the case in ext-mongodb until PHPC-1748 was fixed.
+     */
+    public function testWithWrongIterator()
+    {
+        $nestedIterator = new class implements Iterator {
+            /** @var int */
+            private $i = 0;
+
+            public function current()
+            {
+                return $this->i;
+            }
+
+            public function next()
+            {
+                $this->i++;
+            }
+
+            public function key()
+            {
+                return $this->i;
+            }
+
+            public function valid()
+            {
+                return $this->i == 0;
+            }
+
+            public function rewind()
+            {
+                $this->i = 0;
+            }
+        };
+
+        $iterator = new CachingIterator($nestedIterator);
+        $this->assertCount(1, $iterator);
+    }
+
     private function getTraversable($items)
     {
         foreach ($items as $item) {