[s3] allow sync to skip files that exist

ext/s3/main.php

@@ -40,9 +40,12 @@ public function onCommand(CommandEvent $event)
                 $end = $start;
             }
             foreach(Search::find_images_iterable(tags: ["order=id", "id>=$start", "id<=$end"]) as $image) {
-                print("{$image->id}: {$image->hash}\n");
+                if($this->sync_post($image)) {
+                    print("{$image->id}: {$image->hash}\n");
+                } else {
+                    print("{$image->id}: {$image->hash} (skipped)\n");
+                }
                 ob_flush();
-                $this->sync_post($image);
             }
         }
         if ($event->cmd == "s3-rm") {
@@ -133,20 +136,20 @@ private function hash_to_path(string $hash)
     }
 
     // underlying s3 interaction functions
-    private function sync_post(Image $image, ?array $new_tags = null)
+    private function sync_post(Image $image, ?array $new_tags = null, bool $overwrite = true): bool
     {
         global $config;
 
         // multiple events can trigger a sync,
         // let's only do one per request
         if(in_array($image->id, self::$synced)) {
-            return;
+            return false;
         }
         self::$synced[] = $image->id;
 
         $client = $this->get_client();
         if(is_null($client)) {
-            return;
+            return false;
         }
         $image_bucket = $config->get_string(S3Config::IMAGE_BUCKET);
 
@@ -159,14 +162,20 @@ private function sync_post(Image $image, ?array $new_tags = null)
             $image->tag_array = $_orig_tags;
         }
 
+        $key = $this->hash_to_path($image->hash);
+        if(!$overwrite && $client->doesObjectExist($image_bucket, $key)) {
+            return false;
+        }
+
         $client->putObject([
             'Bucket' => $image_bucket,
-            'Key' => $this->hash_to_path($image->hash),
+            'Key' => $key,
             'Body' => file_get_contents($image->get_image_filename()),
             'ACL' => 'public-read',
             'ContentType' => $image->get_mime(),
             'ContentDisposition' => "inline; filename=\"$friendly\"",
         ]);
+        return true;
     }
 
     private function remove_file(string $hash)