FormatConverter: Copy worlds for backup if rename fails

this can fail if the backups directory points to a different drive than the original worlds location. In this case, we have to copy and delete the files instead, which is much slower, but works.
I REALLY advise against putting backups on a different mount point than worlds if you plan to convert large worlds.

src/utils/Filesystem.php

@@ -23,8 +23,11 @@ declare(strict_types=1);
 
 namespace pocketmine\utils;
 
+use function copy;
+use function dirname;
 use function fclose;
 use function fflush;
+use function file_exists;
 use function flock;
 use function fopen;
 use function ftruncate;
@@ -33,6 +36,7 @@ use function getmypid;
 use function is_dir;
 use function is_file;
 use function ltrim;
+use function mkdir;
 use function preg_match;
 use function realpath;
 use function rmdir;
@@ -88,6 +92,53 @@ final class Filesystem{
 		}
 	}
 
+	/**
+	 * Recursively copies a directory to a new location. The parent directories for the destination must exist.
+	 */
+	public static function recursiveCopy(string $origin, string $destination) : void{
+		if(!is_dir($origin)){
+			throw new \RuntimeException("$origin does not exist, or is not a directory");
+		}
+		if(!is_dir($destination)){
+			if(file_exists($destination)){
+				throw new \RuntimeException("$destination already exists, and is not a directory");
+			}
+			if(!is_dir(dirname($destination))){
+				//if the parent dir doesn't exist, the user most likely made a mistake
+				throw new \RuntimeException("The parent directory of $destination does not exist, or is not a directory");
+			}
+			if(!@mkdir($destination) && !is_dir($destination)){
+				throw new \RuntimeException("Failed to create output directory $destination (permission denied?)");
+			}
+		}
+		self::recursiveCopyInternal($origin, $destination);
+	}
+
+	private static function recursiveCopyInternal(string $origin, string $destination) : void{
+		if(is_dir($origin)){
+			if(!is_dir($destination)){
+				if(file_exists($destination)){
+					throw new \RuntimeException("Path $destination does not exist, or is not a directory");
+				}
+				mkdir($destination); //TODO: access permissions?
+			}
+			$objects = scandir($origin, SCANDIR_SORT_NONE);
+			if($objects === false) throw new AssumptionFailedError("scandir() shouldn't return false when is_dir() returns true");
+			foreach($objects as $object){
+				if($object === "." || $object === ".."){
+					continue;
+				}
+				self::recursiveCopyInternal($origin . "/" . $object, $destination . "/" . $object);
+			}
+		}else{
+			$dirName = dirname($destination);
+			if(!is_dir($dirName)){ //the destination folder should already exist
+				throw new AssumptionFailedError("The destination folder should have been created in the parent call");
+			}
+			copy($origin, $destination);
+		}
+	}
+
 	public static function addCleanedPath(string $path, string $replacement) : void{
 		self::$cleanedPaths[$path] = $replacement;
 		uksort(self::$cleanedPaths, function(string $str1, string $str2) : int{

src/world/format/io/FormatConverter.php

@@ -91,8 +91,17 @@ class FormatConverter{
 		$new->close();
 
 		$this->logger->info("Backing up pre-conversion world to " . $this->backupPath);
-		rename($path, $this->backupPath);
+		if(!@rename($path, $this->backupPath)){
-		rename($new->getPath(), $path);
+			$this->logger->warning("Moving old world files for backup failed, attempting copy instead. This might take a long time.");
+			Filesystem::recursiveCopy($path, $this->backupPath);
+			Filesystem::recursiveUnlink($path);
+		}
+		if(!@rename($new->getPath(), $path)){
+			//we don't expect this to happen because worlds/ should most likely be all on the same FS, but just in case...
+			$this->logger->debug("Relocation of new world files to location failed, attempting copy and delete instead");
+			Filesystem::recursiveCopy($new->getPath(), $path);
+			Filesystem::recursiveUnlink($new->getPath());
+		}
 
 		$this->logger->info("Conversion completed");
 		/**