Disallow file system root zipping.

The StudentFileAwareZipper includes the provided root directory in the zip
archive as a parent directory. File system roots aren't named directories,
so they cannot be added as such.

Author: cxcorp

tmc-langs-framework/src/main/java/fi/helsinki/cs/tmc/langs/io/zip/StudentFileAwareZipper.java

@@ -40,10 +40,16 @@ public byte[] zip(Path rootDirectory) throws IOException {
         log.debug("Starting to zip {}", rootDirectory);
 
         if (!Files.exists(rootDirectory)) {
-            log.error("Attempted to zip nonexistent directory {}", rootDirectory);
+            log.error("Attempted to zip nonexistent directory \"{}\"", rootDirectory);
             throw new FileNotFoundException("Attempted to zip nonexistent directory");
         }
 
+        if (rootDirectory.toAbsolutePath().getNameCount() == 0) {
+            // getNameCount returns 0 if the path only represents a root component
+            log.error("Attempted to zip a root \"{}\"", rootDirectory);
+            throw new IllegalArgumentException("Filesystem root zipping is not supported");
+        }
+
         ByteArrayOutputStream buffer = new ByteArrayOutputStream();
         try (ZipArchiveOutputStream zipStream = new ZipArchiveOutputStream(buffer)) {
             zipRecursively(rootDirectory, zipStream, rootDirectory);