Raise proper error when you try to load an empty qcow2 file

Fix #637

gns3server/modules/qemu/qcow2.py

@@ -62,7 +62,10 @@ class Qcow2:
         with open(self.path, 'rb') as f:
             content = f.read(struct.calcsize(struct_format))
 
-            self.magic, self.version, self.backing_file_offset, self.backing_file_size = struct.unpack_from(struct_format, content)
+            try:
+                self.magic, self.version, self.backing_file_offset, self.backing_file_size = struct.unpack_from(struct_format, content)
+            except struct.error:
+                raise Qcow2Error("Invalid file header for {}".format(self.path))
 
         if self.magic != 1363560955:  # The first 4 bytes contain the characters 'Q', 'F', 'I' followed by 0xfb.
             raise Qcow2Error("Invalid magic for {}".format(self.path))

tests/modules/qemu/test_qcow2.py

@@ -56,6 +56,12 @@ def test_invalid_file():
         Qcow2("tests/resources/nvram_iou")
 
 
+def test_invalid_empty_file(tmpdir):
+    open(str(tmpdir / 'a'), 'w+').close()
+    with pytest.raises(Qcow2Error):
+        Qcow2(str(tmpdir / 'a'))
+
+
 @pytest.mark.skipif(qemu_img() is None, reason="qemu-img is not available")
 def test_rebase(tmpdir, loop):
     shutil.copy("tests/resources/empty8G.qcow2", str(tmpdir / "empty16G.qcow2"))