Use explicit exception when shutil._copytree should group exceptions

This resolves a bug where a non-recursive error is assumed to be a
list of exceptions and presenting the error as:
```
shutil.Error: ['<', 'D', 'i', 'r', 'E', 'n', 't', 'r', 'y', ' ', ...,
               's', 'a', 'm', 'e', ' ', 'f', 'i', 'l', 'e']
```

Author: berland

Lib/shutil.py

@@ -75,6 +75,9 @@
 class Error(OSError):
     pass
 
+class ErrorGroup(Error):
+    """Raised when multiple exceptions have been caught"""
+
 class SameFileError(Error):
     """Raised when source and destination are the same file."""
 
@@ -590,12 +593,9 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
                 copytree(srcobj, dstname, symlinks, ignore, copy_function,
                          ignore_dangling_symlinks, dirs_exist_ok)
             else:
-                # Will raise a SpecialFileError for unsupported file types
                 copy_function(srcobj, dstname)
-        # catch the Error from the recursive copytree so that we can
-        # continue with other files
-        except Error as err:
-            errors.extend(err.args[0])
+        except ErrorGroup as err_group:
+            errors.extend(err_group.args[0])
         except OSError as why:
             errors.append((srcname, dstname, str(why)))
     try:
@@ -605,7 +605,7 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
         if getattr(why, 'winerror', None) is None:
             errors.append((src, dst, str(why)))
     if errors:
-        raise Error(errors)
+        raise ErrorGroup(errors)
     return dst
 
 def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,

Lib/test/test_shutil.py

@@ -1099,6 +1099,39 @@ def test_copytree_subdirectory(self):
         rv = shutil.copytree(src_dir, dst_dir)
         self.assertEqual(['pol'], os.listdir(rv))
 
+    def test_copytree_to_itself_gives_sensible_error_message(self):
+        base_dir = self.mkdtemp()
+        self.addCleanup(shutil.rmtree, base_dir, ignore_errors=True)
+        src_dir = os.path.join(base_dir, "src")
+        os.makedirs(src_dir)
+        create_file((src_dir, "somefilename"), "somecontent")
+        self._assert_are_the_same_file_is_raised(src_dir, src_dir)
+
+    @os_helper.skip_unless_symlink
+    def test_copytree_to_backpointing_symlink_gives_sensible_error_message(self):
+        base_dir = self.mkdtemp()
+        self.addCleanup(shutil.rmtree, base_dir, ignore_errors=True)
+        src_dir = os.path.join(base_dir, "src")
+        target_dir = os.path.join(base_dir, "target")
+        os.makedirs(src_dir)
+        os.makedirs(target_dir)
+        some_file = os.path.join(src_dir, "somefilename")
+        create_file(some_file, "somecontent")
+        os.symlink(some_file, os.path.join(target_dir, "somefilename"))
+        self._assert_are_the_same_file_is_raised(src_dir, target_dir)
+
+    def _assert_are_the_same_file_is_raised(self, src_dir, target_dir):
+        try:
+            shutil.copytree(src_dir, target_dir, dirs_exist_ok=True)
+            self.fail("shutil.Error should have been raised")
+        except Error as error:
+            self.assertEqual(len(error.args[0]), 1)
+            if sys.platform == "win32":
+                self.assertIn("it is being used by another process", error.args[0][0][2])
+            else:
+                self.assertIn("are the same file", error.args[0][0][2])
+
+
 class TestCopy(BaseTest, unittest.TestCase):
 
     ### shutil.copymode

Misc/NEWS.d/next/Library/2024-12-28-16-25-42.gh-issue-102931.55o5kb.rst

@@ -0,0 +1,2 @@
+Make exception from :func:`shutil.copytree` readable when a
+:exc:`shutil.SameFileError` is raised.