Repository: pre-commit/pre-commit-hooks
Pull request #511 (2020-08-02 19:10:14)

New hook 'destroyed-symlinks'
The hook detects symlinks which are changed to regular files with a content of a path which that symlink was pointing to.
This usually happens on Windows in case when user without a permission for creating symlinks clones repository with symlinks.

~~Some implementation notes: `bytes` is used for filenames in order to avoid having any troubles with encodings. After all, the hook check for destroyed symlinks, not for bad file encoding. `;)`~~

P. S. If you don't think that this hook would be useful for general audience, I'm fine with hosting it in the separate repo. Just wanted to try.

m-khvoinitsky: No idea why coverage report is different for pypy, will investigate it.
m-khvoinitsky: I've finally managed to find a time to dig into pypy issue. The reason was the following. In CPython in code like this:
```python
print('something', file=open(somefile, 'a'))
```
file object losts its last reference it this function call and it is destroyed forcing file buffers to flush. So, for some reason, this doesn't happen in pypy. Workaround would be using explicit `flush=True` or using context manager which I did.
@asottile, please, take a look.

Patch:
diff --git a/.pre-commit-hooks.yaml b/.pre-commit-hooks.yaml
index a47f733..fa617b9 100644
--- a/.pre-commit-hooks.yaml
+++ b/.pre-commit-hooks.yaml
@@ -100,6 +100,12 @@
     entry: debug-statement-hook
     language: python
     types: [python]
+-   id: destroyed-symlinks
+    name: Detect Destroyed Symlinks
+    description: Detects symlinks which are changed to regular files with a content of a path which that symlink was pointing to.
+    entry: destroyed-symlinks
+    language: python
+    types: [file]
 -   id: detect-aws-credentials
     name: Detect AWS Credentials
     description: Detects *your* aws credentials from the aws cli credentials file
diff --git a/README.md b/README.md
index 2a76268..3a87e9c 100644
--- a/README.md
+++ b/README.md
@@ -87,6 +87,12 @@ Attempts to load all yaml files to verify syntax.
 #### `debug-statements`
 Check for debugger imports and py37+ `breakpoint()` calls in python source.
 
+#### `destroyed-symlinks`
+Detects symlinks which are changed to regular files with a content of a path
+which that symlink was pointing to.
+This usually happens on Windows when a user clones a repository that has
+symlinks but they do not have the permission to create symlinks.
+
 #### `detect-aws-credentials`
 Checks for the existence of AWS secrets that you have set up with the AWS CLI.
 The following arguments are available:
diff --git a/pre_commit_hooks/check_executables_have_shebangs.py b/pre_commit_hooks/check_executables_have_shebangs.py
index a02d2a9..2d2bd7d 100644
--- a/pre_commit_hooks/check_executables_have_shebangs.py
+++ b/pre_commit_hooks/check_executables_have_shebangs.py
@@ -8,18 +8,11 @@ from typing import Sequence
 from typing import Set
 
 from pre_commit_hooks.util import cmd_output
+from pre_commit_hooks.util import zsplit
 
 EXECUTABLE_VALUES = frozenset(('1', '3', '5', '7'))
 
 
-def zsplit(s: str) -> List[str]:
-    s = s.strip('\0')
-    if s:
-        return s.split('\0')
-    else:
-        return []
-
-
 def check_executables(paths: List[str]) -> int:
     if sys.platform == 'win32':  # pragma: win32 cover
         return _check_git_filemode(paths)
diff --git a/pre_commit_hooks/destroyed_symlinks.py b/pre_commit_hooks/destroyed_symlinks.py
new file mode 100755
index 0000000..cfaf4e5
--- /dev/null
+++ b/pre_commit_hooks/destroyed_symlinks.py
@@ -0,0 +1,96 @@
+import argparse
+import shlex
+import subprocess
+from typing import List
+from typing import Optional
+from typing import Sequence
+
+from pre_commit_hooks.util import cmd_output
+from pre_commit_hooks.util import zsplit
+
+ORDINARY_CHANGED_ENTRIES_MARKER = '1'
+PERMS_LINK = '120000'
+PERMS_NONEXIST = '000000'
+
+
+def find_destroyed_symlinks(files: Sequence[str]) -> List[str]:
+    destroyed_links: List[str] = []
+    if not files:
+        return destroyed_links
+    for line in zsplit(
+        cmd_output('git', 'status', '--porcelain=v2', '-z', '--', *files),
+    ):
+        splitted = line.split(' ')
+        if splitted and splitted[0] == ORDINARY_CHANGED_ENTRIES_MARKER:
+            # https://git-scm.com/docs/git-status#_changed_tracked_entries
+            (
+                _, _, _,
+                mode_HEAD,
+                mode_index,
+                _,
+                hash_HEAD,
+                hash_index,
+                *path_splitted,
+            ) = splitted
+            path = ' '.join(path_splitted)
+            if (
+                    mode_HEAD == PERMS_LINK and
+                    mode_index != PERMS_LINK and
+                    mode_index != PERMS_NONEXIST
+            ):
+                if hash_HEAD == hash_index:
+                    # if old and new hashes are equal, it's not needed to check
+                    # anything more, we've found a destroyed symlink for sure
+                    destroyed_links.append(path)
+                else:
+                    # if old and new hashes are *not* equal, it doesn't mean
+                    # that everything is OK - new file may be altered
+                    # by something like trailing-whitespace and/or
+                    # mixed-line-ending hooks so we need to go deeper
+                    SIZE_CMD = ('git', 'cat-file', '-s')
+                    size_index = int(cmd_output(*SIZE_CMD, hash_index).strip())
+                    size_HEAD = int(cmd_output(*SIZE_CMD, hash_HEAD).strip())
+
+                    # in the worst case new file may have CRLF added
+                    # so check content only if new file is bigger
+                    # not more than 2 bytes compared to the old one
+                    if size_index <= size_HEAD + 2:
+                        head_content = subprocess.check_output(
+                            ('git', 'cat-file', '-p', hash_HEAD),
+                        ).rstrip()
+                        index_content = subprocess.check_output(
+                            ('git', 'cat-file', '-p', hash_index),
+                        ).rstrip()
+                        if head_content == index_content:
+                            destroyed_links.append(path)
+    return destroyed_links
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to check.')
+    args = parser.parse_args(argv)
+    destroyed_links = find_destroyed_symlinks(files=args.filenames)
+    if destroyed_links:
+        print('Destroyed symlinks:')
+        for destroyed_link in destroyed_links:
+            print(f'- {destroyed_link}')
+        print('You should unstage affected files:')
+        print(
+            '\tgit reset HEAD -- {}'.format(
+                ' '.join(shlex.quote(link) for link in destroyed_links),
+            ),
+        )
+        print(
+            'And retry commit. As a long term solution '
+            'you may try to explicitly tell git that your '
+            'environment does not support symlinks:',
+        )
+        print('\tgit config core.symlinks false')
+        return 1
+    else:
+        return 0
+
+
+if __name__ == '__main__':
+    exit(main())
diff --git a/pre_commit_hooks/util.py b/pre_commit_hooks/util.py
index e04b015..402e33e 100644
--- a/pre_commit_hooks/util.py
+++ b/pre_commit_hooks/util.py
@@ -1,5 +1,6 @@
 import subprocess
 from typing import Any
+from typing import List
 from typing import Optional
 from typing import Set
 
@@ -22,3 +23,11 @@ def cmd_output(*cmd: str, retcode: Optional[int] = 0, **kwargs: Any) -> str:
     if retcode is not None and proc.returncode != retcode:
         raise CalledProcessError(cmd, retcode, proc.returncode, stdout, stderr)
     return stdout
+
+
+def zsplit(s: str) -> List[str]:
+    s = s.strip('\0')
+    if s:
+        return s.split('\0')
+    else:
+        return []
diff --git a/setup.cfg b/setup.cfg
index 3c401fc..e2cad1a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -44,6 +44,7 @@ console_scripts =
     check-xml = pre_commit_hooks.check_xml:main
     check-yaml = pre_commit_hooks.check_yaml:main
     debug-statement-hook = pre_commit_hooks.debug_statement_hook:main
+    destroyed-symlinks = pre_commit_hooks.destroyed_symlinks:main
     detect-aws-credentials = pre_commit_hooks.detect_aws_credentials:main
     detect-private-key = pre_commit_hooks.detect_private_key:main
     double-quote-string-fixer = pre_commit_hooks.string_fixer:main


Test patch:
diff --git a/tests/check_executables_have_shebangs_test.py b/tests/check_executables_have_shebangs_test.py
index 7046081..5703ede 100644
--- a/tests/check_executables_have_shebangs_test.py
+++ b/tests/check_executables_have_shebangs_test.py
@@ -102,16 +102,6 @@ def test_check_git_filemode_failing(tmpdir):
         assert check_executables_have_shebangs._check_git_filemode(files) == 1
 
 
-@pytest.mark.parametrize('out', ('\0f1\0f2\0', '\0f1\0f2', 'f1\0f2\0'))
-def test_check_zsplits_correctly(out):
-    assert check_executables_have_shebangs.zsplit(out) == ['f1', 'f2']
-
-
-@pytest.mark.parametrize('out', ('\0\0', '\0', ''))
-def test_check_zsplit_returns_empty(out):
-    assert check_executables_have_shebangs.zsplit(out) == []
-
-
 @pytest.mark.parametrize(
     ('content', 'mode', 'expected'),
     (
diff --git a/tests/destroyed_symlinks_test.py b/tests/destroyed_symlinks_test.py
new file mode 100644
index 0000000..d2c9031
--- /dev/null
+++ b/tests/destroyed_symlinks_test.py
@@ -0,0 +1,74 @@
+import os
+import subprocess
+
+import pytest
+
+from pre_commit_hooks.destroyed_symlinks import find_destroyed_symlinks
+from pre_commit_hooks.destroyed_symlinks import main
+
+TEST_SYMLINK = 'test_symlink'
+TEST_SYMLINK_TARGET = '/doesnt/really/matters'
+TEST_FILE = 'test_file'
+TEST_FILE_RENAMED = f'{TEST_FILE}_renamed'
+
+
+@pytest.fixture
+def repo_with_destroyed_symlink(tmpdir):
+    source_repo = tmpdir.join('src')
+    os.makedirs(source_repo, exist_ok=True)
+    test_repo = tmpdir.join('test')
+    with source_repo.as_cwd():
+        subprocess.check_call(('git', 'init'))
+        os.symlink(TEST_SYMLINK_TARGET, TEST_SYMLINK)
+        with open(TEST_FILE, 'w') as f:
+            print('some random content', file=f)
+        subprocess.check_call(('git', 'add', '.'))
+        subprocess.check_call(
+            ('git', 'commit', '--no-gpg-sign', '-m', 'initial'),
+        )
+        assert b'120000 ' in subprocess.check_output(
+            ('git', 'cat-file', '-p', 'HEAD^{tree}'),
+        )
+    subprocess.check_call(
+        ('git', '-c', 'core.symlinks=false', 'clone', source_repo, test_repo),
+    )
+    with test_repo.as_cwd():
+        subprocess.check_call(
+            ('git', 'config', '--local', 'core.symlinks', 'true'),
+        )
+        subprocess.check_call(('git', 'mv', TEST_FILE, TEST_FILE_RENAMED))
+    assert not os.path.islink(test_repo.join(TEST_SYMLINK))
+    yield test_repo
+
+
+def test_find_destroyed_symlinks(repo_with_destroyed_symlink):
+    with repo_with_destroyed_symlink.as_cwd():
+        assert find_destroyed_symlinks([]) == []
+        assert main([]) == 0
+
+        subprocess.check_call(('git', 'add', TEST_SYMLINK))
+        assert find_destroyed_symlinks([TEST_SYMLINK]) == [TEST_SYMLINK]
+        assert find_destroyed_symlinks([]) == []
+        assert main([]) == 0
+        assert find_destroyed_symlinks([TEST_FILE_RENAMED, TEST_FILE]) == []
+        ALL_STAGED = [TEST_SYMLINK, TEST_FILE_RENAMED]
+        assert find_destroyed_symlinks(ALL_STAGED) == [TEST_SYMLINK]
+        assert main(ALL_STAGED) != 0
+
+        with open(TEST_SYMLINK, 'a') as f:
+            print(file=f)  # add trailing newline
+        subprocess.check_call(['git', 'add', TEST_SYMLINK])
+        assert find_destroyed_symlinks(ALL_STAGED) == [TEST_SYMLINK]
+        assert main(ALL_STAGED) != 0
+
+        with open(TEST_SYMLINK, 'w') as f:
+            print('0' * len(TEST_SYMLINK_TARGET), file=f)
+        subprocess.check_call(('git', 'add', TEST_SYMLINK))
+        assert find_destroyed_symlinks(ALL_STAGED) == []
+        assert main(ALL_STAGED) == 0
+
+        with open(TEST_SYMLINK, 'w') as f:
+            print('0' * (len(TEST_SYMLINK_TARGET) + 3), file=f)
+        subprocess.check_call(('git', 'add', TEST_SYMLINK))
+        assert find_destroyed_symlinks(ALL_STAGED) == []
+        assert main(ALL_STAGED) == 0
diff --git a/tests/util_test.py b/tests/util_test.py
index b42ee6f..7f48816 100644
--- a/tests/util_test.py
+++ b/tests/util_test.py
@@ -2,6 +2,7 @@ import pytest
 
 from pre_commit_hooks.util import CalledProcessError
 from pre_commit_hooks.util import cmd_output
+from pre_commit_hooks.util import zsplit
 
 
 def test_raises_on_error():
@@ -12,3 +13,13 @@ def test_raises_on_error():
 def test_output():
     ret = cmd_output('sh', '-c', 'echo hi')
     assert ret == 'hi\n'
+
+
+@pytest.mark.parametrize('out', ('\0f1\0f2\0', '\0f1\0f2', 'f1\0f2\0'))
+def test_check_zsplits_str_correctly(out):
+    assert zsplit(out) == ['f1', 'f2']
+
+
+@pytest.mark.parametrize('out', ('\0\0', '\0', ''))
+def test_check_zsplit_returns_empty(out):
+    assert zsplit(out) == []