Issue #17222: Raise FileExistsError when py_compile.compile would

overwrite a symlink or non-regular file with a regular file.

2 files changed, 34 insertions, 0 deletions

diff --git a/Lib/py_compile.py b/Lib/py_compile.py
index 701e8ac..cee35a5 100644
--- a/Lib/py_compile.py
+++ b/Lib/py_compile.py
@@ -7,6 +7,7 @@ import imp
 import importlib._bootstrap
 import importlib.machinery
 import os
+import os.path
 import sys
 import traceback
 
@@ -96,12 +97,25 @@ def compile(file, cfile=None, dfile=None, doraise=False, optimize=-1):
     See compileall.py for a script/module that uses this module to
     byte-compile all installed files (or all files in selected
     directories).
+
+    Do note that FileExistsError is raised if cfile ends up pointing at a
+    non-regular file or symlink. Because the compilation uses a file renaming,
+    the resulting file would be regular and thus not the same type of file as
+    it was previously.
     """
     if cfile is None:
         if optimize >= 0:
             cfile = imp.cache_from_source(file, debug_override=not optimize)
         else:
             cfile = imp.cache_from_source(file)
+    if os.path.islink(cfile):
+        msg = ('{} is a symlink and will be changed into a regular file if '
+               'import writes a byte-compiled file to it')
+        raise FileExistsError(msg.format(file, cfile))
+    elif os.path.exists(cfile) and not os.path.isfile(cfile):
+        msg = ('{} is a non-regular file and will be changed into a regular '
+               'one if import writes a byte-compiled file to it')
+        raise FileExistsError(msg.format(file, cfile))
     loader = importlib.machinery.SourceFileLoader('<py_compile>', file)
     source_bytes = loader.get_data(file)
     try:
diff --git a/Lib/test/test_py_compile.py b/Lib/test/test_py_compile.py
index e179554..54dc596 100644
--- a/Lib/test/test_py_compile.py
+++ b/Lib/test/test_py_compile.py
@@ -36,6 +36,26 @@ class PyCompileTests(unittest.TestCase):
         self.assertTrue(os.path.exists(self.pyc_path))
         self.assertFalse(os.path.exists(self.cache_path))
 
+    def test_do_not_overwrite_symlinks(self):
+        # In the face of a cfile argument being a symlink, bail out.
+        # Issue #17222
+        try:
+            os.symlink(self.pyc_path + '.actual', self.pyc_path)
+        except OSError:
+            self.skipTest('need to be able to create a symlink for a file')
+        else:
+            assert os.path.islink(self.pyc_path)
+            with self.assertRaises(FileExistsError):
+                py_compile.compile(self.source_path, self.pyc_path)
+
+    @unittest.skipIf(not os.path.exists(os.devnull) or os.path.isfile(os.devnull),
+                     'requires os.devnull and for it to be a non-regular file')
+    def test_do_not_overwrite_nonregular_files(self):
+        # In the face of a cfile argument being a non-regular file, bail out.
+        # Issue #17222
+        with self.assertRaises(FileExistsError):
+            py_compile.compile(self.source_path, os.devnull)
+
     def test_cache_path(self):
         py_compile.compile(self.source_path)
         self.assertTrue(os.path.exists(self.cache_path))