GH-87235: Make sure "python /dev/fd/9 9</path/to/script.py" works on macOS (#99768)

On macOS all file descriptors for a particular file in /dev/fd
share the same file offset, that is ``open("/dev/fd/9", "r")`` behaves
more like ``dup(9)`` than a regular open.

This causes problems when a user tries to run "/dev/fd/9" as a script
because zipimport changes the file offset to try to read a zipfile
directory. Therefore change zipimport to reset the file offset after
trying to read the zipfile directory.

Lib/test/test_cmd_line_script.py

@@ -752,6 +752,20 @@ class CmdLineTest(unittest.TestCase):
         self.assertIn(": can't open file ", err)
         self.assertNotEqual(proc.returncode, 0)
 
+    @unittest.skipUnless(os.path.exists('/dev/fd/0'), 'requires /dev/fd platform')
+    def test_script_as_dev_fd(self):
+        # GH-87235: On macOS passing a non-trivial script to /dev/fd/N can cause
+        # problems because all open /dev/fd/N file descriptors share the same
+        # offset.
+        script = 'print("12345678912345678912345")'
+        with os_helper.temp_dir() as work_dir:
+            script_name = _make_test_script(work_dir, 'script.py', script)
+            with open(script_name, "r") as fp:
+                p = spawn_python(f"/dev/fd/{fp.fileno()}", close_fds=False, pass_fds=(0,1,2,fp.fileno()))
+                out, err = p.communicate()
+                self.assertEqual(out, b"12345678912345678912345\n")
+
+
 
 def tearDownModule():
     support.reap_children()

Lib/zipimport.py

@@ -347,6 +347,11 @@ def _read_directory(archive):
         raise ZipImportError(f"can't open Zip file: {archive!r}", path=archive)
 
     with fp:
+        # GH-87235: On macOS all file descriptors for /dev/fd/N share the same
+        # file offset, reset the file offset after scanning the zipfile diretory
+        # to not cause problems when some runs 'python3 /dev/fd/9 9<some_script'
+        start_offset = fp.tell()
+        try:
             try:
                 fp.seek(-END_CENTRAL_DIR_SIZE, 2)
                 header_position = fp.tell()
@@ -455,6 +460,8 @@ def _read_directory(archive):
                 t = (path, compress, data_size, file_size, file_offset, time, date, crc)
                 files[name] = t
                 count += 1
+        finally:
+            fp.seek(start_offset)
     _bootstrap._verbose_message('zipimport: found {} names in {!r}', count, archive)
     return files
 

Misc/NEWS.d/next/macOS/2022-11-25-09-23-20.gh-issue-87235.SifjCD.rst

@@ -0,0 +1 @@
+On macOS ``python3 /dev/fd/9 9</path/to/script.py`` failed for any script longer than a couple of bytes.