closes bpo-37964: add F_GETPATH command to fcntl

Doc/library/fcntl.rst

@@ -33,6 +33,10 @@ descriptor.
    ``F_SEAL_*`` constants for sealing of :func:`os.memfd_create` file
    descriptors.
 
+.. versionchanged:: 3.9
+   On macOS, the fcntl module exposes the ``F_GETPATH`` constant, which obtains
+   the path of a file from a file descriptor.
+
 The module defines the following functions:
 
 

Lib/test/test_fcntl.py

@@ -144,6 +144,12 @@ def test_flock_overflow(self):
         self.assertRaises(OverflowError, fcntl.flock, _testcapi.INT_MAX+1,
                           fcntl.LOCK_SH)
 
+    @unittest.skipIf(sys.platform != 'darwin', "F_GETPATH is only available on macos")
+    def test_fcntl_f_getpath(self):
+        self.f = open(TESTFN, 'wb')
+        abspath = os.path.abspath(TESTFN)
+        res = fcntl.fcntl(self.f.fileno(), fcntl.F_GETPATH, bytes(len(abspath)))
+        self.assertEqual(abspath, res.decode('utf-8'))
 
 def test_main():
     run_unittest(TestFcntl)

Misc/NEWS.d/next/Library/2019-08-27-21-19-28.bpo-37964.SxdnsF.rst

@@ -0,0 +1 @@
+Add ``F_GETPATH`` command to :mod:`fcntl`.

Modules/fcntlmodule.c

@@ -501,6 +501,9 @@ all_ins(PyObject* m)
 #ifdef F_SETOWN
     if (PyModule_AddIntMacro(m, F_SETOWN)) return -1;
 #endif
+#ifdef F_GETPATH
+    if (PyModule_AddIntMacro(m, F_GETPATH)) return -1;
+#endif
 #ifdef F_GETSIG
     if (PyModule_AddIntMacro(m, F_GETSIG)) return -1;
 #endif