fix: FileIO.seekable incorrectly returning false

Author: KowalskiThomas

Lib/test/test_io/test_fileio.py

@@ -62,6 +62,10 @@ def testSeekTell(self):
         self.f.seek(-5, 2)
         self.assertEqual(self.f.tell(), 15)
 
+    def testSeekableAfterFailedSeek(self) -> None:
+        self.assertRaises(OSError, self.f.seek, -1)
+        self.assertTrue(self.f.seekable())
+
     def testAttributes(self):
         # verify expected attributes exist
         f = self.f

Modules/_io/fileio.c

@@ -1013,7 +1013,7 @@ portable_lseek(fileio *self, PyObject *posobj, int whence, bool suppress_pipe_er
     _Py_END_SUPPRESS_IPH
     Py_END_ALLOW_THREADS
 
-    if (self->seekable < 0) {
+    if (self->seekable < 0 && (res >= 0 || errno == ESPIPE || posobj == NULL)) {
         self->seekable = (res >= 0);
     }