gh-86768: check if fd is seekable in os.lseek on Windows

Lib/test/test_os/test_os.py

@@ -2788,6 +2788,14 @@ def test_ftruncate(self):
     def test_lseek(self):
         self.check(os.lseek, 0, 0)
 
+    @unittest.skipUnless(hasattr(os, 'lseek'), 'test needs os.lseek()')
+    @unittest.skipUnless(hasattr(os, 'pipe'), "need os.pipe()")
+    def test_lseek_on_pipe(self):
+        rfd, wfd = os.pipe()
+        self.addCleanup(os.close, rfd)
+        self.addCleanup(os.close, wfd)
+        self.assertRaises(OSError, os.lseek, rfd, 123, os.SEEK_END)
+
     @unittest.skipUnless(hasattr(os, 'read'), 'test needs os.read()')
     def test_read(self):
         self.check(os.read, 1)

Lib/test/test_winapi.py

@@ -144,7 +144,7 @@ def test_namedpipe(self):
         # Pipe instance is available, so this passes
         _winapi.WaitNamedPipe(pipe_name, 0)
 
-        with open(pipe_name, 'w+b') as pipe2:
+        with open(pipe_name, 'w+b', buffering=0) as pipe2:
             # No instances available, so this times out
             # (WinError 121 does not get mapped to TimeoutError)
             with self.assertRaises(OSError):

Misc/NEWS.d/next/Windows/2025-04-29-17-55-55.gh-issue-86768.uIDTHc.rst

@@ -0,0 +1,2 @@
+:exc:`OSError` will be raised if call :func:`os.lseek` with non seekable
+object on Windows.

Modules/_io/fileio.c

@@ -989,7 +989,14 @@ portable_lseek(fileio *self, PyObject *posobj, int whence, bool suppress_pipe_er
     Py_BEGIN_ALLOW_THREADS
     _Py_BEGIN_SUPPRESS_IPH
 #ifdef MS_WINDOWS
-    res = _lseeki64(fd, pos, whence);
+    HANDLE h = (HANDLE)_get_osfhandle(fd);
+    if (h != INVALID_HANDLE_VALUE && GetFileType(h) == FILE_TYPE_PIPE) {
+        res = -1;
+        errno = ESPIPE;
+    }
+    else {
+        res = _lseeki64(fd, pos, whence);
+    }
 #else
     res = lseek(fd, pos, whence);
 #endif

Modules/posixmodule.c

@@ -11915,7 +11915,7 @@ static Py_off_t
 os_lseek_impl(PyObject *module, int fd, Py_off_t position, int how)
 /*[clinic end generated code: output=971e1efb6b30bd2f input=4a3de549f07e1c40]*/
 {
-    Py_off_t result;
+    Py_off_t result = -1;
 
 #ifdef SEEK_SET
     /* Turn 0, 1, 2 into SEEK_{SET,CUR,END} */
@@ -11929,14 +11929,21 @@ os_lseek_impl(PyObject *module, int fd, Py_off_t position, int how)
     Py_BEGIN_ALLOW_THREADS
     _Py_BEGIN_SUPPRESS_IPH
 #ifdef MS_WINDOWS
-    result = _lseeki64(fd, position, how);
+    HANDLE h = (HANDLE)_get_osfhandle(fd);
+    if (h != INVALID_HANDLE_VALUE && GetFileType(h) == FILE_TYPE_PIPE) {
+        errno = ESPIPE;
+    }
+    else {
+        result = _lseeki64(fd, position, how);
+    }
 #else
     result = lseek(fd, position, how);
 #endif
     _Py_END_SUPPRESS_IPH
     Py_END_ALLOW_THREADS
-    if (result < 0)
+    if (result < 0) {
         posix_error();
+    }
 
     return result;
 }