Issue #23668: Adds support for os.truncate and os.ftruncate on Windows

Doc/library/io.rst

@@ -339,8 +339,11 @@ I/O Base Classes
       if *size* is not specified).  The current stream position isn't changed.
       This resizing can extend or reduce the current file size.  In case of
       extension, the contents of the new file area depend on the platform
-      (on most systems, additional bytes are zero-filled, on Windows they're
-      undetermined).  The new file size is returned.
+      (on most systems, additional bytes are zero-filled).  The new file size
+      is returned.
+
+   .. versionchanged:: 3.5
+      Windows will now zero-fill files when extending.
 
    .. method:: writable()
 

Doc/library/os.rst

@@ -805,8 +805,10 @@ as internal buffering of data.
    most *length* bytes in size.  As of Python 3.3, this is equivalent to
    ``os.truncate(fd, length)``.
 
-   Availability: Unix.
+   Availability: Unix, Windows.
 
+   .. versionchanged:: 3.5
+      Added support for Windows
 
 .. function:: get_blocking(fd)
 
@@ -2492,10 +2494,12 @@ features:
 
    This function can support :ref:`specifying a file descriptor <path_fd>`.
 
-   Availability: Unix.
+   Availability: Unix, Windows.
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.5
+      Added support for Windows
 
 .. function:: unlink(path, *, dir_fd=None)
 

Misc/NEWS

@@ -286,6 +286,8 @@ Library
 
 - Issue #2052: Add charset parameter to HtmlDiff.make_file().
 
+- Issue #23668: Support os.truncate and os.ftruncate on Windows.
+
 - Issue #23138: Fixed parsing cookies with absent keys or values in cookiejar.
   Patch by Demian Brecht.
 

Modules/_io/fileio.c

@@ -839,9 +839,7 @@ static PyObject *
 fileio_truncate(fileio *self, PyObject *args)
 {
     PyObject *posobj = NULL; /* the new size wanted by the user */
-#ifndef MS_WINDOWS
     Py_off_t pos;
-#endif
     int ret;
     int fd;
 
@@ -864,52 +862,6 @@ fileio_truncate(fileio *self, PyObject *args)
         Py_INCREF(posobj);
     }
 
-#ifdef MS_WINDOWS
-    /* MS _chsize doesn't work if newsize doesn't fit in 32 bits,
-       so don't even try using it. */
-    {
-        PyObject *oldposobj, *tempposobj;
-        HANDLE hFile;
-
-        /* we save the file pointer position */
-        oldposobj = portable_lseek(fd, NULL, 1);
-        if (oldposobj == NULL) {
-            Py_DECREF(posobj);
-            return NULL;
-        }
-
-        /* we then move to the truncation position */
-        tempposobj = portable_lseek(fd, posobj, 0);
-        if (tempposobj == NULL) {
-            Py_DECREF(oldposobj);
-            Py_DECREF(posobj);
-            return NULL;
-        }
-        Py_DECREF(tempposobj);
-
-        /* Truncate.  Note that this may grow the file! */
-        Py_BEGIN_ALLOW_THREADS
-        errno = 0;
-        hFile = (HANDLE)_get_osfhandle(fd);
-        ret = hFile == (HANDLE)-1; /* testing for INVALID_HANDLE value */
-        if (ret == 0) {
-            ret = SetEndOfFile(hFile) == 0;
-            if (ret)
-                errno = EACCES;
-        }
-        Py_END_ALLOW_THREADS
-
-        /* we restore the file pointer position in any case */
-        tempposobj = portable_lseek(fd, oldposobj, 0);
-        Py_DECREF(oldposobj);
-        if (tempposobj == NULL) {
-            Py_DECREF(posobj);
-            return NULL;
-        }
-        Py_DECREF(tempposobj);
-    }
-#else
-
 #if defined(HAVE_LARGEFILE_SUPPORT)
     pos = PyLong_AsLongLong(posobj);
 #else
@@ -922,11 +874,13 @@ fileio_truncate(fileio *self, PyObject *args)
 
     Py_BEGIN_ALLOW_THREADS
     errno = 0;
+#ifdef MS_WINDOWS
+    ret = _chsize_s(fd, pos);
+#else
     ret = ftruncate(fd, pos);
+#endif
     Py_END_ALLOW_THREADS
 
-#endif /* !MS_WINDOWS */
-
     if (ret != 0) {
         Py_DECREF(posobj);
         PyErr_SetFromErrno(PyExc_IOError);

Modules/posixmodule.c

@@ -2315,6 +2315,10 @@ FTRUNCATE
 #endif
 /*[python end generated code: output=4bd4f6f7d41267f1 input=80b4c890b6774ea5]*/
 
+#ifdef MS_WINDOWS
+    #undef PATH_HAVE_FTRUNCATE
+    #define PATH_HAVE_FTRUNCATE 1
+#endif
 
 /*[python input]
 
@@ -8753,7 +8757,7 @@ os_makedev_impl(PyModuleDef *module, int major, int minor)
 #endif /* HAVE_DEVICE_MACROS */
 
 
-#ifdef HAVE_FTRUNCATE
+#if defined HAVE_FTRUNCATE || defined MS_WINDOWS
 /*[clinic input]
 os.ftruncate
 
@@ -8771,9 +8775,16 @@ os_ftruncate_impl(PyModuleDef *module, int fd, Py_off_t length)
     int result;
     int async_err = 0;
 
+    if (!_PyVerify_fd(fd))
+        return posix_error();
+
     do {
         Py_BEGIN_ALLOW_THREADS
+#ifdef MS_WINDOWS
+        result = _chsize_s(fd, length);
+#else
         result = ftruncate(fd, length);
+#endif
         Py_END_ALLOW_THREADS
     } while (result != 0 && errno == EINTR &&
              !(async_err = PyErr_CheckSignals()));
@@ -8781,10 +8792,10 @@ os_ftruncate_impl(PyModuleDef *module, int fd, Py_off_t length)
         return (!async_err) ? posix_error() : NULL;
     Py_RETURN_NONE;
 }
-#endif /* HAVE_FTRUNCATE */
+#endif /* HAVE_FTRUNCATE || MS_WINDOWS */
 
 
-#ifdef HAVE_TRUNCATE
+#if defined HAVE_TRUNCATE || defined MS_WINDOWS
 /*[clinic input]
 os.truncate
     path: path_t(allow_fd='PATH_HAVE_FTRUNCATE')
@@ -8801,21 +8812,37 @@ os_truncate_impl(PyModuleDef *module, path_t *path, Py_off_t length)
 /*[clinic end generated code: output=f60a9e08370e9e2e input=77229cf0b50a9b77]*/
 {
     int result;
+#ifdef MS_WINDOWS
+    int fd;
+#endif
+
+    if (path->fd != -1)
+        return os_ftruncate_impl(module, path->fd, length);
 
     Py_BEGIN_ALLOW_THREADS
-#ifdef HAVE_FTRUNCATE
-    if (path->fd != -1)
-        result = ftruncate(path->fd, length);
+#ifdef MS_WINDOWS
+    if (path->wide)
+        fd = _wopen(path->wide, _O_WRONLY | _O_BINARY | _O_NOINHERIT);
     else
+        fd = _open(path->narrow, _O_WRONLY | _O_BINARY | _O_NOINHERIT);
+    if (fd < 0) 
+        result = -1;
+    else {
+        result = _chsize_s(fd, length);
+        close(fd);
+        if (result < 0)
+            errno = result;
+    }
+#else
+    result = truncate(path->narrow, length);
 #endif
-        result = truncate(path->narrow, length);
     Py_END_ALLOW_THREADS
     if (result < 0)
         return path_error(path);
 
     Py_RETURN_NONE;
 }
-#endif /* HAVE_TRUNCATE */
+#endif /* HAVE_TRUNCATE || MS_WINDOWS */
 
 
 /* Issue #22396: On 32-bit AIX platform, the prototypes of os.posix_fadvise()
@@ -12771,7 +12798,7 @@ static char *have_functions[] = {
     "HAVE_FSTATVFS",
 #endif
 
-#ifdef HAVE_FTRUNCATE
+#if defined HAVE_FTRUNCATE || defined MS_WINDOWS
     "HAVE_FTRUNCATE",
 #endif