bpo-42606: Support POSIX atomicity guarantee of O_APPEND on Windows

Include/cpython/fileutils.h

@@ -91,6 +91,13 @@ PyAPI_FUNC(int) _Py_open_noraise(
     const char *pathname,
     int flags);
 
+#ifdef MS_WINDOWS
+PyAPI_FUNC(int) _Py_wopen_noraise(
+    const wchar_t *pathname,
+    int flags,
+    int mode);
+#endif
+
 PyAPI_FUNC(FILE *) _Py_wfopen(
     const wchar_t *path,
     const wchar_t *mode);

Misc/NEWS.d/next/Windows/2020-12-09-03-03-02.bpo-42606.IJ9e6N.rst

@@ -0,0 +1,2 @@
+Support POSIX atomicity guarantee of ``O_APPEND`` on Windows for :func:`open`
+and :func:`os.open`.

Modules/_io/fileio.c

@@ -381,7 +381,7 @@ _Py_COMP_DIAG_POP
             do {
                 Py_BEGIN_ALLOW_THREADS
 #ifdef MS_WINDOWS
-                self->fd = _wopen(widename, flags, 0666);
+                self->fd = _Py_wopen_noraise(widename, flags, 0666);
 #else
                 self->fd = open(name, flags, 0666);
 #endif

Modules/posixmodule.c

@@ -9060,7 +9060,7 @@ os_open_impl(PyObject *module, path_t *path, int flags, int mode, int dir_fd)
     do {
         Py_BEGIN_ALLOW_THREADS
 #ifdef MS_WINDOWS
-        fd = _wopen(path->wide, flags, mode);
+        fd = _Py_wopen_noraise(path->wide, flags, mode);
 #else
 #ifdef HAVE_OPENAT
         if (dir_fd != DEFAULT_DIR_FD) {

Python/fileutils.c

@@ -1415,6 +1415,76 @@ _Py_open_noraise(const char *pathname, int flags)
     return _Py_open_impl(pathname, flags, 0);
 }
 
+#ifdef MS_WINDOWS
+/* Open a file like _wopen(). In case it is a normal disk file and O_APPEND is
+   in flags, ensure that for each write() the file offset update and the write
+   are performed as a single atomic step, so that two processes concurrently
+   appending to the same file can never overwrite each others data.
+
+   Return a file descriptor on success. Set errno and return -1 on error. */
+int
+_Py_wopen_noraise(const wchar_t *pathname, int flags, int mode)
+{
+    int fd;
+    _Py_BEGIN_SUPPRESS_IPH
+    fd = _wopen(pathname, flags, mode);
+    _Py_END_SUPPRESS_IPH
+
+    if (fd < 0 || !(flags & O_APPEND) || !(flags & (O_WRONLY | O_RDWR))) {
+        return fd;
+    }
+
+    HANDLE src;
+    _Py_BEGIN_SUPPRESS_IPH
+    src = (HANDLE)_get_osfhandle(fd);
+    _Py_END_SUPPRESS_IPH
+
+    if (src == INVALID_HANDLE_VALUE) {
+        goto error;
+    }
+
+    /* FILE_APPEND_DATA is meaningful only for normal files. */
+    if (GetFileType(src) != FILE_TYPE_DISK) {
+        return fd;
+    }
+
+    /* Use the access rights corresponding to GENERIC_WRITE for files,
+     * but remove FILE_WRITE_DATA, leaving only FILE_APPEND_DATA. */
+    DWORD access = FILE_READ_ATTRIBUTES | FILE_GENERIC_WRITE ^ FILE_WRITE_DATA;
+    assert(access & FILE_APPEND_DATA);
+    if (flags & O_RDWR) {
+        access |= FILE_GENERIC_READ;
+    }
+
+    HANDLE dst;
+    if (!DuplicateHandle(GetCurrentProcess(), src, GetCurrentProcess(), &dst,
+                         access, !(flags & O_NOINHERIT), 0)) {
+        errno = winerror_to_errno(GetLastError());
+        goto error;
+    }
+
+    _Py_BEGIN_SUPPRESS_IPH
+    (void)close(fd);
+     fd = _open_osfhandle((intptr_t)dst, flags & ~O_APPEND);
+    _Py_END_SUPPRESS_IPH
+
+    if (fd < 0) {
+        CloseHandle(dst);
+        return -1;
+    }
+
+    return fd;
+
+error:;
+    int err = errno;
+    _Py_BEGIN_SUPPRESS_IPH
+    (void)close(fd);
+    _Py_END_SUPPRESS_IPH
+    errno = err;
+    return -1;
+}
+#endif
+
 /* Open a file. Use _wfopen() on Windows, encode the path to the locale
    encoding and use fopen() otherwise.