bug #1461855: make os.fdopen() add the O_APPEND flag if using "a" mode.

glibc, for example, does this already on its own, but it seems that
the solaris libc doesn't. This leads to Python code being able to over-
write file contents even though having specified "a" mode.

Modules/posixmodule.c

@@ -5768,9 +5768,20 @@ posix_fdopen(PyObject *self, PyObject *args)
 			     "invalid file mode '%s'", mode);
 		return NULL;
 	}
-
 	Py_BEGIN_ALLOW_THREADS
-	fp = fdopen(fd, mode);
+	if (mode[0] == 'a') {
+		/* try to make sure the O_APPEND flag is set */
+		int flags;
+		flags = fcntl(fd, F_GETFL);
+		if (flags != -1)
+			fcntl(fd, F_SETFL, flags | O_APPEND);
+		fp = fdopen(fd, mode);
+		if (fp == NULL)
+			/* restore old mode if fdopen failed */
+			fcntl(fd, F_SETFL, flags);
+	} else {
+		fp = fdopen(fd, mode);
+	}
 	Py_END_ALLOW_THREADS
 	if (fp == NULL)
 		return posix_error();