Checking in the new, improve file.writelines() code.

This (1) avoids thread unsafety whereby another thread could zap the
list while we were using it, and (2) now supports writing arbitrary
sequences of strings.

Objects/fileobject.c

@@ -890,40 +890,94 @@ file_writelines(f, args)
 	PyFileObject *f;
 	PyObject *args;
 {
-	int i, n;
+#define CHUNKSIZE 1000
+	PyObject *list, *line;
+	PyObject *result;
+	int i, j, index, len, nwritten, islist;
+
 	if (f->f_fp == NULL)
 		return err_closed();
-	if (args == NULL || !PyList_Check(args)) {
+	if (args == NULL || !PySequence_Check(args)) {
 		PyErr_SetString(PyExc_TypeError,
-			   "writelines() requires list of strings");
+			   "writelines() requires sequence of strings");
 		return NULL;
 	}
-	n = PyList_Size(args);
-	f->f_softspace = 0;
-	Py_BEGIN_ALLOW_THREADS
-	errno = 0;
-	for (i = 0; i < n; i++) {
-		PyObject *line = PyList_GetItem(args, i);
-		int len;
-		int nwritten;
-		if (!PyString_Check(line)) {
-			Py_BLOCK_THREADS
-			PyErr_SetString(PyExc_TypeError,
-				   "writelines() requires list of strings");
+	islist = PyList_Check(args);
+
+	/* Strategy: slurp CHUNKSIZE lines into a private list,
+	   checking that they are all strings, then write that list
+	   without holding the interpreter lock, then come back for more. */
+	index = 0;
+	if (islist)
+		list = NULL;
+	else {
+		list = PyList_New(CHUNKSIZE);
+		if (list == NULL)
 			return NULL;
+	}
+	result = NULL;
+
+	for (;;) {
+		if (islist) {
+			Py_XDECREF(list);
+			list = PyList_GetSlice(args, index, index+CHUNKSIZE);
+			if (list == NULL)
+				return NULL;
+			j = PyList_GET_SIZE(list);
 		}
-		len = PyString_Size(line);
-		nwritten = fwrite(PyString_AsString(line), 1, len, f->f_fp);
-		if (nwritten != len) {
-			Py_BLOCK_THREADS
-			PyErr_SetFromErrno(PyExc_IOError);
-			clearerr(f->f_fp);
-			return NULL;
+		else {
+			for (j = 0; j < CHUNKSIZE; j++) {
+				line = PySequence_GetItem(args, index+j);
+				if (line == NULL) {
+					if (PyErr_ExceptionMatches(
+						PyExc_IndexError))
+					{
+						PyErr_Clear();
+						break;
+					}
+					/* Some other error occurred.
+					   XXX We may lose some output. */
+					goto error;
+				}
+				if (!PyString_Check(line)) {
+					Py_DECREF(line);
+					PyErr_SetString(PyExc_TypeError,
+				 "writelines() requires sequences of strings");
+					goto error;
+				}
+				PyList_SetItem(list, j, line);
+			}
 		}
+		if (j == 0)
+			break;
+
+		Py_BEGIN_ALLOW_THREADS
+		f->f_softspace = 0;
+		errno = 0;
+		for (i = 0; i < j; i++) {
+			line = PyList_GET_ITEM(list, i);
+			len = PyString_GET_SIZE(line);
+			nwritten = fwrite(PyString_AS_STRING(line),
+					  1, len, f->f_fp);
+			if (nwritten != len) {
+				Py_BLOCK_THREADS
+				PyErr_SetFromErrno(PyExc_IOError);
+				clearerr(f->f_fp);
+				goto error;
+			}
+		}
+		Py_END_ALLOW_THREADS
+
+		if (j < CHUNKSIZE)
+			break;
+		index += CHUNKSIZE;
 	}
-	Py_END_ALLOW_THREADS
+
 	Py_INCREF(Py_None);
-	return Py_None;
+	result = Py_None;
+  error:
+	Py_XDECREF(list);
+	return result;
 }
 
 static PyMethodDef file_methods[] = {