gh-78274: Produce consistent output from marshal.dumps().

Misc/NEWS.d/next/Core and Builtins/2021-09-15-22-33-31.bpo-34093.O-gZHM.rst

@@ -0,0 +1,2 @@
+Make marshal output the same whether or not it's a debug build. The fix has
+a side-effect of making un-marshaling a little faster.

Python/marshal.c

@@ -87,6 +87,7 @@ typedef struct{
  const char *end;
  char *buf;
  _Py_hashtable_t *hashtable;
+ Py_ssize_t refs_numobjects;
  int version;
 } WFILE;
 
@@ -305,32 +306,63 @@ w_ref(PyObject *v, char *flag, WFILE *p)
 
  entry = _Py_hashtable_get_entry(p->hashtable, v);
  if (entry != NULL){
- /* write the reference index to the stream */
  w = (int)(uintptr_t)entry->value;
- /* we don't store "long" indices in the dict */
- assert(0 <= w && w <= 0x7fffffff);
- w_byte(TYPE_REF, p);
- w_long(w, p);
- return 1;
- } else{
- size_t s = p->hashtable->nentries;
+ if (p->refs_numobjects < 0){
+ /* It's an extra ref on the first pass. */
+ w -= 1;
+ entry->value = (void *)(uintptr_t)w;
+ w_byte(TYPE_REF, p);
+ w_long(-1, p);
+ return 1;
+ }
+ else if (w >= 0){
+ /* It's an extra ref on the second (or only) pass. */
+ /* write the reference index to the stream */
+ /* we don't store "long" indices in the dict */
+ assert(w <= 0x7fffffff);
+ w_byte(TYPE_REF, p);
+ w_long(w, p);
+ return 1;
+ }
+ else if (w == -1){
+ /* It's the first ref on the second pass, with no extra refs.
+ So we treat it as though it has a refcount of 1. */
+ return 0;
+ }
+ else{
+ /* It's the first ref on the second pass, with extra refs, */
+ assert(w < 0);
+ entry->value = (void *)(uintptr_t)p->refs_numobjects;
+ p->refs_numobjects += 1;
+ *flag |= FLAG_REF;
+ return 0;
+ }
+ }
+
+ /* It's the object's first ref. */
+ if (p->refs_numobjects < 0){
+ /* It's the first pass. */
+ w = -1;
+ }
+ else{
+ /* It's the second (or only) pass. */
  /* we don't support long indices */
- if (s >= 0x7fffffff){
+ if (p->refs_numobjects >= 0x7fffffff){
  PyErr_SetString(PyExc_ValueError, "too many objects");
- goto err;
- }
- w = (int)s;
- Py_INCREF(v);
- if (_Py_hashtable_set(p->hashtable, v, (void *)(uintptr_t)w) < 0){
- Py_DECREF(v);
- goto err;
+ p->error = WFERR_UNMARSHALLABLE;
+ return 1;
  }
-*flag |= FLAG_REF;
-return 0;
+w = (int)p->refs_numobjects;
+p->refs_numobjects += 1;
  }
-err:
- p->error = WFERR_UNMARSHALLABLE;
- return 1;
+ Py_INCREF(v);
+ if (_Py_hashtable_set(p->hashtable, v, (void *)(uintptr_t)w) < 0){
+ Py_DECREF(v);
+ p->error = WFERR_UNMARSHALLABLE;
+ return 1;
+ }
+ *flag |= FLAG_REF;
+ return 0;
 }
 
 static void
@@ -1645,8 +1677,8 @@ PyMarshal_ReadObjectFromString(const char *str, Py_ssize_t len)
  return result;
 }
 
-PyObject *
-PyMarshal_WriteObjectToString(PyObject *x, int version)
+static PyObject *
+write_object_to_string(PyObject *x, int version, int stable)
 {
  WFILE wf;
 
@@ -1665,7 +1697,17 @@ PyMarshal_WriteObjectToString(PyObject *x, int version)
  Py_DECREF(wf.str);
  return NULL;
  }
+ if (stable){
+ /* Make a first pass tracking which objects have multiple refs. */
+ wf.refs_numobjects = -1;
+ w_object(x, &wf);
+ /* Make a second pass de-duplicating the multiple refs. */
+ wf.refs_numobjects = 0;
+ wf.ptr = wf.buf = PyBytes_AS_STRING(wf.str);
+ wf.end = wf.ptr + PyBytes_GET_SIZE(wf.str);
+ }
  w_object(x, &wf);
+
  w_clear_refs(&wf);
  if (wf.str != NULL){
  const char *base = PyBytes_AS_STRING(wf.str);
@@ -1685,6 +1727,12 @@ PyMarshal_WriteObjectToString(PyObject *x, int version)
  return wf.str;
 }
 
+PyObject *
+PyMarshal_WriteObjectToString(PyObject *x, int version)
+{
+ return write_object_to_string(x, version, 0);
+}
+
 /* And an interface for Python programs... */
 /*[clinic input]
 marshal.dump
@@ -1789,6 +1837,8 @@ marshal.dumps
  version: int(c_default="Py_MARSHAL_VERSION") = version
  Indicates the data format that dumps should use.
  /
+ *
+ stable: bool = True
 
 Return the bytes object that would be written to a file by dump(value, file).
 
@@ -1797,10 +1847,11 @@ unsupported type.
 [clinic start generated code]*/
 
 static PyObject *
-marshal_dumps_impl(PyObject *module, PyObject *value, int version)
-/*[clinic end generated code: output=9c200f98d7256cad input=a2139ea8608e9b27]*/
+marshal_dumps_impl(PyObject *module, PyObject *value, int version,
+ int stable)
+/*[clinic end generated code: output=87276039e6c75faf input=b460e8ba0aa325ac]*/
 {
- return PyMarshal_WriteObjectToString(value, version);
+ return write_object_to_string(value, version, stable);
 }
 
 /*[clinic input]