[New-bugs-announce] [issue19503] does gc_list_merge merge properly?

[martenjan]

New submission from martenjan:

The code for gc_list_merge is given in gcmodule.c. I retrieved it from
http://hg.python.org/cpython/file/tip/Modules/gcmodule.c#l287

The code seems to merge list `from` incompletely: the first entry of `from` is omitted in the merged list. 

The issue is in line 295:   tail->gc.gc_next = from->gc.gc_next;
I fixed it as :             tail->gc.gc_next = from;

Please check if my analysis is correct.


See below for the context.

Original: lines 287 to 301

/* append list `from` onto list `to`; `from` becomes an empty list */
static void
gc_list_merge(PyGC_Head *from, PyGC_Head *to)
{
    PyGC_Head *tail;
    assert(from != to);
    if (!gc_list_is_empty(from)) {
        tail = to->gc.gc_prev;
        tail->gc.gc_next = from->gc.gc_next;
        tail->gc.gc_next->gc.gc_prev = tail;
        to->gc.gc_prev = from->gc.gc_prev;
        to->gc.gc_prev->gc.gc_next = to;
    }
    gc_list_init(from);
}


Fix:
/* append list `from` onto list `to`; `from` becomes an empty list */
static void
gc_list_merge(PyGC_Head *from, PyGC_Head *to)
{
    PyGC_Head *tail;
    assert(from != to);
    if (!gc_list_is_empty(from)) {
        tail = to->gc.gc_prev;
        tail->gc.gc_next = from;
        tail->gc.gc_next->gc.gc_prev = tail;
        to->gc.gc_prev = from->gc.gc_prev;
        to->gc.gc_prev->gc.gc_next = to;
    }
    gc_list_init(from);
}

----------
components: Interpreter Core
messages: 202206
nosy: martenjan
priority: normal
severity: normal
status: open
title: does gc_list_merge merge properly?
type: behavior
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19503>
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