Re: [PATCH 6/7] gdb/testsuite/gdb.debuginfod: Add lazy downloading tests

[Aaron Merey <amerey@redhat.com>]

Hi Andrew,

On Tue, May 2, 2023 at 11:48 AM Andrew Burgess <aburgess@redhat.com> wrote:
> I compile GDB with both '-D_GLIBCXX_DEBUG=1' and
> '-D_GLIBCXX_DEBUG_PEDANTIC=1' defined.
>
> After applying this series I see the following GDB crash when running
> the fetch_src_and_symbols.exp test:
>
>   (gdb) file
>   No executable file now.
>   /usr/include/c++/9/debug/safe_iterator.h:597:
>   In function:
>       __gnu_debug::_Safe_iterator<_Iterator, _Sequence,
>       std::bidirectional_iterator_tag>& __gnu_debug::_Safe_iterator<_Iterator,
>       _Sequence, std::bidirectional_iterator_tag>::operator--() [with
>       _Iterator = std::__cxx1998::_List_iterator<std::unique_ptr<objfile> >;
>       _Sequence = std::__debug::list<std::unique_ptr<objfile> >]
>
>   Error: attempt to decrement a dereferenceable (start-of-sequence) iterator.

Thanks for spotting this. It looks like the culprit is from the following change
in patch 5/7 [1]:

diff --git a/gdbsupport/iterator-range.h b/gdbsupport/iterator-range.h
index e934f5ebf01..853ebb9074a 100644
--- a/gdbsupport/iterator-range.h
+++ b/gdbsupport/iterator-range.h
@@ -61,4 +61,49 @@ struct iterator_range
+template <typename IteratorType>
+struct reverse_iterator_range
+{
+  using iterator = IteratorType;
+
+  /* Create a reverse iterator range using explicit BEGIN and END
iterators.  */
+  template <typename... Args>
+  reverse_iterator_range (IteratorType begin, IteratorType end)
+    : m_begin (std::move (end)), m_end (std::move (begin))
+  {
+    if (m_begin != m_end)
+      {
+ /* M_BEGIN is the base iterator's one-past-the-end.  Increment it
+   so it points to the ranges's last element.  */
+ ++m_begin;
+
+ /* M_END points to the range's first element.  Increment it so it's
+   one-past-the-end.  */
+ ++m_end;
+      }
+  }
[...]

'++m_end' has the effect of decrementing a std::list begin() iterator, though it
should never be dereferenced. I'll fix this and post a follow up to patch 5/7.

Aaron

[1] https://sourceware.org/pipermail/gdb-patches/2023-April/199076.html