Re: [PATCH 5/7 v2] gdb/progspace: Add reverse safe iterator and template for unwrapping iterator

[Aaron Merey <amerey@redhat.com>]

Ping

Thanks,
Aaron

On Wed, Aug 16, 2023 at 12:43 AM Aaron Merey <amerey@redhat.com> wrote:
>
> v1: https://sourceware.org/pipermail/gdb-patches/2023-June/199984.html
>
> v2 removes unwrapping_reverse_objfile_iterator and adds
> basic_safe_reverse_range and basic_safe_reverse_iterator.
>
> Commit message:
>
> This patch changes progspace objfile_list insertion so that separate
> debug objfiles are placed into the list after the parent objfile,
> instead of before.  Additionally qf_require_partial_symbols now returns
> a safe_range.
>
> These changes are intended to prepare gdb for on-demand debuginfo
> downloading and the downloading of .gdb_index sections.
>
> With on-demand downloading enabled, gdb might need to delete a
> .gdb_index quick_symbol_functions from a parent objfile while looping
> the objfile's list of quick_symbol_functions becasue the separate
> debug objfile has just been downloaded.  The use of a safe_range
> prevents this removal from causing iterator invalidation.
>
> gdb might also download a debuginfo file during symtab expansion.
> In this case an objfile will be added to the current progspace's
> objfiles_list during iteration over the list (for example, in
> iterate_over_symtabs).  We want these loops to also iterate over
> newly downloaded objfiles.  So objfiles need to be inserted into
> objfiles_list after their parent since it is during the search of
> the parent objfile for some symbol or filename that the separate
> debug objfile might be downloaded.
>
> To facilitate the safe deletion of objfiles, this patch also adds
> basic_safe_reverse_range and basic_safe_reverse_iterator.  This allows
> objfiles to be removed from the objfiles_list in a loop without iterator
> invalidation.
>
> If a forward safe iterator were to be used, the deletion of an
> objfile could invalidate the safe iterator's reference to the next
> objfile in the objfiles_list.  This can happen when the deletion
> of an objfile causes the deletion of a separate debug objfile that
> happens to the be next element in the objfiles_list.
>
> The standard reverse iterator is not suitable for safe objfile deletion.
> In order to safely delete the first objfile in the objfiles_list, the
> standard reverse iterator's underlying begin iterator would have to be
> decremented, resulting in undefined behavior.
>
> A small change was also made to a testcase in py-objfile.exp to
> account for the new placement of separate debug objfiles in
> objfiles_list.
> ---
>  gdb/jit.c                               |   7 +-
>  gdb/objfiles.c                          |   8 +-
>  gdb/objfiles.h                          |   8 +-
>  gdb/progspace.c                         |  19 ++++-
>  gdb/progspace.h                         |  31 ++++---
>  gdb/testsuite/gdb.python/py-objfile.exp |   2 +-
>  gdbsupport/safe-iterator.h              | 106 ++++++++++++++++++++++++
>  7 files changed, 154 insertions(+), 27 deletions(-)
>
> diff --git a/gdb/jit.c b/gdb/jit.c
> index 804c832f47d..091e0b4a771 100644
> --- a/gdb/jit.c
> +++ b/gdb/jit.c
> @@ -1239,11 +1239,10 @@ jit_breakpoint_re_set (void)
>  static void
>  jit_inferior_exit_hook (struct inferior *inf)
>  {
> -  for (objfile *objf : current_program_space->objfiles_safe ())
> +  current_program_space->unlink_objfiles_if ([&] (const objfile *objf)
>      {
> -      if (objf->jited_data != nullptr && objf->jited_data->addr != 0)
> -       objf->unlink ();
> -    }
> +      return (objf->jited_data != nullptr) && (objf->jited_data->addr != 0);
> +    });
>  }
>
>  void
> diff --git a/gdb/objfiles.c b/gdb/objfiles.c
> index 5ba5f0a616d..f862182458d 100644
> --- a/gdb/objfiles.c
> +++ b/gdb/objfiles.c
> @@ -794,14 +794,12 @@ have_full_symbols (void)
>  void
>  objfile_purge_solibs (void)
>  {
> -  for (objfile *objf : current_program_space->objfiles_safe ())
> +  current_program_space->unlink_objfiles_if ([&] (const objfile *objf)
>      {
>        /* We assume that the solib package has been purged already, or will
>          be soon.  */
> -
> -      if (!(objf->flags & OBJF_USERLOADED) && (objf->flags & OBJF_SHARED))
> -       objf->unlink ();
> -    }
> +      return !(objf->flags & OBJF_USERLOADED) && (objf->flags & OBJF_SHARED);
> +    });
>  }
>
>
> diff --git a/gdb/objfiles.h b/gdb/objfiles.h
> index 189856f0a51..4a396af17da 100644
> --- a/gdb/objfiles.h
> +++ b/gdb/objfiles.h
> @@ -699,13 +699,17 @@ struct objfile
>
>  private:
>
> +  using qf_list = std::forward_list<quick_symbol_functions_up>;
> +  using qf_range = iterator_range<qf_list::iterator>;
> +  using qf_safe_range = basic_safe_range<qf_range>;
> +
>    /* Ensure that partial symbols have been read and return the "quick" (aka
>       partial) symbol functions for this symbol reader.  */
> -  const std::forward_list<quick_symbol_functions_up> &
> +  qf_safe_range
>    qf_require_partial_symbols ()
>    {
>      this->require_partial_symbols (true);
> -    return qf;
> +    return qf_safe_range (qf_range (qf.begin (), qf.end ()));
>    }
>
>  public:
> diff --git a/gdb/progspace.c b/gdb/progspace.c
> index 5cf8334ee67..2ab1a799fe7 100644
> --- a/gdb/progspace.c
> +++ b/gdb/progspace.c
> @@ -139,19 +139,19 @@ program_space::free_all_objfiles ()
>
>  void
>  program_space::add_objfile (std::unique_ptr<objfile> &&objfile,
> -                           struct objfile *before)
> +                           struct objfile *after)
>  {
> -  if (before == nullptr)
> +  if (after == nullptr)
>      objfiles_list.push_back (std::move (objfile));
>    else
>      {
>        auto iter = std::find_if (objfiles_list.begin (), objfiles_list.end (),
>                                 [=] (const std::unique_ptr<::objfile> &objf)
>                                 {
> -                                 return objf.get () == before;
> +                                 return objf.get () == after;
>                                 });
>        gdb_assert (iter != objfiles_list.end ());
> -      objfiles_list.insert (iter, std::move (objfile));
> +      objfiles_list.insert (++iter, std::move (objfile));
>      }
>  }
>
> @@ -180,6 +180,17 @@ program_space::remove_objfile (struct objfile *objfile)
>
>  /* See progspace.h.  */
>
> +void
> +program_space::unlink_objfiles_if
> +  (gdb::function_view<bool (const objfile *objfile)> predicate)
> +{
> +  for (auto &it : objfiles_safe ())
> +    if (predicate (it.get ()))
> +      it->unlink ();
> +}
> +
> +/* See progspace.h.  */
> +
>  struct objfile *
>  program_space::objfile_for_address (CORE_ADDR address)
>  {
> diff --git a/gdb/progspace.h b/gdb/progspace.h
> index ee12d89c173..a730c1334eb 100644
> --- a/gdb/progspace.h
> +++ b/gdb/progspace.h
> @@ -213,28 +213,32 @@ struct program_space
>         unwrapping_objfile_iterator (objfiles_list.end ()));
>    }
>
> -  using objfiles_safe_range = basic_safe_range<objfiles_range>;
> +  using objfiles_safe_range = iterator_range<objfile_list::iterator>;
> +  using objfiles_safe_reverse_range
> +    = basic_safe_reverse_range<objfiles_safe_range>;
>
>    /* An iterable object that can be used to iterate over all objfiles.
>       The basic use is in a foreach, like:
>
>       for (objfile *objf : pspace->objfiles_safe ()) { ... }
>
> -     This variant uses a basic_safe_iterator so that objfiles can be
> -     deleted during iteration.  */
> -  objfiles_safe_range objfiles_safe ()
> +     This variant uses a basic_safe_reverse_iterator so that objfiles
> +     can be deleted during iteration.
> +
> +     The use of a reverse iterator helps ensure that separate debug
> +     objfiles are deleted before their parent objfile.  This prevents
> +     iterator invalidation due to the deletion of a parent objfile.  */
> + objfiles_safe_reverse_range objfiles_safe ()
>    {
> -    return objfiles_safe_range
> -      (objfiles_range
> -        (unwrapping_objfile_iterator (objfiles_list.begin ()),
> -         unwrapping_objfile_iterator (objfiles_list.end ())));
> +    return objfiles_safe_reverse_range
> +      (objfiles_safe_range (objfiles_list.begin (), objfiles_list.end ()));
>    }
>
> -  /* Add OBJFILE to the list of objfiles, putting it just before
> -     BEFORE.  If BEFORE is nullptr, it will go at the end of the
> +  /* Add OBJFILE to the list of objfiles, putting it just after
> +     AFTER.  If AFTER is nullptr, it will go at the end of the
>       list.  */
>    void add_objfile (std::unique_ptr<objfile> &&objfile,
> -                   struct objfile *before);
> +                   struct objfile *after);
>
>    /* Remove OBJFILE from the list of objfiles.  */
>    void remove_objfile (struct objfile *objfile);
> @@ -249,6 +253,11 @@ struct program_space
>    /* Free all the objfiles associated with this program space.  */
>    void free_all_objfiles ();
>
> +  /* Unlink all objfiles associated with this program space for which
> +     PREDICATE evaluates to true.  */
> +  void unlink_objfiles_if
> +    (gdb::function_view<bool (const objfile *objfile)> predicate);
> +
>    /* Return the objfile containing ADDRESS, or nullptr if the address
>       is outside all objfiles in this progspace.  */
>    struct objfile *objfile_for_address (CORE_ADDR address);
> diff --git a/gdb/testsuite/gdb.python/py-objfile.exp b/gdb/testsuite/gdb.python/py-objfile.exp
> index 61b9942de79..0bf49976b73 100644
> --- a/gdb/testsuite/gdb.python/py-objfile.exp
> +++ b/gdb/testsuite/gdb.python/py-objfile.exp
> @@ -135,7 +135,7 @@ gdb_test "p main" "= {<text variable, no debug info>} $hex <main>" \
>  gdb_py_test_silent_cmd "python objfile.add_separate_debug_file(\"${binfile}\")" \
>      "Add separate debug file file" 1
>
> -gdb_py_test_silent_cmd "python sep_objfile = gdb.objfiles()\[0\]" \
> +gdb_py_test_silent_cmd "python sep_objfile = gdb.objfiles()\[1\]" \
>      "Get separate debug info objfile" 1
>
>  gdb_test "python print (sep_objfile.owner.filename)" "${testfile}2" \
> diff --git a/gdbsupport/safe-iterator.h b/gdbsupport/safe-iterator.h
> index ccd772ca2a5..9f57c1543cf 100644
> --- a/gdbsupport/safe-iterator.h
> +++ b/gdbsupport/safe-iterator.h
> @@ -136,4 +136,110 @@ class basic_safe_range
>    Range m_range;
>  };
>
> +/* A reverse basic_safe_iterator.  See basic_safe_iterator for intended use.  */
> +
> +template<typename Iterator>
> +class basic_safe_reverse_iterator
> +{
> +public:
> +  typedef basic_safe_reverse_iterator self_type;
> +  typedef typename Iterator::value_type value_type;
> +  typedef typename Iterator::reference reference;
> +  typedef typename Iterator::pointer pointer;
> +  typedef typename Iterator::iterator_category iterator_category;
> +  typedef typename Iterator::difference_type difference_type;
> +
> +  /* Construct the iterator using ARG, and construct the end iterator
> +     using ARG2.  */
> +  template<typename Arg>
> +  explicit basic_safe_reverse_iterator (Arg &&arg, Arg &&arg2)
> +    : m_begin (std::forward<Arg> (arg)),
> +      m_end (std::forward<Arg> (arg2)),
> +      m_it (m_end),
> +      m_next (m_end)
> +  {
> +    /* M_IT and M_NEXT are initialized as one-past-end.  Set M_IT to point
> +       to the last element and set M_NEXT to point to the second last element,
> +       if such elements exist.  */
> +    if (m_it != m_begin)
> +      {
> +       --m_it;
> +
> +       if (m_it != m_begin)
> +         {
> +           --m_next;
> +           --m_next;
> +         }
> +      }
> +  }
> +
> +  typename gdb::invoke_result<decltype(&Iterator::operator*), Iterator>::type
> +    operator* () const
> +  { return *m_it; }
> +
> +  self_type &operator++ ()
> +  {
> +    m_it = m_next;
> +
> +    if (m_it != m_end)
> +      {
> +       /* Use M_BEGIN only if we sure that it is valid.  */
> +       if (m_it == m_begin)
> +         m_next = m_end;
> +       else
> +         --m_next;
> +      }
> +
> +    return *this;
> +  }
> +
> +  bool operator== (const self_type &other) const
> +  { return m_it == other.m_it; }
> +
> +  bool operator!= (const self_type &other) const
> +  { return m_it != other.m_it; }
> +
> +private:
> +  /* The first element.  */
> +  Iterator m_begin {};
> +
> +  /* A one-past-end iterator.  */
> +  Iterator m_end {};
> +
> +  /* The current element.  */
> +  Iterator m_it {};
> +
> +  /* The next element.  Always one element ahead of M_IT.  */
> +  Iterator m_next {};
> +};
> +
> +/* A range adapter that wraps a forward range, and then returns
> +   safe reverse iterators wrapping the original range's iterators.  */
> +
> +template<typename Range>
> +class basic_safe_reverse_range
> +{
> +public:
> +
> +  typedef basic_safe_reverse_iterator<typename Range::iterator> iterator;
> +
> +  explicit basic_safe_reverse_range (Range range)
> +    : m_range (range)
> +  {
> +  }
> +
> +  iterator begin ()
> +  {
> +    return iterator (m_range.begin (), m_range.end ());
> +  }
> +
> +  iterator end ()
> +  {
> +    return iterator (m_range.end (), m_range.end ());
> +  }
> +
> +private:
> +
> +  Range m_range;
> +};
>  #endif /* COMMON_SAFE_ITERATOR_H */
> --
> 2.41.0
>