Re: [PATCH][gdb/tui,c++17] Fix NULL string_view in tui_partial_win_by_name

[Simon Marchi <simark@simark.ca>]

On 2020-07-05 5:40 p.m., Tom de Vries wrote:
> Hi,
> 
> When building gdb with CFLAGS=-std=gnu17 and CXXFLAGS=-std=gnu++17 and running
> test-case gdb.tui/new-layout.exp, we run into:
> ...
> UNRESOLVED: gdb.tui/new-layout.exp: left window box after shrink (ll corner)
> FAIL: gdb.tui/new-layout.exp: right window box after shrink (ll corner)
> ...
> 
> In a minimal form, we run into an abort when issuing a winheight command:
> ...
> $ gdb -tui -ex "winheight src - 5"
>    <tui stuff>
> Aborted (core dumped)
> $
> ...
> with this backtrace at the abort:
> ...
> \#0  0x0000000000438db0 in std::char_traits<char>::length (__s=0x0)
>      at /usr/include/c++/9/bits/char_traits.h:335
> \#1  0x000000000043b72e in std::basic_string_view<char, \
>    std::char_traits<char> >::basic_string_view (this=0x7fffffffd4f0, \
>    __str=0x0) at /usr/include/c++/9/string_view:124
> \#2  0x000000000094971b in tui_partial_win_by_name (name="src")
>      at src/gdb/tui/tui-win.c:663
> ...
> due to a NULL comparison which constructs a string_view object from NULL:
> ...
>    657  /* Answer the window represented by name.  */
>    658  static struct tui_win_info *
>    659  tui_partial_win_by_name (gdb::string_view name)
>    660  {
>    661    struct tui_win_info *best = nullptr;
>    662
>    663    if (name != NULL)
> ...
> 
> In gdbsupport/gdb_string_view.h, we either use:
> - gdb's copy of libstdc++-v3/include/experimental/string_view, or
> - the standard implementation of string_view, when built with C++17 or later
>   (which in gcc's case comes from libstdc++-v3/include/std/string_view)
> 
> In the first case, there's support for constructing a string_view from a NULL
> pointer:
> ...
>       /*constexpr*/ basic_string_view(const _CharT* __str)
>       : _M_len{__str == nullptr ? 0 : traits_type::length(__str)},
>         _M_str{__str}
>       { }
> ...
> but in the second case, there's not:
> ...
>       __attribute__((__nonnull__)) constexpr
>       basic_string_view(const _CharT* __str) noexcept
>       : _M_len{traits_type::length(__str)},
>         _M_str{__str}
>       { }
> ...
> 
> Fix this by removing the NULL comparison altogether.
> 
> Build on x86_64-linux with CFLAGS=-std=gnu17 and CXXFLAGS=-std=gnu++17, and
> tested.
> 
> Any comments?

This is the second string_view problem in a short amount of time, the other being:

  https://sourceware.org/bugzilla/show_bug.cgi?id=26183

When importing the string_view support, I ended up copying experimental/string_view
(which is a version from before it was a standard) from libstdc++, rather than
std/string_view.  In this message:

  https://sourceware.org/legacy-ml/gdb-patches/2018-04/msg00111.html

I said it was because "it is easier to adapt to the c++ standard we follow", but I
don't remember the specific reason.  I think we should change our string_view to
be an adapted copy of std/string_view instead, to minimize the chances of differences
with the standard version.  I started to do it the other day (but did not finish
adapting the tests), and didn't see any blocker.

I still think your patch is good (it would be necessary in any case).

Simon