Re: [PATCH 4/7 v5] gdb: Buffer output streams during events that might download debuginfo

[Aaron Merey <amerey@redhat.com>]

Ping

Thanks,
Aaron

On Wed, Aug 16, 2023 at 12:43 AM Aaron Merey <amerey@redhat.com> wrote:
>
> v4: https://sourceware.org/pipermail/gdb-patches/2023-June/200175.html
>
> v5 replaces buffer_file with buffered_stream and buffer_group to
> facilitate buffering of all gdb_std* output streams.
>
> v5 also adds prefix_state to progress_update objects to track when
> a newline prefix needs to be added to a download progress message.
> This is used to correctly format progress messages that occur during
> command autocompletion, which is added in a later patch in this series.
>
> Commit message:
>
> Introduce new ui_file buffering_file to temporarily collect output
> written to gdb_std* output streams during print_thread, print_frame_info
> and print_stop_event.
>
> This ensures that output during these functions is not interrupted
> by debuginfod progress messages.
>
> With the addition of deferred debuginfo downloading it is possible
> for download progress messages to print during these events.
> Without any intervention we can end up with poorly formatted output:
>
>     (gdb) backtrace
>     [...]
>     #8  0x00007fbe8af7d7cf in pygi_invoke_c_callable (Downloading separate debug info for /lib64/libpython3.11.so.1.0
>     function_cache=0x561221b224d0, state=<optimized out>...
>
> To fix this we buffer writes to gdb_std* output streams while allowing
> debuginfod progress messages to skip the buffers and print to the
> underlying output streams immediately.  Buffered output is then written
> to the output streams.  This ensures that progress messages print first,
> followed by uninterrupted frame/thread/stop info:
>
>     (gdb) backtrace
>     [...]
>     Downloading separate debug info for /lib64/libpython3.11.so.1.0
>     #8  0x00007fbe8af7d7cf in pygi_invoke_c_callable (function_cache=0x561221b224d0, state=<optimized out>...
>
> Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
> ---
>  gdb/cli-out.c            |  21 +++--
>  gdb/cli-out.h            |   3 +
>  gdb/debuginfod-support.c |  15 ++-
>  gdb/infrun.c             |  16 +++-
>  gdb/mi/mi-out.c          |   9 +-
>  gdb/mi/mi-out.h          |   3 +
>  gdb/python/py-mi.c       |   3 +
>  gdb/stack.c              |  35 +++++--
>  gdb/thread.c             | 171 +++++++++++++++++++---------------
>  gdb/ui-file.h            |   2 +-
>  gdb/ui-out.c             | 123 +++++++++++++++++++++++++
>  gdb/ui-out.h             | 194 +++++++++++++++++++++++++++++++++++++++
>  12 files changed, 497 insertions(+), 98 deletions(-)
>
> diff --git a/gdb/cli-out.c b/gdb/cli-out.c
> index 20d3d93f1ad..a570a2d939d 100644
> --- a/gdb/cli-out.c
> +++ b/gdb/cli-out.c
> @@ -299,7 +299,7 @@ cli_ui_out::do_progress_notify (const std::string &msg,
>                                 double howmuch, double total)
>  {
>    int chars_per_line = get_chars_per_line ();
> -  struct ui_file *stream = m_streams.back ();
> +  struct ui_file *stream = get_unbuffered (m_streams.back ());
>    cli_progress_info &info (m_progress_info.back ());
>
>    if (chars_per_line > MAX_CHARS_PER_LINE)
> @@ -307,16 +307,23 @@ cli_ui_out::do_progress_notify (const std::string &msg,
>
>    if (info.state == progress_update::START)
>      {
> +      std::string prefix;
> +      if (cur_prefix_state == prefix_state_t::NEWLINE_NEEDED)
> +       {
> +         prefix = "\n";
> +         cur_prefix_state = prefix_state_t::NEWLINE_PRINTED;
> +       }
> +
>        if (stream->isatty ()
>           && current_ui->input_interactive_p ()
>           && chars_per_line >= MIN_CHARS_PER_LINE)
>         {
> -         gdb_printf (stream, "%s\n", msg.c_str ());
> +         gdb_printf (stream, "%s\n", (prefix + msg).c_str ());
>           info.state = progress_update::BAR;
>         }
>        else
>         {
> -         gdb_printf (stream, "%s...\n", msg.c_str ());
> +         gdb_printf (stream, "%s...\n", (prefix + msg).c_str ());
>           info.state = progress_update::WORKING;
>         }
>      }
> @@ -384,7 +391,7 @@ cli_ui_out::do_progress_notify (const std::string &msg,
>  void
>  cli_ui_out::clear_progress_notify ()
>  {
> -  struct ui_file *stream = m_streams.back ();
> +  struct ui_file *stream = get_unbuffered (m_streams.back ());
>    int chars_per_line = get_chars_per_line ();
>
>    scoped_restore save_pagination
> @@ -413,10 +420,12 @@ void
>  cli_ui_out::do_progress_end ()
>  {
>    struct ui_file *stream = m_streams.back ();
> -  m_progress_info.pop_back ();
> +  cli_progress_info &info (m_progress_info.back ());
>
> -  if (stream->isatty ())
> +  if (stream->isatty () && info.state != progress_update::START)
>      clear_progress_notify ();
> +
> +  m_progress_info.pop_back ();
>  }
>
>  /* local functions */
> diff --git a/gdb/cli-out.h b/gdb/cli-out.h
> index 34016182269..89b4aa40870 100644
> --- a/gdb/cli-out.h
> +++ b/gdb/cli-out.h
> @@ -35,6 +35,9 @@ class cli_ui_out : public ui_out
>
>    bool can_emit_style_escape () const override;
>
> +  ui_file *current_stream () const override
> +  { return m_streams.back (); }
> +
>  protected:
>
>    virtual void do_table_begin (int nbrofcols, int nr_rows,
> diff --git a/gdb/debuginfod-support.c b/gdb/debuginfod-support.c
> index c0794212a31..e041815019b 100644
> --- a/gdb/debuginfod-support.c
> +++ b/gdb/debuginfod-support.c
> @@ -155,7 +155,8 @@ progressfn (debuginfod_client *c, long cur, long total)
>
>    if (check_quit_flag ())
>      {
> -      gdb_printf ("Cancelling download of %s %s...\n",
> +      ui_file *outstream = get_unbuffered (gdb_stdout);
> +      gdb_printf (outstream, _("Cancelling download of %s %s...\n"),
>                   data->desc, styled_fname.c_str ());
>        return 1;
>      }
> @@ -296,10 +297,14 @@ static void
>  print_outcome (int fd, const char *desc, const char *fname)
>  {
>    if (fd < 0 && fd != -ENOENT)
> -    gdb_printf (_("Download failed: %s.  Continuing without %s %ps.\n"),
> -               safe_strerror (-fd),
> -               desc,
> -               styled_string (file_name_style.style (), fname));
> +    {
> +      ui_file *outstream = get_unbuffered (gdb_stdout);
> +      gdb_printf (outstream,
> +                 _("Download failed: %s.  Continuing without %s %ps.\n"),
> +                 safe_strerror (-fd),
> +                 desc,
> +                 styled_string (file_name_style.style (), fname));
> +    }
>  }
>
>  /* See debuginfod-support.h  */
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 8286026e6c6..68f47af0224 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -8783,10 +8783,10 @@ print_stop_location (const target_waitstatus &ws)
>      print_stack_frame (get_selected_frame (nullptr), 0, source_flag, 1);
>  }
>
> -/* See infrun.h.  */
> +/* See `print_stop_event` in infrun.h.  */
>
> -void
> -print_stop_event (struct ui_out *uiout, bool displays)
> +static void
> +do_print_stop_event (struct ui_out *uiout, bool displays)
>  {
>    struct target_waitstatus last;
>    struct thread_info *tp;
> @@ -8815,6 +8815,16 @@ print_stop_event (struct ui_out *uiout, bool displays)
>      }
>  }
>
> +/* See infrun.h.  This function itself sets up buffered output for the
> +   duration of do_print_stop_event, which performs the actual event
> +   printing.  */
> +
> +void
> +print_stop_event (struct ui_out *uiout, bool displays)
> +{
> +  do_with_buffered_output (do_print_stop_event, uiout, displays);
> +}
> +
>  /* See infrun.h.  */
>
>  void
> diff --git a/gdb/mi/mi-out.c b/gdb/mi/mi-out.c
> index 29a416a426d..dc30b65d7a8 100644
> --- a/gdb/mi/mi-out.c
> +++ b/gdb/mi/mi-out.c
> @@ -278,7 +278,14 @@ mi_ui_out::do_progress_notify (const std::string &msg, const char *unit,
>
>    if (info.state == progress_update::START)
>      {
> -      gdb_printf ("%s...\n", msg.c_str ());
> +      std::string prefix;
> +      if (cur_prefix_state == prefix_state_t::NEWLINE_NEEDED)
> +       {
> +         prefix = "\n";
> +         cur_prefix_state = prefix_state_t::NEWLINE_PRINTED;
> +       }
> +
> +      gdb_printf ("%s...\n", (prefix + msg).c_str ());
>        info.state = progress_update::WORKING;
>      }
>  }
> diff --git a/gdb/mi/mi-out.h b/gdb/mi/mi-out.h
> index 10c9f8a4585..85604c328d1 100644
> --- a/gdb/mi/mi-out.h
> +++ b/gdb/mi/mi-out.h
> @@ -45,6 +45,9 @@ class mi_ui_out : public ui_out
>      return false;
>    }
>
> +  ui_file *current_stream () const override
> +  { return m_streams.back (); }
> +
>  protected:
>
>    virtual void do_table_begin (int nbrofcols, int nr_rows, const char *tblid)
> diff --git a/gdb/python/py-mi.c b/gdb/python/py-mi.c
> index 0fcd57844e7..0bc46132458 100644
> --- a/gdb/python/py-mi.c
> +++ b/gdb/python/py-mi.c
> @@ -55,6 +55,9 @@ class py_ui_out : public ui_out
>      return current ().obj.release ();
>    }
>
> +  ui_file *current_stream () const override
> +  { return nullptr; }
> +
>  protected:
>
>    void do_progress_end () override { }
> diff --git a/gdb/stack.c b/gdb/stack.c
> index 002bf580634..ff76436358e 100644
> --- a/gdb/stack.c
> +++ b/gdb/stack.c
> @@ -220,7 +220,8 @@ static void print_frame_local_vars (frame_info_ptr frame,
>                                     const char *regexp, const char *t_regexp,
>                                     int num_tabs, struct ui_file *stream);
>
> -static void print_frame (const frame_print_options &opts,
> +static void print_frame (struct ui_out *uiout,
> +                        const frame_print_options &opts,
>                          frame_info_ptr frame, int print_level,
>                          enum print_what print_what,  int print_args,
>                          struct symtab_and_line sal);
> @@ -1020,16 +1021,15 @@ get_user_print_what_frame_info (gdb::optional<enum print_what> *what)
>     Used in "where" output, and to emit breakpoint or step
>     messages.  */
>
> -void
> -print_frame_info (const frame_print_options &fp_opts,
> -                 frame_info_ptr frame, int print_level,
> -                 enum print_what print_what, int print_args,
> -                 int set_current_sal)
> +static void
> +do_print_frame_info (struct ui_out *uiout, const frame_print_options &fp_opts,
> +                    frame_info_ptr frame, int print_level,
> +                    enum print_what print_what, int print_args,
> +                    int set_current_sal)
>  {
>    struct gdbarch *gdbarch = get_frame_arch (frame);
>    int source_print;
>    int location_print;
> -  struct ui_out *uiout = current_uiout;
>
>    if (!current_uiout->is_mi_like_p ()
>        && fp_opts.print_frame_info != print_frame_info_auto)
> @@ -1105,7 +1105,8 @@ print_frame_info (const frame_print_options &fp_opts,
>                     || print_what == LOC_AND_ADDRESS
>                     || print_what == SHORT_LOCATION);
>    if (location_print || !sal.symtab)
> -    print_frame (fp_opts, frame, print_level, print_what, print_args, sal);
> +    print_frame (uiout, fp_opts, frame, print_level,
> +                print_what, print_args, sal);
>
>    source_print = (print_what == SRC_LINE || print_what == SRC_AND_LOC);
>
> @@ -1185,6 +1186,20 @@ print_frame_info (const frame_print_options &fp_opts,
>    gdb_flush (gdb_stdout);
>  }
>
> +/* Redirect output to a temporary buffer for the duration
> +   of do_print_frame_info.  */
> +
> +void
> +print_frame_info (const frame_print_options &fp_opts,
> +                 frame_info_ptr frame, int print_level,
> +                 enum print_what print_what, int print_args,
> +                 int set_current_sal)
> +{
> +  do_with_buffered_output (do_print_frame_info, current_uiout,
> +                          fp_opts, frame, print_level, print_what,
> +                          print_args, set_current_sal);
> +}
> +
>  /* See stack.h.  */
>
>  void
> @@ -1309,13 +1324,13 @@ find_frame_funname (frame_info_ptr frame, enum language *funlang,
>  }
>
>  static void
> -print_frame (const frame_print_options &fp_opts,
> +print_frame (struct ui_out *uiout,
> +            const frame_print_options &fp_opts,
>              frame_info_ptr frame, int print_level,
>              enum print_what print_what, int print_args,
>              struct symtab_and_line sal)
>  {
>    struct gdbarch *gdbarch = get_frame_arch (frame);
> -  struct ui_out *uiout = current_uiout;
>    enum language funlang = language_unknown;
>    struct value_print_options opts;
>    struct symbol *func;
> diff --git a/gdb/thread.c b/gdb/thread.c
> index 63ed87e9aa6..baa774c27b1 100644
> --- a/gdb/thread.c
> +++ b/gdb/thread.c
> @@ -1040,6 +1040,103 @@ thread_target_id_str (thread_info *tp)
>      return target_id;
>  }
>
> +/* Print thread TP.  GLOBAL_IDS indicates whether REQUESTED_THREADS
> +   is a list of global or per-inferior thread ids.  */
> +
> +static void
> +do_print_thread (ui_out *uiout, const char *requested_threads,
> +                int global_ids, int pid, int show_global_ids,
> +                int default_inf_num, thread_info *tp,
> +                thread_info *current_thread)
> +{
> +  int core;
> +
> +  if (!should_print_thread (requested_threads, default_inf_num,
> +                           global_ids, pid, tp))
> +    return;
> +
> +  ui_out_emit_tuple tuple_emitter (uiout, NULL);
> +
> +  if (!uiout->is_mi_like_p ())
> +    {
> +      if (tp == current_thread)
> +       uiout->field_string ("current", "*");
> +      else
> +       uiout->field_skip ("current");
> +
> +      uiout->field_string ("id-in-tg", print_thread_id (tp));
> +    }
> +
> +  if (show_global_ids || uiout->is_mi_like_p ())
> +    uiout->field_signed ("id", tp->global_num);
> +
> +  /* Switch to the thread (and inferior / target).  */
> +  switch_to_thread (tp);
> +
> +  /* For the CLI, we stuff everything into the target-id field.
> +     This is a gross hack to make the output come out looking
> +     correct.  The underlying problem here is that ui-out has no
> +     way to specify that a field's space allocation should be
> +     shared by several fields.  For MI, we do the right thing
> +     instead.  */
> +
> +  if (uiout->is_mi_like_p ())
> +    {
> +      uiout->field_string ("target-id", target_pid_to_str (tp->ptid));
> +
> +      const char *extra_info = target_extra_thread_info (tp);
> +      if (extra_info != nullptr)
> +       uiout->field_string ("details", extra_info);
> +
> +      const char *name = thread_name (tp);
> +      if (name != NULL)
> +       uiout->field_string ("name", name);
> +    }
> +  else
> +    {
> +      uiout->field_string ("target-id", thread_target_id_str (tp));
> +    }
> +
> +  if (tp->state == THREAD_RUNNING)
> +    uiout->text ("(running)\n");
> +  else
> +    {
> +      /* The switch above put us at the top of the stack (leaf
> +        frame).  */
> +      print_stack_frame (get_selected_frame (NULL),
> +                        /* For MI output, print frame level.  */
> +                        uiout->is_mi_like_p (),
> +                        LOCATION, 0);
> +    }
> +
> +  if (uiout->is_mi_like_p ())
> +    {
> +      const char *state = "stopped";
> +
> +      if (tp->state == THREAD_RUNNING)
> +       state = "running";
> +      uiout->field_string ("state", state);
> +    }
> +
> +  core = target_core_of_thread (tp->ptid);
> +  if (uiout->is_mi_like_p () && core != -1)
> +    uiout->field_signed ("core", core);
> +}
> +
> +/* Redirect output to a temporary buffer for the duration
> +   of do_print_thread.  */
> +
> +static void
> +print_thread (ui_out *uiout, const char *requested_threads,
> +             int global_ids, int pid, int show_global_ids,
> +             int default_inf_num, thread_info *tp, thread_info *current_thread)
> +
> +{
> +  do_with_buffered_output (do_print_thread, uiout, requested_threads,
> +                          global_ids, pid, show_global_ids,
> +                          default_inf_num, tp, current_thread);
> +}
> +
>  /* Like print_thread_info, but in addition, GLOBAL_IDS indicates
>     whether REQUESTED_THREADS is a list of global or per-inferior
>     thread ids.  */
> @@ -1123,82 +1220,12 @@ print_thread_info_1 (struct ui_out *uiout, const char *requested_threads,
>      for (inferior *inf : all_inferiors ())
>        for (thread_info *tp : inf->threads ())
>         {
> -         int core;
> -
>           any_thread = true;
>           if (tp == current_thread && tp->state == THREAD_EXITED)
>             current_exited = true;
>
> -         if (!should_print_thread (requested_threads, default_inf_num,
> -                                   global_ids, pid, tp))
> -           continue;
> -
> -         ui_out_emit_tuple tuple_emitter (uiout, NULL);
> -
> -         if (!uiout->is_mi_like_p ())
> -           {
> -             if (tp == current_thread)
> -               uiout->field_string ("current", "*");
> -             else
> -               uiout->field_skip ("current");
> -
> -             uiout->field_string ("id-in-tg", print_thread_id (tp));
> -           }
> -
> -         if (show_global_ids || uiout->is_mi_like_p ())
> -           uiout->field_signed ("id", tp->global_num);
> -
> -         /* Switch to the thread (and inferior / target).  */
> -         switch_to_thread (tp);
> -
> -         /* For the CLI, we stuff everything into the target-id field.
> -            This is a gross hack to make the output come out looking
> -            correct.  The underlying problem here is that ui-out has no
> -            way to specify that a field's space allocation should be
> -            shared by several fields.  For MI, we do the right thing
> -            instead.  */
> -
> -         if (uiout->is_mi_like_p ())
> -           {
> -             uiout->field_string ("target-id", target_pid_to_str (tp->ptid));
> -
> -             const char *extra_info = target_extra_thread_info (tp);
> -             if (extra_info != nullptr)
> -               uiout->field_string ("details", extra_info);
> -
> -             const char *name = thread_name (tp);
> -             if (name != NULL)
> -               uiout->field_string ("name", name);
> -           }
> -         else
> -           {
> -             uiout->field_string ("target-id", thread_target_id_str (tp));
> -           }
> -
> -         if (tp->state == THREAD_RUNNING)
> -           uiout->text ("(running)\n");
> -         else
> -           {
> -             /* The switch above put us at the top of the stack (leaf
> -                frame).  */
> -             print_stack_frame (get_selected_frame (NULL),
> -                                /* For MI output, print frame level.  */
> -                                uiout->is_mi_like_p (),
> -                                LOCATION, 0);
> -           }
> -
> -         if (uiout->is_mi_like_p ())
> -           {
> -             const char *state = "stopped";
> -
> -             if (tp->state == THREAD_RUNNING)
> -               state = "running";
> -             uiout->field_string ("state", state);
> -           }
> -
> -         core = target_core_of_thread (tp->ptid);
> -         if (uiout->is_mi_like_p () && core != -1)
> -           uiout->field_signed ("core", core);
> +         print_thread (uiout, requested_threads, global_ids, pid,
> +                       show_global_ids, default_inf_num, tp, current_thread);
>         }
>
>      /* This end scope restores the current thread and the frame
> diff --git a/gdb/ui-file.h b/gdb/ui-file.h
> index 31f87ffd51d..8385033b441 100644
> --- a/gdb/ui-file.h
> +++ b/gdb/ui-file.h
> @@ -224,7 +224,7 @@ class string_file : public ui_file
>    bool empty () const { return m_string.empty (); }
>    void clear () { return m_string.clear (); }
>
> -private:
> +protected:
>    /* The internal buffer.  */
>    std::string m_string;
>
> diff --git a/gdb/ui-out.c b/gdb/ui-out.c
> index 9380630c320..005e1faa059 100644
> --- a/gdb/ui-out.c
> +++ b/gdb/ui-out.c
> @@ -32,6 +32,9 @@
>  #include <memory>
>  #include <string>
>
> +/* Current state of newline prefixing for progress update messages.  */
> +prefix_state_t cur_prefix_state = prefix_state_t::NEWLINE_OFF;
> +
>  namespace {
>
>  /* A header of a ui_out_table.  */
> @@ -871,3 +874,123 @@ ui_out::ui_out (ui_out_flags flags)
>  ui_out::~ui_out ()
>  {
>  }
> +
> +/* See ui-out.h.  */
> +
> +void
> +buffer_group::output_unit::flush () const
> +{
> +  m_stream->puts (m_msg.c_str ());
> +
> +  if (m_wrap_hint >= 0)
> +    m_stream->wrap_here (m_wrap_hint);
> +}
> +
> +/* See ui-out.h.  */
> +
> +void
> +buffer_group::write (const char *buf, long length_buf, ui_file *stream)
> +{
> +  /* Record each line separately.  */
> +  for (size_t prev = 0, cur = 0; cur < length_buf; ++cur)
> +    if (buf[cur] == '\n' || cur == length_buf - 1)
> +      {
> +       std::string msg (buf + prev, cur - prev + 1);
> +
> +       if (m_buffered_output.size () > 0
> +           && m_buffered_output.back ().m_wrap_hint == -1
> +           && m_buffered_output.back ().m_stream == stream
> +           && m_buffered_output.back ().m_msg.size () > 0
> +           && m_buffered_output.back ().m_msg.back () != '\n')
> +         m_buffered_output.back ().m_msg.append (msg);
> +       else
> +         {
> +           m_buffered_output.emplace_back (msg);
> +           m_buffered_output.back ().m_stream = stream;
> +         }
> +       prev = cur + 1;
> +      }
> +}
> +
> +/* See ui-out.h.  */
> +
> +void
> +buffer_group::wrap_here (int indent, ui_file *stream)
> +{
> +  m_buffered_output.emplace_back ("", indent);
> +  m_buffered_output.back ().m_stream = stream;
> +}
> +
> +/* See ui-out.h.  */
> +
> +ui_file *
> +get_unbuffered (ui_file * stream)
> +{
> +  buffering_file *buf = dynamic_cast<buffering_file *> (stream);
> +
> +  if (buf == nullptr)
> +    return stream;
> +
> +  return buf->stream ();
> +}
> +
> +buffered_streams::buffered_streams (buffer_group *group)
> +    : m_buffered_stdout (group, gdb_stdout),
> +      m_buffered_stderr (group, gdb_stderr),
> +      m_buffered_stdlog (group, gdb_stdlog),
> +      m_buffered_stdtarg (group, gdb_stdtarg),
> +      m_buffered_stdtargerr (group, gdb_stdtargerr)
> +  {
> +    gdb_stdout = &m_buffered_stdout;
> +    gdb_stderr = &m_buffered_stderr;
> +    gdb_stdlog = &m_buffered_stdlog;
> +    gdb_stdtarg = &m_buffered_stdtarg;
> +    gdb_stdtargerr = &m_buffered_stdtargerr;
> +
> +    ui_file *stream = current_uiout->current_stream ();
> +    if (stream != nullptr)
> +      {
> +       m_buffered_uiout.emplace (group, stream);
> +       current_uiout->redirect (&(*m_buffered_uiout));
> +      }
> +
> +    m_buffers_in_place = true;
> +  };
> +
> +/* See ui-out.h.  */
> +
> +void
> +buffered_streams::remove_buffers ()
> +  {
> +    if (!m_buffers_in_place)
> +      return;
> +
> +    m_buffers_in_place = false;
> +
> +    gdb_stdout = m_buffered_stdout.stream ();
> +    gdb_stderr = m_buffered_stderr.stream ();
> +    gdb_stdlog = m_buffered_stdlog.stream ();
> +    gdb_stdtarg = m_buffered_stdtarg.stream ();
> +    gdb_stdtargerr = m_buffered_stdtargerr.stream ();
> +
> +    if (m_buffered_uiout.has_value ())
> +      current_uiout->redirect (nullptr);
> +  }
> +
> +buffer_group::buffer_group ()
> +  : m_buffered_streams (new buffered_streams (this))
> +{ /* Nothing.  */ }
> +
> +buffer_group::~buffer_group ()
> +{ /* Nothing.  */ }
> +
> +/* See ui-out.h.  */
> +
> +void
> +buffer_group::flush () const
> +{
> +  m_buffered_streams->remove_buffers ();
> +
> +  for (const output_unit &ou : m_buffered_output)
> +    ou.flush ();
> +}
> diff --git a/gdb/ui-out.h b/gdb/ui-out.h
> index ba5b1de68ab..f7638992bf3 100644
> --- a/gdb/ui-out.h
> +++ b/gdb/ui-out.h
> @@ -278,6 +278,9 @@ class ui_out
>       escapes.  */
>    virtual bool can_emit_style_escape () const = 0;
>
> +  /* Return the ui_file currently used for output.  */
> +  virtual ui_file *current_stream () const = 0;
> +
>    /* An object that starts and finishes displaying progress updates.  */
>    class progress_update
>    {
> @@ -293,6 +296,21 @@ class ui_out
>        BAR
>      };
>
> +    /* Used to communicate the status of a newline prefix for the next progress
> +       update message.  */
> +    enum prefix_state
> +    {
> +      /* Do not modify the next progress update message.  */
> +      NEWLINE_OFF,
> +
> +      /* The next progress update message should include a newline prefix.  */
> +      NEWLINE_NEEDED,
> +
> +      /* A newline prefix was included in a debuginfod progress update
> +        message.  */
> +      NEWLINE_PRINTED
> +    };
> +
>      /* SHOULD_PRINT indicates whether something should be printed for a tty.  */
>      progress_update ()
>      {
> @@ -390,6 +408,11 @@ class ui_out
>    ui_out_level *current_level () const;
>  };
>
> +typedef ui_out::progress_update::prefix_state prefix_state_t;
> +
> +/* Current state of the newline prefix.  */
> +extern prefix_state_t cur_prefix_state;
> +
>  /* Start a new tuple or list on construction, and end it on
>     destruction.  Normally this is used via the typedefs
>     ui_out_emit_tuple and ui_out_emit_list.  */
> @@ -470,4 +493,175 @@ class ui_out_redirect_pop
>    struct ui_out *m_uiout;
>  };
>
> +struct buffered_streams;
> +
> +/* Organizes writes to a collection of buffered output streams
> +   so that when flushed, output is written to all streams in
> +   chronological order.  */
> +
> +struct buffer_group
> +{
> +  buffer_group ();
> +
> +  ~buffer_group ();
> +
> +  /* Flush all buffered writes to the underlying output streams.  */
> +  void flush () const;
> +
> +  /* Record contents of BUF and associate it with STREAM.  */
> +  void write (const char *buf, long length_buf, ui_file *stream);
> +
> +  /* Record a wrap_here and associate it with STREAM.  */
> +  void wrap_here (int indent, ui_file *stream);
> +
> +private:
> +
> +  struct output_unit
> +  {
> +    output_unit (std::string msg, int wrap_hint = -1)
> +      : m_msg (msg), m_wrap_hint (wrap_hint)
> +    {}
> +
> +    /* Write contents of this output_unit to the underlying stream.  */
> +    void flush () const;
> +
> +    /* Underlying stream for which this output unit will be written to.  */
> +    ui_file *m_stream;
> +
> +    /* String to be written to underlying buffer.  */
> +    std::string m_msg;
> +
> +    /* Argument to wrap_here.  -1 indicates no wrap.  Used to call wrap_here
> +       at appropriate times during flush.  */
> +    int m_wrap_hint;
> +  };
> +
> +  /* Output_units to be written to buffered output streams.  */
> +  std::vector<output_unit> m_buffered_output;
> +
> +  /* Buffered output streams.  */
> +  std::unique_ptr<buffered_streams> m_buffered_streams;
> +};
> +
> +/* If FILE is a buffering_file, return it's underlying stream.  */
> +
> +extern ui_file *get_unbuffered (ui_file *file);
> +
> +/* Buffer output to gdb_stdout and gdb_stderr for the duration of FUNC.  */
> +
> +template<typename F, typename... Arg>
> +void
> +do_with_buffered_output (F func, ui_out *uiout, Arg... args)
> +{
> +  buffer_group g;
> +
> +  try
> +    {
> +      func (uiout, std::forward<Arg> (args)...);
> +    }
> +  catch (gdb_exception &ex)
> +    {
> +      /* Ideally flush would be called in the destructor of buffer_group,
> +        however flushing might cause an exception to be thrown.  Catch it
> +        and ensure the first exception propagates.  */
> +      try
> +       {
> +         g.flush ();
> +       }
> +      catch (const gdb_exception &)
> +       {
> +       }
> +
> +      throw_exception (std::move (ex));
> +    }
> +
> +  /* Try was successful.  Let any further exceptions propagate.  */
> +  g.flush ();
> +}
> +
> +/* Accumulate writes to an underlying ui_file.  Output to the
> +   underlying file is deferred until required.  */
> +
> +struct buffering_file : public ui_file
> +{
> +  buffering_file (buffer_group *group, ui_file *stream)
> +    : m_group (group),
> +      m_stream (stream)
> +  { /* Nothing.  */ }
> +
> +  /* Return the underlying output stream.  */
> +  ui_file *stream () const
> +  {
> +    return m_stream;
> +  }
> +
> +  /* Record the contents of BUF.  */
> +  void write (const char *buf, long length_buf) override
> +  {
> +    m_group->write (buf, length_buf, m_stream);
> +  }
> +
> +  /* Record a wrap_here call with argument INDENT.  */
> +  void wrap_here (int indent) override
> +  {
> +    m_group->wrap_here (indent, m_stream);
> +  }
> +
> +  /* Return true if the underlying stream is a tty.  */
> +  bool isatty () override
> +  {
> +    return m_stream->isatty ();
> +  }
> +
> +  /* Return true if ANSI escapes can be used on the underlying stream.  */
> +  bool can_emit_style_escape () override
> +  {
> +    return m_stream->can_emit_style_escape ();
> +  }
> +
> +  /* Flush the underlying output stream.  */
> +  void flush () override
> +  {
> +    return m_stream->flush ();
> +  }
> +
> +private:
> +
> +  /* Coordinates buffering across multiple buffering_files.  */
> +  buffer_group *m_group;
> +
> +  /* The underlying output stream.  */
> +  ui_file *m_stream;
> +};
> +
> +/* Attaches and detaches buffers for each of the gdb_std* streams.  */
> +
> +struct buffered_streams
> +{
> +  buffered_streams (buffer_group *group);
> +
> +  ~buffered_streams ()
> +  {
> +    this->remove_buffers ();
> +  }
> +
> +  /* Remove buffering_files from all underlying streams.  */
> +  void remove_buffers ();
> +
> +private:
> +
> +  /* True if buffers are still attached to each underlying output stream.  */
> +  bool m_buffers_in_place;
> +
> +  /* Buffers for each gdb_std* output stream.  */
> +  buffering_file m_buffered_stdout;
> +  buffering_file m_buffered_stderr;
> +  buffering_file m_buffered_stdlog;
> +  buffering_file m_buffered_stdtarg;
> +  buffering_file m_buffered_stdtargerr;
> +
> +  /* Buffer for current_uiout's output stream.  */
> +  gdb::optional<buffering_file> m_buffered_uiout;
> +};
> +
>  #endif /* UI_OUT_H */
> --
> 2.41.0
>