[PATCH] gdb/cli: if redirecting output, also redirect top-level interpreter's ui-out

[Aktemur, Tankut Baris]

On Thursday, October 22, 2020 6:09 PM, Pedro Alves wrote:
> On 10/6/20 4:40 PM, Tankut Baris Aktemur via Gdb-patches wrote:
> > When executing commands from Python, the API provides an option to
> > capture the command output in a string and return this to the user.
> > See the `gdb.execute` function at
> >
> >   https://sourceware.org/gdb/onlinedocs/gdb/Basic-Python.html#Basic-Python
> >
> > When the `to_string` argument is True, the output of the executed
> > command is supposed to be captured and returned to the caller.  For
> > this, GDB temporarily redirects the ui-outs to a string file.
> >
> > However, output does not get captured if the executed command prints
> > its output through the top-level interpreter's ui-out.  E.g., in the
> > following we still see the output being printed.
> >
> >   (gdb) python gdb.execute("inferior 1", from_tty=False, to_string=True)
> >   [Switching to inferior 1 [<null>] (<noexec>)]
> 
> Note this output is not really considered output of the command.
> It's asynchronous output sent to all UIs in reaction to the
> inferior-switching event.
> 
> If e.g., the top interpreter is MI (say, you run that python from a breakpoint
> command), then you get:
> 
>  (gdb)
>  python gdb.execute("inferior 1", from_tty=False, to_string=True)
>  &"python gdb.execute(\"inferior 1\", from_tty=False, to_string=True)\n"
>  ~"[Switching to inferior 1 [<null>] (/home/pedro/gdb/binutils-gdb/build/gdb/gdb)]\n"
>  ^done
>  (gdb)
> 
> And your patch won't capture that:
> 
>  python str=gdb.execute("inferior 1", from_tty=False, to_string=True)
>  &"python str=gdb.execute(\"inferior 1\", from_tty=False, to_string=True)\n"
>  ~"[Switching to inferior 1 [<null>] (<noexec>)]\n"
>  ^done
>  (gdb)
>  python print(str)
>  &"python print(str)\n"
>  ~"\n"
>  ^done
>  (gdb)

Sorry for the delayed response.  The motivation behind this patch is this: 
We have a Python script via which we create an additional inferior, connect
it to a target, and later remove this inferior.  For the actions for which a
dedicated Python API does not exist, we use gdb.execute().  Switching the
inferior as well as connecting to a target produces output that we want to
hide from the user.  In principle, the goal is to suppress the output on the
CLI to obtain clean command-line interaction, but additionally we would like
to avoid temporary context switches in an IDE, if connected via MI.

Potential solutions:
1.  Expose "cli_suppress_notification" and "mi_suppress_notification" via
maintenance commands.  Use these commands in the Python script via gdb.execute()
to suppress the CLI and MI outputs.

2.  Extend the "add-inferior" command to accept a target string so that an
explicit switch is not necessary to make the target connection.  This would
avoid context switch-related output, but the target connection's output would
still be visible.  Perhaps CLI/MI suppress mechanism has to be used here, again.

3.  Extend the Python API to allow the actions we need (e.g. add a new inferior,
connect it to a target, etc.) instead of gdb.execute().  The Python API suppresses
the output before performing the actual actions.

Suggestions are welcome and appreciated.

Thanks
-Baris


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