Re: [PATCH 2/3] qga: Add optional `merge-output` flag to guest-exec qapi

[Daniel Xu]

Hi,

On Mon, Feb 27, 2023, at 1:22 AM, Marc-André Lureau wrote:
> Hi
>
> On Fri, Feb 24, 2023 at 8:31 AM Daniel Xu <dxu@dxuuu.xyz> wrote:
>>
>> Currently, the captured output (via `capture-output`) is segregated into
>> separate GuestExecStatus fields (`out-data` and `err-data`). This means
>> that downstream consumers have no way to reassemble the captured data
>> back into the original stream.
>>
>> This is relevant for chatty and semi-interactive (ie. read only) CLI
>> tools.  Such tools may deliberately interleave stdout and stderr for
>> visual effect. If segregated, the output becomes harder to visually
>> understand.
>>
>> This commit adds a new optional flag to the guest-exec qapi to merge the
>> output streams such that consumers can have a pristine view of the
>> original command output.
>>
>> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
>> ---
>>  qga/commands.c       | 13 ++++++++++++-
>>  qga/qapi-schema.json |  6 +++++-
>>  2 files changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/qga/commands.c b/qga/commands.c
>> index 360077364e..14b970e768 100644
>> --- a/qga/commands.c
>> +++ b/qga/commands.c
>> @@ -274,6 +274,15 @@ static void guest_exec_child_watch(GPid pid, gint 
>> status, gpointer data)
>>  /** Reset ignored signals back to default. */
>>  static void guest_exec_task_setup(gpointer data)
>>  {
>> +    bool has_merge = *(bool *)data;
>> +
>> +    if (has_merge) {
>> +        if (dup2(STDOUT_FILENO, STDERR_FILENO) != 0) {
>> +            slog("dup2() failed to merge stderr into stdout: %s",
>> +                 strerror(errno));
>> +        }
>> +    }
>
> https://docs.gtk.org/glib/callback.SpawnChildSetupFunc.html
>
> "On Windows, the function is called in the parent. Its usefulness on
> Windows is thus questionable. In many cases executing the child setup
> function in the parent can have ill effects, and you should be very
> careful when porting software to Windows that uses child setup
> functions."
>
> It looks like this would be bad.

Ah that's a good catch. I'm not very familiar with windows APIs so
unfortunately I don't have any good ideas here.

Best I can tell g_spawn_async_with_pipes_and_fds() work with it's
source_fds and target_fds mapping. But it looks like that came in
glib 2.68 so we cannot use it yet.

How about limiting this merge-output flag to linux/unix systems
for now? Could document this in the qapi doc string.

>
>> +
>>  #if !defined(G_OS_WIN32)
>>      struct sigaction sigact;
>>
>> @@ -385,6 +394,7 @@ GuestExec *qmp_guest_exec(const char *path,
>>                         bool has_env, strList *env,
>>                         const char *input_data,
>>                         bool has_capture_output, bool capture_output,
>> +                       bool has_merge_output, bool merge_output,
>>                         Error **errp)
>>  {
>>      GPid pid;
>> @@ -398,6 +408,7 @@ GuestExec *qmp_guest_exec(const char *path,
>>      GIOChannel *in_ch, *out_ch, *err_ch;
>>      GSpawnFlags flags;
>>      bool has_output = (has_capture_output && capture_output);
>> +    bool has_merge = (has_merge_output && merge_output);
>>      g_autofree uint8_t *input = NULL;
>>      size_t ninput = 0;
>>
>> @@ -421,7 +432,7 @@ GuestExec *qmp_guest_exec(const char *path,
>>      }
>>
>>      ret = g_spawn_async_with_pipes(NULL, argv, envp, flags,
>> -            guest_exec_task_setup, NULL, &pid, input_data ? &in_fd : NULL,
>> +            guest_exec_task_setup, &has_merge, &pid, input_data ? &in_fd : 
>> NULL,
>>              has_output ? &out_fd : NULL, has_output ? &err_fd : NULL, 
>> &gerr);
>>      if (!ret) {
>>          error_setg(errp, QERR_QGA_COMMAND_FAILED, gerr->message);
>> diff --git a/qga/qapi-schema.json b/qga/qapi-schema.json
>> index 796434ed34..4192fcc5a4 100644
>> --- a/qga/qapi-schema.json
>> +++ b/qga/qapi-schema.json
>> @@ -1211,6 +1211,9 @@
>>  # @input-data: data to be passed to process stdin (base64 encoded)
>>  # @capture-output: bool flag to enable capture of
>>  #                  stdout/stderr of running process. defaults to false.
>> +# @merge-output: bool flag to merge stdout/stderr of running process
>> +#                into stdout. only effective if used with @capture-output.
>> +#                defaults to false.
>
> Add (since: 8.0)

Ack.

[...]

Thanks,
Daniel