Re: [PATCH v2 1/2] tools subcmd: Add non-waitpid check_if_command_finished()

[Ian Rogers <irogers@google.com>]

On Tue, Jul 2, 2024 at 8:24 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hi Ian,
>
> On Sun, Jun 30, 2024 at 09:42:35PM -0700, Ian Rogers wrote:
> > Using waitpid can cause stdout/stderr of the child process to be
> > lost. Use Linux's /prod/<pid>/status file to determine if the process
> > has reached the zombie state. Use the 'status' file rather than 'stat'
> > to avoid issues around skipping the process name.
> >
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  tools/lib/subcmd/run-command.c | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/tools/lib/subcmd/run-command.c b/tools/lib/subcmd/run-command.c
> > index 4e3a557a2f37..ec06683e77a0 100644
> > --- a/tools/lib/subcmd/run-command.c
> > +++ b/tools/lib/subcmd/run-command.c
> > @@ -2,6 +2,7 @@
> >  #include <unistd.h>
> >  #include <sys/types.h>
> >  #include <sys/stat.h>
> > +#include <ctype.h>
> >  #include <fcntl.h>
> >  #include <string.h>
> >  #include <linux/string.h>
> > @@ -217,8 +218,40 @@ static int wait_or_whine(struct child_process *cmd, bool block)
> >
> >  int check_if_command_finished(struct child_process *cmd)
> >  {
> > +#ifdef __linux__
>
> Is this really necessary?  I don't think we plan to support other OS..

I don't think it'd be unreasonable to say run "perf report" on
Windows, or using wasm inside a web browser. Part of the reason for
doing things this way was to keep the WNOHANG logic although this
change no longer uses it for __linux__.

Thanks,
Ian

> Thanks,
> Namhyung
>
>
> > +     char filename[FILENAME_MAX + 12];
> > +     char status_line[256];
> > +     FILE *status_file;
> > +
> > +     /*
> > +      * Check by reading /proc/<pid>/status as calling waitpid causes
> > +      * stdout/stderr to be closed and data lost.
> > +      */
> > +     sprintf(filename, "/proc/%d/status", cmd->pid);
> > +     status_file = fopen(filename, "r");
> > +     if (status_file == NULL) {
> > +             /* Open failed assume finish_command was called. */
> > +             return true;
> > +     }
> > +     while (fgets(status_line, sizeof(status_line), status_file) != NULL) {
> > +             char *p;
> > +
> > +             if (strncmp(status_line, "State:", 6))
> > +                     continue;
> > +
> > +             fclose(status_file);
> > +             p = status_line + 6;
> > +             while (isspace(*p))
> > +                     p++;
> > +             return *p == 'Z';
> > +     }
> > +     /* Read failed assume finish_command was called. */
> > +     fclose(status_file);
> > +     return true;
> > +#else
> >       wait_or_whine(cmd, /*block=*/false);
> >       return cmd->finished;
> > +#endif
> >  }
> >
> >  int finish_command(struct child_process *cmd)
> > --
> > 2.45.2.803.g4e1b14247a-goog
> >