[libvirt] [PATCH v3 01/13] virprocess: Introduce virProcessRunInFork

[Daniel P. Berrangé]

On Wed, Oct 17, 2018 at 11:06:43AM +0200, Michal Privoznik wrote:
> This new helper can be used to spawn a child process and run
> passed callback from it. This will come handy esp. if the
> callback is not thread safe.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/libvirt_private.syms |  1 +
>  src/util/virprocess.c    | 81 ++++++++++++++++++++++++++++++++++++++++
>  src/util/virprocess.h    | 16 ++++++++
>  3 files changed, 98 insertions(+)

> +/**
> + * virProcessRunInFork:
> + * @cb: callback to run
> + * @opaque: opaque data to @cb
> + *
> + * Do the fork and run @cb in the child. This can be used when
> + * @cb does something thread unsafe, for instance. However, due
> + * to possible signal propagation @cb should only call async
> + * signal safe functions. On return, the returned value is either
> + * -1 with error message reported if something went bad in the
> + *  parent, if child has died from a signal or if the child
> + *  returned EXIT_CANCELED. Otherwise the returned value is the
> + *  exit status of the child.

Instead of;

  However, due to possible signal propagation @cb should only call
  async signal safe functions.

Use:

  All signals will be reset to have their platform default handlers
  and unmasked. @cb must only use async signal safe functions. In
  particular no mutexes should be used in @cb, unless steps were
  taken before forking to guarantee a predictable state. @cb must
  not exec any external binaries, instead virCommand/virExec should
  be used for that purpose.


> + */
> +int
> +virProcessRunInFork(virProcessForkCallback cb,
> +                    void *opaque)
> +{
> +    int ret = -1;
> +    pid_t child = -1;
> +    pid_t parent = getpid();
> +    int errfd[2] = { -1, -1 };
> +
> +    if (pipe2(errfd, O_CLOEXEC) < 0) {
> +        virReportSystemError(errno, "%s",
> +                             _("Cannot create pipe for child"));
> +        return -1;
> +    }
> +
> +    if ((child = virFork()) < 0)
> +        goto cleanup;
> +
> +    if (child == 0) {
> +        VIR_FORCE_CLOSE(errfd[0]);
> +        ret = virProcessForkHelper(errfd[1], parent, cb, opaque);
> +        VIR_FORCE_CLOSE(errfd[1]);
> +        _exit(ret < 0 ? EXIT_CANCELED : ret);
> +    } else {
> +        int status;
> +        VIR_AUTOFREE(char *) buf = NULL;
> +
> +        VIR_FORCE_CLOSE(errfd[1]);
> +        ignore_value(virFileReadHeaderFD(errfd[0], 1024, &buf));
> +        ret = virProcessWait(child, &status, false);
> +        if (!ret) {
> +            ret = status == EXIT_CANCELED ? -1 : status;
> +            if (ret) {
> +                virReportError(VIR_ERR_INTERNAL_ERROR,
> +                               _("child reported: %s"),
> +                               NULLSTR(buf));

It would be desirable to include the exit status value in the message

> +            }
> +        }
> +    }
> +
> + cleanup:
> +    VIR_FORCE_CLOSE(errfd[0]);
> +    VIR_FORCE_CLOSE(errfd[1]);
> +    return ret;
> +}

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|