Re: [PATCH v15 08/20] multi-process: add qio channel read function

[Jag Raman]

> On Dec 23, 2020, at 6:24 AM, Marc-André Lureau <marcandre.lureau@gmail.com> 
> wrote:
> 
> Hi
> 
> On Wed, Dec 23, 2020 at 10:17 AM <elena.ufimtseva@oracle.com> wrote:
> From: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> 
> Adds qio_channel_readv_full_all() to read both data and FDs.
> Refactors existing code to use this function.
> 
> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> ---
>  include/io/channel.h | 25 +++++++++++++
>  io/channel.c         | 85 +++++++++++++++++++++++++++++++-------------
>  2 files changed, 85 insertions(+), 25 deletions(-)
> 
> diff --git a/include/io/channel.h b/include/io/channel.h
> index 2378567d4b..429ece9a05 100644
> --- a/include/io/channel.h
> +++ b/include/io/channel.h
> @@ -774,6 +774,31 @@ void qio_channel_set_aio_fd_handler(QIOChannel *ioc,
>                                      IOHandler *io_write,
>                                      void *opaque);
> 
> +/**
> + * qio_channel_readv_full_all:
> + * @ioc: the channel object
> + * @iov: the array of memory regions to read data to
> + * @niov: the length of the @iov array
> + * @fds: an array of file handles to read
> + * @nfds: number of file handles in @fds
> + * @errp: pointer to a NULL-initialized error object
> + *
> + *
> + * Behaves like qio_channel_readv_full but will attempt
> + * to read all data specified (file handles and memory regions).
> + * The function will wait for all requested data
> + * to be read, yielding from the current coroutine
> + * if required.
> + *
> + * Returns: 0 if all bytes were read, or -1 on error
> 
> It may also returns -ECANCEL. I am not sure it's a good idea. 
> 
> + */
> +
> +int qio_channel_readv_full_all(QIOChannel *ioc,
> +                                const struct iovec *iov,
> +                                size_t niov,
> +                                int **fds, size_t *nfds,
> +                                Error **errp);
> +
>  /**
>   * qio_channel_writev_full_all:
>   * @ioc: the channel object
> diff --git a/io/channel.c b/io/channel.c
> index bde1f6d0f4..5edaea1fac 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -91,11 +91,49 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
>                                const struct iovec *iov,
>                                size_t niov,
>                                Error **errp)
> +{
> +    int ret = qio_channel_readv_full_all(ioc, iov, niov, NULL, NULL, errp);
> +
> +    if (ret == -ECANCELED) {
> 
>  No io/ functions use -errno return values so far.
> 
> Maybe the simplest is to use the same return values as read_all_eof:
>  * Returns: 1 if all bytes were read, 0 if end-of-file occurs
>  *          without data, or -1 on error

Hi Marc-Andre,

qio_channel_readv_all_eof() also sets the Error variable when the channel
is closed in the middle of reading (partial read).

qio_channel_readv_full_all() needs to return a special value to
qio_channel_readv_all_eof() in the case of partial reads, aside
from '-1' for all other error scenarios. qio_channel_readv_full_all()
returns '-ECANCEL' to identify partial reads.

qio_channel_readv_full_all() could directly set this error variable for partial
read scenarios, but then there wouldn't be any difference between the
_full_all() version and _all_eof() version. Is that alright?

Thank you!
—
Jag


> 
> +        error_prepend(errp,
> +                      "Unexpected end-of-file before all bytes were read: ");
> +        ret = -1;
> +    }
> +
> +    return ret;
> +}
> +
> +int qio_channel_readv_all(QIOChannel *ioc,
> +                          const struct iovec *iov,
> +                          size_t niov,
> +                          Error **errp)
> +{
> +    int ret = qio_channel_readv_all_eof(ioc, iov, niov, errp); 
> +
> 
> It looks like it would make more sense to call readv_full_all directly 
> instead now.
> 
> +    if (ret == 0) {
> +        error_setg(errp,
> +                   "Unexpected end-of-file before all bytes were read");
> +        return -1;
> +    }
> +    if (ret == 1) {
> +        return 0;
> +    }
> +
> +    return ret;
> +}
> +
> +int qio_channel_readv_full_all(QIOChannel *ioc,
> +                                const struct iovec *iov,
> +                                size_t niov,
> +                                int **fds, size_t *nfds,
> +                                Error **errp)
>  {
>      int ret = -1;
>      struct iovec *local_iov = g_new(struct iovec, niov);
>      struct iovec *local_iov_head = local_iov;
>      unsigned int nlocal_iov = niov;
> +    int **local_fds = fds;
> +    size_t *local_nfds = nfds;
>      bool partial = false;
> 
>      nlocal_iov = iov_copy(local_iov, nlocal_iov,
> @@ -104,7 +142,8 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
> 
>      while (nlocal_iov > 0) {
>          ssize_t len;
> -        len = qio_channel_readv(ioc, local_iov, nlocal_iov, errp);
> +        len = qio_channel_readv_full(ioc, local_iov, nlocal_iov, local_fds,
> +                                     local_nfds, errp);
>          if (len == QIO_CHANNEL_ERR_BLOCK) {
>              if (qemu_in_coroutine()) {
>                  qio_channel_yield(ioc, G_IO_IN);
> @@ -112,20 +151,33 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
>                  qio_channel_wait(ioc, G_IO_IN);
>              }
>              continue;
> -        } else if (len < 0) {
> -            goto cleanup;
> -        } else if (len == 0) {
> -            if (partial) {
> -                error_setg(errp,
> -                           "Unexpected end-of-file before all bytes were 
> read");
> +        }
> +
> +        if (len <= 0) {
> +            size_t fd_idx = nfds ? *nfds : 0;
> +            if (partial && (len == 0)) {
> +                ret = -ECANCELED;
>              } else {
> -                ret = 0;
> +                ret = len;
> +            }
> +
> +            while (fds && fd_idx) {
> +                close(*fds[fd_idx - 1]);
> +                fd_idx--;
> +            }
> +
> +            if (fds) {
> +                g_free(*fds);
>              }
> +
>              goto cleanup;
>          }
> 
>          partial = true;
>          iov_discard_front(&local_iov, &nlocal_iov, len);
> +
> +        local_fds = NULL;
> +        local_nfds = 0;
>      }
> 
>      ret = 1;
> @@ -135,23 +187,6 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
>      return ret;
>  }
> 
> -int qio_channel_readv_all(QIOChannel *ioc,
> -                          const struct iovec *iov,
> -                          size_t niov,
> -                          Error **errp)
> -{
> -    int ret = qio_channel_readv_all_eof(ioc, iov, niov, errp);
> -
> -    if (ret == 0) {
> -        ret = -1;
> -        error_setg(errp,
> -                   "Unexpected end-of-file before all bytes were read");
> -    } else if (ret == 1) {
> -        ret = 0;
> -    }
> -    return ret;
> -}
> -
>  int qio_channel_writev_all(QIOChannel *ioc,
>                             const struct iovec *iov,
>                             size_t niov,
> -- 
> 2.25.GIT
> 
> 
> 
> -- 
> Marc-André Lureau