Re: [PATCH 2/3] util: support detailed error reporting for qemu_open

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> Create a "qemu_open_err" method which does the same as "qemu_open",
> but with a "Error **errp" for error reporting. There should be no
> behavioural difference for existing callers at this stage.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  include/qemu/osdep.h |  1 +
>  util/osdep.c         | 71 +++++++++++++++++++++++++++++++++++---------
>  2 files changed, 58 insertions(+), 14 deletions(-)
>
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 0d26a1b9bd..e41701a308 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -483,6 +483,7 @@ int qemu_madvise(void *addr, size_t len, int advice);
>  int qemu_mprotect_rwx(void *addr, size_t size);
>  int qemu_mprotect_none(void *addr, size_t size);
>  
> +int qemu_open_err(const char *name, int flags, Error **errp, ...);
>  int qemu_open(const char *name, int flags, ...);
>  int qemu_close(int fd);
>  int qemu_unlink(const char *name);
> diff --git a/util/osdep.c b/util/osdep.c
> index 4bdbe81cec..450b3a5da3 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -22,6 +22,7 @@
>   * THE SOFTWARE.
>   */
>  #include "qemu/osdep.h"
> +#include "qapi/error.h"
>  
>  /* Needed early for CONFIG_BSD etc. */
>  
> @@ -282,7 +283,7 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive)
>  /*
>   * Opens a file with FD_CLOEXEC set
>   */
> -int qemu_open(const char *name, int flags, ...)
> +static int qemu_openv(const char *name, int flags, Error **errp, va_list ap)
>  {
>      int ret;
>      int mode = 0;
> @@ -297,24 +298,31 @@ int qemu_open(const char *name, int flags, ...)
>  
>          fdset_id = qemu_parse_fdset(fdset_id_str);
>          if (fdset_id == -1) {
> +            error_setg(errp, "Unable to parse fdset %s", name);
>              errno = EINVAL;
>              return -1;
>          }
>  
>          fd = monitor_fdset_get_fd(fdset_id, flags);
>          if (fd < 0) {
> +            error_setg_errno(errp, -fd, "Unable acquire FD for %s flags %x",
> +                             name, flags);
>              errno = -fd;
>              return -1;
>          }
>  
>          dupfd = qemu_dup_flags(fd, flags);
>          if (dupfd == -1) {
> +            error_setg_errno(errp, errno, "Unable dup FD for %s flags %x",
> +                             name, flags);
>              return -1;
>          }
>  
>          ret = monitor_fdset_dup_fd_add(fdset_id, dupfd);
>          if (ret == -1) {
>              close(dupfd);
> +            error_setg(errp, "Unable save FD for %s flags %x",
> +                       name, flags);
>              errno = EINVAL;
>              return -1;
>          }
> @@ -324,11 +332,7 @@ int qemu_open(const char *name, int flags, ...)
>  #endif
>  
>      if (flags & O_CREAT) {
> -        va_list ap;
> -
> -        va_start(ap, flags);
>          mode = va_arg(ap, int);
> -        va_end(ap);
>      }
>  
>  #ifdef O_CLOEXEC
> @@ -340,25 +344,64 @@ int qemu_open(const char *name, int flags, ...)
>      }
>  #endif
>  
> +    if (ret == -1) {
> +        const char *action = "open";
> +        if (flags & O_CREAT) {
> +            action = "create";
> +        }
>  #ifdef O_DIRECT
> -    if (ret == -1 && errno == EINVAL && (flags & O_DIRECT)) {
> -        int newflags = flags & ~O_DIRECT;
> +        if (errno == EINVAL && (flags & O_DIRECT)) {
> +            int newflags = flags & ~O_DIRECT;
>  # ifdef O_CLOEXEC
> -        ret = open(name, newflags | O_CLOEXEC, mode);
> +            ret = open(name, newflags | O_CLOEXEC, mode);
>  # else
> -        ret = open(name, newflags, mode);
> +            ret = open(name, newflags, mode);
>  # endif
> -        if (ret != -1) {
> -            close(ret);
> -            error_report("file system does not support O_DIRECT");
> -            errno = EINVAL;
> +            if (ret != -1) {
> +                close(ret);
> +                error_setg(errp, "Unable to %s '%s' flags 0x%x: "
> +                           "filesystem does not support O_DIRECT",
> +                           action, name, flags);
> +                if (!errp) {

If the caller ignores errors, ...

> +                    error_report("file system does not support O_DIRECT");

... we report this error to stderr (but not any of the other ones).
This is weird.  I figure you do it here to reproduce the weirdness of
qemu_open() before the patch.  Goes back to

    commit a5813077aac7865f69b7ee46a594f3705429f7cd
    Author: Stefan Hajnoczi <stefanha@redhat.com>
    Date:   Thu Aug 22 11:29:03 2013 +0200

        osdep: warn if open(O_DIRECT) on fails with EINVAL

        Print a warning when opening a file O_DIRECT fails with EINVAL.  This
        saves users a lot of time trying to figure out the EINVAL error, which
        is typical when attempting to open a file O_DIRECT on Linux tmpfs.

        Reported-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
        Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
        Reviewed-by: Eric Blake <eblake@redhat.com>

The message isn't phrased as a warning, though.  Should it use
warn_report()?  Stefan?

Regardless, since use of error_report() in a function that returns
errors through its Error ** parameter is an anti-pattern, this use needs
a comment.  I'd make it a TODO comment: convert all callers where the
warning is wanted to qemu_open_err(), then drop it.

> +                }
> +                errno = EINVAL;
> +                return -1;
> +            }
>          }
> -    }
>  #endif /* O_DIRECT */
> +        error_setg_errno(errp, errno, "Unable to %s '%s' flags 0x%x",
> +                         action, name, flags);
> +    }
> +
>  
>      return ret;
>  }
>  
> +int qemu_open_err(const char *name, int flags, Error **errp, ...)
> +{
> +    va_list ap;
> +    int rv;
> +
> +    va_start(ap, errp);
> +    rv = qemu_openv(name, flags, errp, ap);
> +    va_end(ap);
> +
> +    return rv;
> +}
> +
> +int qemu_open(const char *name, int flags, ...)
> +{
> +    va_list ap;
> +    int rv;
> +
> +    va_start(ap, flags);
> +    rv = qemu_openv(name, flags, NULL, ap);
> +    va_end(ap);
> +
> +    return rv;
> +}

I'd rename this to qemu_open_with_bad_error_messages().

For better ones, callers can use

    if (qemu_open_err(name, flags, &err) < 0) {
        error_report_err(err);
        ...
    }

or, where the error is fatal
    
    qemu_open_err(name, flags, &error_fatal);

If you prefer not to rename it now, please add a comment why it should
not be used in new code, and existing uses should be converted.

If you rename, call the new one qemu_open().

> +
>  int qemu_close(int fd)
>  {
>      int64_t fdset_id;