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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jul 02, 2020 at 07:13:08AM +0200, Markus Armbruster wrote:
> 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?

I was really in two minds as to whether to keep this error_report or
not. It depends depends on whether any other callers of qemu_open
still need it. In fact if nothing outside the block layer uses O_DIRECT
then I think we can remove the error_report line.


> > +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().

My goal was to avoid a big bang conversion of all callers.

> 
> 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().

Maybe it is better to rename upfront though, instead of waiting till
everything uses the qemu_open_err and then renaming back to qemu_open.

Regards,
Daniel
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