Re: [Qemu-devel] [PATCH 1/2] qga: flush implicitely when needed

["Marc-André Lureau" <mlureau@redhat.com>]

Hi

----- Original Message -----
> On 11/24/2015 09:34 AM, marcandre.lureau@redhat.com wrote:
> > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> In the subject: s/implicitely/implicitly/ if you are fixing the typo, or
> s/implicitely/explicitly/ if you are trying to make it match what the
> patch actually does.
> 

ok, I'll switch to explicitely (it depends on the point of view, I was commenting from the qga API user pov, but I get your point)
 
> No 0/2 cover letter?  ALL multi-patch series should include a cover
> letter, as it is easier on tooling to be able to base series-wide
> conversations on the cover letter.
> 

Ok, I didn't know. If I don't have much to say in cover letter, I usually drop it. I'll keep it then.

> > 
> > According to the specification:
> > http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
> > 
> > "the application shall ensure that output is not directly followed by
> > input without an intervening call to fflush() or to a file positioning
> > function (fseek(), fsetpos(), or rewind()), and input is not directly
> > followed by output without an intervening call to a file positioning
> > function, unless the input operation encounters end-of-file."
> > 
> > Without this change, a write() followed by a read() may lose the
> > previously written content, as shown in the following test.
> > 
> > Fixes:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1210246
> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  qga/commands-posix.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> > index 0ebd473..3c86a4e 100644
> > --- a/qga/commands-posix.c
> > +++ b/qga/commands-posix.c
> > @@ -219,6 +219,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns,
> > Error **errp)
> >  typedef struct GuestFileHandle {
> >      uint64_t id;
> >      FILE *fh;
> > +    bool writing;
> >      QTAILQ_ENTRY(GuestFileHandle) next;
> >  } GuestFileHandle;
> >  
> > @@ -460,6 +461,17 @@ struct GuestFileRead *qmp_guest_file_read(int64_t
> > handle, bool has_count,
> >      }
> >  
> >      fh = gfh->fh;
> > +
> > +    /* implicitely flush when switching from writing to reading */
> 
> Again, s/implicitely/explicitly/
> 
> > +    if (gfh->writing) {
> > +        int ret = fflush(fh);
> > +        if (ret == EOF) {
> > +            error_setg_errno(errp, errno, "failed to flush file");
> > +            return NULL;
> > +        }
> > +        gfh->writing = false;
> > +    }
> > +
> >      buf = g_malloc0(count+1);
> >      read_count = fread(buf, 1, count, fh);
> >      if (ferror(fh)) {
> > @@ -496,6 +508,16 @@ GuestFileWrite *qmp_guest_file_write(int64_t handle,
> > const char *buf_b64,
> >      }
> >  
> >      fh = gfh->fh;
> > +
> > +    if (!gfh->writing) {
> > +        int ret = fseek(fh, 0, SEEK_CUR);
> 
> Seems a bit odd to use fflush() in one place and fseek() in the other,
> but the net result is the same either way.

"and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file."

so I tried to follow what the spec said.

> 
> > +        if (ret == -1) {
> > +            error_setg_errno(errp, errno, "failed to seek file");
> > +            return NULL;
> > +        }
> > +        gfh->writing = true;
> > +    }
> > +
> 
> With typos fixed,
> Reviewed-by: Eric Blake <eblake@redhat.com>

thanks

> 
> >      buf = g_base64_decode(buf_b64, &buf_len);
> >  
> >      if (!has_count) {
> > 
> 
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
> 
>