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

[Eric Blake <eblake@redhat.com>]

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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.

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.

> 
> 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.

> +        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>

>      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


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