[libvirt] [PATCH 2/3] util: Introduce virFileWaitForAccess

[Dawid Zamirski]

On Tue, 2017-06-20 at 17:03 +0200, Erik Skultety wrote:
> Since we have a number of places where we workaround timing issues
> with
> devices, attributes (files in general) not being available at the
> time
> of processing them by calling usleep in a loop for a fixed number of
> tries, we could as well have a utility function that would do that.
> Therefore we won't have to duplicate this ugly workaround even more.
> 
> This is a prerequisite for
> https://bugzilla.redhat.com/show_bug.cgi?id=1463285.
> 
> Signed-off-by: Erik Skultety <eskultet at redhat.com>
> ---
>  src/libvirt_private.syms |  1 +
>  src/util/virfile.c       | 36 ++++++++++++++++++++++++++++++++++++
>  src/util/virfile.h       |  2 ++
>  3 files changed, 39 insertions(+)
> 
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index c1e9471c5..53878a30f 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -1698,6 +1698,7 @@ virFileStripSuffix;
>  virFileTouch;
>  virFileUnlock;
>  virFileUpdatePerm;
> +virFileWaitForAccess;
>  virFileWrapperFdClose;
>  virFileWrapperFdFree;
>  virFileWrapperFdNew;
> diff --git a/src/util/virfile.c b/src/util/virfile.c
> index 6bbcc3d15..0b1a91699 100644
> --- a/src/util/virfile.c
> +++ b/src/util/virfile.c
> @@ -4164,3 +4164,39 @@ virFileReadValueString(char **value, const
> char *format, ...)
>      VIR_FREE(str);
>      return ret;
>  }
> +
> +
> +/**
> + * virFileWaitForAccess:
> + * @path: absolute path to a sysfs attribute (can be a symlink)
> + * @ms: how long to wait (in milliseconds)
> + * @tries: how many times should we try to wait for @path to become
> accessible
> + *
> + * Checks the existence of @path. In case the file defined by @path
> + * doesn't exist, we wait for it to appear in 100ms (for up to
> @tries times).
> + *
> + * Returns 0 on success, -1 on error (ENOENT is fine here).
> + */
> +int
> +virFileWaitForAccess(const char *path, size_t ms, size_t tries)
> +{
> +    errno = 0;
> +
> +    /* wait for @path to be accessible in @ms milliseconds, up to
> @tries */
> +    while (tries-- > 0 && !virFileExists(path)) {
> +        if (errno != ENOENT) {
> +            virReportSystemError(errno, "%s", path);
> +            return -1;
> +        } else if (tries == 10) {
> +            virReportError(VIR_ERR_INTERNAL_ERROR,
> +                           _("Failed to access '%s' after %zu
> tries"),
> +                           path, tries);
> +            return -1;
> +        } else {
> +            VIR_DEBUG("Failed to access '%s', re-try in %zu ms",
> path, ms);
> +            usleep(ms * 1000);
> +        }
> +    }
> +
> +    return 0;
> +}

Just FYI, there's another way to address it by calling udevadm settle
before and after "touching" a block device, libguestfs is using this
approach and it works very well:

https://github.com/libguestfs/libguestfs/search?utf8=%E2%9C%93&q=udev_s
ettle&type=

> diff --git a/src/util/virfile.h b/src/util/virfile.h
> index 57ceb8072..42630ebb5 100644
> --- a/src/util/virfile.h
> +++ b/src/util/virfile.h
> @@ -347,6 +347,8 @@ int virFileReadValueScaledInt(unsigned long long
> *value, const char *format, ...
>  int virFileReadValueString(char **value, const char *format, ...)
>   ATTRIBUTE_FMT_PRINTF(2, 3);
>  
> +int virFileWaitForAccess(const char *path, size_t ms, size_t tries);
> +
>  
>  int virFileInData(int fd,
>                    int *inData,