Re: [pbs-devel] [RFC pxar 4/20] fix #3174: metadata: impl fn to calc byte size

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Fri, Sep 22, 2023 at 09:16:05AM +0200, Christian Ebner wrote:
> Add a helper function to calculate the byte size of pxar Metadata
> objects, needed to be able to recalculate offsets when creating archives
> with appendix sections.
> 
> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
> ---
>  src/lib.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/src/lib.rs b/src/lib.rs
> index 210c4b1..ed7ba40 100644
> --- a/src/lib.rs
> +++ b/src/lib.rs
> @@ -144,6 +144,50 @@ impl Metadata {
>      pub fn builder_from_stat(stat: &libc::stat) -> MetadataBuilder {
>          MetadataBuilder::new(0).fill_from_stat(stat)
>      }
> +
> +    /// Calculate the number of bytes when serialized in pxar archive
> +    pub fn calculate_byte_len(&self) -> usize {

This looks like a maintenance nightmare with extra sprinkles.

Can we instead create a specialized `SeqWrite` type that just counts
bytes instead of actually writing anything and call the encoder's
`encode_metadata()` with it?
(Either by factorizing `encode_metadat()` out further or by creating an
actual `EncoderImpl` instance with it...)

> +        let mut bytes = mem::size_of::<format::Header>();
> +        bytes += mem::size_of_val(&self.stat);
> +        for xattr in &self.xattrs {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(xattr);
> +        }
> +        for acl_user in &self.acl.users {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(acl_user);
> +        }
> +        for acl_group in &self.acl.groups {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(acl_group);
> +        }
> +        if let Some(group_obj) = &self.acl.group_obj {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(group_obj);
> +        }
> +        if let Some(default) = &self.acl.default {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(default);
> +        }
> +        for acl_default_user in &self.acl.default_users {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(acl_default_user);
> +        }
> +        for acl_default_group in &self.acl.default_groups {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(acl_default_group);
> +        }
> +        if let Some(fcaps) = &self.fcaps {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(fcaps);
> +        }
> +        if let Some(quota_project_id) = &self.quota_project_id {
> +            bytes += mem::size_of::<format::Header>();
> +            bytes += mem::size_of_val(quota_project_id);
> +        }
> +
> +        bytes
> +    }
>  }
>  
>  impl From<MetadataBuilder> for Metadata {
> -- 
> 2.39.2