[pbs-devel] [PATCH pxar 02/22] decoder: add peek()

[Wolfgang Bumiller]

On Tue, Feb 16, 2021 at 06:06:50PM +0100, Stefan Reiter wrote:
> Allows peeking the current element, but will not advance the state
> (except for contents() and content_size() functions).
> 
> Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
> ---
>  src/accessor/mod.rs |  3 +++
>  src/decoder/aio.rs  | 10 +++++++++-
>  src/decoder/mod.rs  | 19 +++++++++++++++++--
>  src/decoder/sync.rs | 10 +++++++++-
>  4 files changed, 38 insertions(+), 4 deletions(-)
> 
> diff --git a/src/accessor/mod.rs b/src/accessor/mod.rs
> index d02dc13..aa1b3f6 100644
> --- a/src/accessor/mod.rs
> +++ b/src/accessor/mod.rs
> @@ -293,6 +293,7 @@ impl<T: Clone + ReadAt> AccessorImpl<T> {
>          let entry = decoder
>              .next()
>              .await
> +            .transpose()
>              .ok_or_else(|| io_format_err!("unexpected EOF while decoding file entry"))??;
>          Ok(FileEntryImpl {
>              input: self.input.clone(),
> @@ -334,6 +335,7 @@ impl<T: Clone + ReadAt> AccessorImpl<T> {
>          let entry = decoder
>              .next()
>              .await
> +            .transpose()
>              .ok_or_else(|| io_format_err!("unexpected EOF while following a hardlink"))??;
>  
>          match entry.kind() {
> @@ -516,6 +518,7 @@ impl<T: Clone + ReadAt> DirectoryImpl<T> {
>          let entry = decoder
>              .next()
>              .await
> +            .transpose()
>              .ok_or_else(|| io_format_err!("unexpected EOF while decoding directory entry"))??;
>          Ok((entry, decoder))
>      }
> diff --git a/src/decoder/aio.rs b/src/decoder/aio.rs
> index 5cc6694..c553d45 100644
> --- a/src/decoder/aio.rs
> +++ b/src/decoder/aio.rs
> @@ -53,7 +53,15 @@ impl<T: SeqRead> Decoder<T> {
>      #[allow(clippy::should_implement_trait)]
>      /// If this is a directory entry, get the next item inside the directory.
>      pub async fn next(&mut self) -> Option<io::Result<Entry>> {
> -        self.inner.next_do().await.transpose()
> +        self.inner.next().await.transpose()
> +    }
> +
> +    /// If this is a directory entry, get the next item inside the directory.
> +    /// Do not advance the cursor, so multiple calls to peek() will return the same entry,
> +    /// and the next call to next() will read the item once again before moving on.
> +    /// NOTE: This *will* advance the state for contents() and content_size()!

^ Which is why I'm wondering whether we should maybe leave this up to
the *user* rather than provide a sort-of broken API here?

I'd rather have this be guarded by a Seek trait, but that too is
something we won't get from `std` and so we'd have to add one.

Why do we need this exactly?

And would this be solved by simply *generally* storing a
"current_entry"? Then we can have a `.current_entry() -> Option<&Entry>`
which works after at least `next()` call, and `.next()` working as
usual.  And we may just have `next()` also return a reference instead.
The user can `.clone()` if necessary. Or we return a mutable reference
and allow `.take()`, then the user is responsible for knowing whether
calling `.current_entry()` makes sense ;-)

> +    pub async fn peek(&mut self) -> Option<io::Result<Entry>> {
> +        self.inner.peek().await.transpose()
>      }
>  
>      /// Get a reader for the contents of the current entry, if the entry has contents.
> diff --git a/src/decoder/mod.rs b/src/decoder/mod.rs
> index 2a5e79a..041226d 100644
> --- a/src/decoder/mod.rs
> +++ b/src/decoder/mod.rs
> @@ -155,6 +155,7 @@ pub(crate) struct DecoderImpl<T> {
>      path_lengths: Vec<usize>,
>      state: State,
>      with_goodbye_tables: bool,
> +    peeked: Option<io::Result<Option<Entry>>>,
>  
>      /// The random access code uses decoders for sub-ranges which may not end in a `PAYLOAD` for
>      /// entries like FIFOs or sockets, so there we explicitly allow an item to terminate with EOF.
> @@ -218,6 +219,7 @@ impl<I: SeqRead> DecoderImpl<I> {
>              path_lengths: Vec::new(),
>              state: State::Begin,
>              with_goodbye_tables: false,
> +            peeked: None,
>              eof_after_entry,
>          };
>  
> @@ -227,8 +229,21 @@ impl<I: SeqRead> DecoderImpl<I> {
>      }
>  
>      /// Get the next file entry, recursing into directories.
> -    pub async fn next(&mut self) -> Option<io::Result<Entry>> {
> -        self.next_do().await.transpose()
> +    pub async fn next(&mut self) -> io::Result<Option<Entry>> {
> +        if let Some(ent) = self.peeked.take() {
> +            return ent;
> +        }
> +        self.next_do().await
> +    }
> +
> +    pub async fn peek(&mut self) -> io::Result<Option<Entry>> {
> +        self.peeked = Some(self.next().await);
> +        match &self.peeked {
> +            Some(Ok(ent)) => Ok(ent.clone()),
> +            // io::Error does not implement Clone...
> +            Some(Err(err)) => Err(io_format_err!("{}", err)),
> +            None => unreachable!()
> +        }
>      }
>  
>      async fn next_do(&mut self) -> io::Result<Option<Entry>> {
> diff --git a/src/decoder/sync.rs b/src/decoder/sync.rs
> index 85b4865..c6a1bc3 100644
> --- a/src/decoder/sync.rs
> +++ b/src/decoder/sync.rs
> @@ -63,7 +63,15 @@ impl<T: SeqRead> Decoder<T> {
>      #[allow(clippy::should_implement_trait)]
>      /// If this is a directory entry, get the next item inside the directory.
>      pub fn next(&mut self) -> Option<io::Result<Entry>> {
> -        poll_result_once(self.inner.next_do()).transpose()
> +        poll_result_once(self.inner.next()).transpose()
> +    }
> +
> +    /// If this is a directory entry, get the next item inside the directory.
> +    /// Do not advance the cursor, so multiple calls to peek() will return the same entry,
> +    /// and the next call to next() will read the item once again before moving on.
> +    /// NOTE: This *will* advance the state for contents() and content_size()!
> +    pub async fn peek(&mut self) -> Option<io::Result<Entry>> {
> +        poll_result_once(self.inner.peek()).transpose()
>      }
>  
>      /// Get a reader for the contents of the current entry, if the entry has contents.
> -- 
> 2.20.1