ssh-encoding: remove NestedReader

Changes `Reader::read_prefixed` from a provided method to one which must
be impl'd by all of the `Reader` types, of which there are currently
three:

- `&[u8]`
- `Base64Reader`
- `PemReader`

This should reduce monomorphization bloat from `NestedReader` which
encapsulates other reader types, including other `NestedReader`s,
depending on the level of nesting, which is generally low, but good to
eliminate entirely.

This does lead to a small amount of duplication between `Base64Reader`
and `PemReader` which was previously encapsulated in `NestedReader`.
It can possibly be eliminated in followup work.

ssh-encoding/src/base64/reader.rs

@@ -1,13 +1,17 @@
 //! Base64 reader support (constant-time).
 
-use crate::{Reader, Result};
+use crate::{Decode, Error, Reader, Result};
 
 /// Inner constant-time Base64 reader type from the `base64ct` crate.
 type Inner<'i> = base64ct::Decoder<'i, base64ct::Base64>;
 
 /// Constant-time Base64 reader implementation.
 pub struct Base64Reader<'i> {
+    /// Inner Base64 reader.
     inner: Inner<'i>,
+
+    /// Custom length of remaining data, used for nested length-prefixed reading.
+    remaining_len: usize,
 }
 
 impl<'i> Base64Reader<'i> {
@@ -18,18 +22,52 @@ impl<'i> Base64Reader<'i> {
     /// - `Ok(reader)` on success.
     /// - `Err(Error::Base64)` if the input buffer is empty.
     pub fn new(input: &'i [u8]) -> Result<Self> {
+        let inner = Inner::new(input)?;
+        let remaining_len = inner.remaining_len();
+
         Ok(Self {
-            inner: Inner::new(input)?,
+            inner,
+            remaining_len,
         })
     }
 }
 
 impl Reader for Base64Reader<'_> {
     fn read<'o>(&mut self, out: &'o mut [u8]) -> Result<&'o [u8]> {
-        Ok(self.inner.decode(out)?)
+        if out.is_empty() {
+            return Ok(out);
+        }
+
+        let remaining_len = self
+            .remaining_len
+            .checked_sub(out.len())
+            .ok_or(Error::Length)?;
+
+        let ret = self.inner.decode(out)?;
+        self.remaining_len = remaining_len;
+        Ok(ret)
+    }
+
+    fn read_prefixed<T, E, F>(&mut self, f: F) -> core::result::Result<T, E>
+    where
+        E: From<Error>,
+        F: FnOnce(&mut Self) -> core::result::Result<T, E>,
+    {
+        let prefix_len = usize::decode(self)?;
+        let new_remaining_len = self
+            .remaining_len
+            .checked_sub(prefix_len)
+            .ok_or(Error::Length)?;
+
+        self.remaining_len = prefix_len;
+        let ret = f(self)?;
+        self.ensure_finished()?;
+
+        self.remaining_len = new_remaining_len;
+        Ok(ret)
     }
 
     fn remaining_len(&self) -> usize {
-        self.inner.remaining_len()
+        self.remaining_len
     }
 }

ssh-encoding/src/lib.rs

@@ -42,7 +42,7 @@ pub use crate::{
     encode::Encode,
     error::{Error, Result},
     label::{Label, LabelError},
-    reader::{NestedReader, Reader},
+    reader::Reader,
     writer::Writer,
 };
 

ssh-encoding/src/pem/reader.rs

@@ -1,19 +1,29 @@
 use super::LINE_WIDTH;
-use crate::{Reader, Result};
+use crate::{Decode, Error, Reader, Result};
 
 /// Inner PEM decoder.
 type Inner<'i> = pem_rfc7468::Decoder<'i>;
 
 /// Constant-time PEM reader.
 pub struct PemReader<'i> {
+    /// Inner PEM reader.
     inner: Inner<'i>,
+
+    /// Custom length of remaining data, used for nested length-prefixed reading.
+    remaining_len: usize,
 }
 
 impl<'i> PemReader<'i> {
-    /// TODO
+    /// Create a new PEM reader.
+    ///
+    /// Uses [`LINE_WIDTH`] as the default line width (i.e. 70 chars).
     pub fn new(pem: &'i [u8]) -> Result<Self> {
+        let inner = Inner::new_wrapped(pem, LINE_WIDTH)?;
+        let remaining_len = inner.remaining_len();
+
         Ok(Self {
-            inner: Inner::new_wrapped(pem, LINE_WIDTH)?,
+            inner,
+            remaining_len,
         })
     }
 
@@ -25,10 +35,40 @@ impl<'i> PemReader<'i> {
 
 impl Reader for PemReader<'_> {
     fn read<'o>(&mut self, out: &'o mut [u8]) -> Result<&'o [u8]> {
-        Ok(self.inner.decode(out)?)
+        if out.is_empty() {
+            return Ok(out);
+        }
+
+        let remaining_len = self
+            .remaining_len
+            .checked_sub(out.len())
+            .ok_or(Error::Length)?;
+
+        let ret = self.inner.decode(out)?;
+        self.remaining_len = remaining_len;
+        Ok(ret)
+    }
+
+    fn read_prefixed<T, E, F>(&mut self, f: F) -> core::result::Result<T, E>
+    where
+        E: From<Error>,
+        F: FnOnce(&mut Self) -> core::result::Result<T, E>,
+    {
+        let prefix_len = usize::decode(self)?;
+        let new_remaining_len = self
+            .remaining_len
+            .checked_sub(prefix_len)
+            .ok_or(Error::Length)?;
+
+        self.remaining_len = prefix_len;
+        let ret = f(self)?;
+        self.ensure_finished()?;
+
+        self.remaining_len = new_remaining_len;
+        Ok(ret)
     }
 
     fn remaining_len(&self) -> usize {
-        self.inner.remaining_len()
+        self.remaining_len
     }
 }

ssh-encoding/src/reader.rs

@@ -29,18 +29,10 @@ pub trait Reader: Sized {
     /// Decodes a `uint32` which identifies the length of some encapsulated
     /// data, then calls the given reader function with the length of the
     /// remaining data.
-    fn read_prefixed<'r, T, E, F>(&'r mut self, f: F) -> core::result::Result<T, E>
+    fn read_prefixed<T, E, F>(&mut self, f: F) -> core::result::Result<T, E>
     where
         E: From<Error>,
-        F: FnOnce(&mut NestedReader<'r, Self>) -> core::result::Result<T, E>,
-    {
-        let len = usize::decode(self)?;
-
-        f(&mut NestedReader {
-            inner: self,
-            remaining_len: len,
-        })
-    }
+        F: FnOnce(&mut Self) -> core::result::Result<T, E>;
 
     /// Decodes `[u8]` from `byte[n]` as described in [RFC4251 § 5]:
     ///
@@ -111,17 +103,27 @@ pub trait Reader: Sized {
         })
     }
 
-    /// Finish decoding, returning the given value if there is no remaining
-    /// data, or an error otherwise.
-    fn finish<T>(self, value: T) -> Result<T> {
+    /// Ensure that decoding is finished.
+    ///
+    /// # Errors
+    ///
+    /// - Returns `Error::TrailingData` if there is data remaining in the encoder.
+    fn ensure_finished(&self) -> Result<()> {
         if self.is_finished() {
-            Ok(value)
+            Ok(())
         } else {
             Err(Error::TrailingData {
                 remaining: self.remaining_len(),
             })
         }
     }
+
+    /// Finish decoding, returning the given value if there is no remaining
+    /// data, or an error otherwise.
+    fn finish<T>(self, value: T) -> Result<T> {
+        self.ensure_finished()?;
+        Ok(value)
+    }
 }
 
 impl Reader for &[u8] {
@@ -136,37 +138,24 @@ impl Reader for &[u8] {
         }
     }
 
-    fn remaining_len(&self) -> usize {
-        self.len()
-    }
-}
-
-/// Reader type used by [`Reader::read_prefixed`].
-pub struct NestedReader<'r, R: Reader> {
-    /// Inner reader type.
-    inner: &'r mut R,
-
-    /// Remaining length in the prefixed reader.
-    remaining_len: usize,
-}
+    fn read_prefixed<T, E, F>(&mut self, f: F) -> core::result::Result<T, E>
+    where
+        E: From<Error>,
+        F: FnOnce(&mut Self) -> core::result::Result<T, E>,
+    {
+        let prefix_len = usize::decode(self)?;
 
-impl<'r, R: Reader> Reader for NestedReader<'r, R> {
-    fn read<'o>(&mut self, out: &'o mut [u8]) -> Result<&'o [u8]> {
-        if out.is_empty() {
-            return Ok(out);
+        if self.len() < prefix_len {
+            return Err(Error::Length.into());
         }
 
-        let remaining_len = self
-            .remaining_len
-            .checked_sub(out.len())
-            .ok_or(Error::Length)?;
-
-        let ret = self.inner.read(out)?;
-        self.remaining_len = remaining_len;
+        let (mut prefix, remaining) = self.split_at(prefix_len);
+        let ret = f(&mut prefix)?;
+        *self = remaining;
         Ok(ret)
     }
 
     fn remaining_len(&self) -> usize {
-        self.remaining_len
+        self.len()
     }
 }