feat(castore): implement FromStr for trait B3Digest

This provides a canonical way to parse a `B3Digest`, to be used in,
e.g., a CLI like `snix-castore`.

Change-Id: If6c7858c1fcd7645721002be7789f5ce2ff97a5c
Reviewed-on: https://cl.snix.dev/c/snix/+/30303
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii

snix/castore/src/digests.rs

@@ -1,5 +1,6 @@
 use bytes::Bytes;
 use data_encoding::BASE64;
+use std::str::FromStr;
 use thiserror::Error;
 
 #[derive(PartialEq, Eq, Hash)]
@@ -15,6 +16,8 @@ impl B3Digest {
 pub enum Error {
     #[error("invalid digest length: {0}")]
     InvalidDigestLen(usize),
+    #[error("invalid hash type: expected a 'blake3-' prefixed digest")]
+    InvalidHashType,
 }
 
 impl AsRef<[u8; B3Digest::LENGTH]> for B3Digest {
@@ -109,3 +112,18 @@ impl std::fmt::Debug for B3Digest {
         BASE64.encode_write(&self.0, f)
     }
 }
+
+impl FromStr for B3Digest {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if !s.starts_with("blake3-") {
+            return Err(Error::InvalidHashType);
+        }
+        let encoded = &s[7..];
+        let decoded = BASE64
+            .decode(encoded.as_bytes())
+            .map_err(|_| Error::InvalidDigestLen(s.len()))?;
+        decoded.as_slice().try_into()
+    }
+}

snix/castore/src/errors.rs

@@ -27,12 +27,16 @@ pub enum ValidateNodeError {
     /// Invalid symlink target
     #[error("Invalid symlink target: {0}")]
     InvalidSymlinkTarget(SymlinkTargetError),
+    /// Invalid hash type encountered
+    #[error("invalid hash type: expected a 'blake3-' prefixed digest")]
+    InvalidHashType,
 }
 
 impl From<crate::digests::Error> for ValidateNodeError {
     fn from(e: crate::digests::Error) -> Self {
         match e {
             crate::digests::Error::InvalidDigestLen(n) => ValidateNodeError::InvalidDigestLen(n),
+            crate::digests::Error::InvalidHashType => ValidateNodeError::InvalidHashType,
         }
     }
 }