Implement ToString for RelativeOid

For the purposes of computing the tlog key ID, the log ID is used as the
key name. Add a method for encoding the arcs as a string so that we can
properly construct the key name.

Implementation note: Currently we only keep around the DER encoding of
the arcs. We could decode the arcs from the DER blob, but it's a bit
more convenient to just keep the arcs around in the data structure.

Author: cjpatton

crates/mtc_api/src/relative_oid.rs

@@ -7,6 +7,7 @@ use std::str::FromStr;
 #[derive(Clone)]
 pub struct RelativeOid {
     ber: Vec<u8>,
+    arcs: Vec<u32>,
 }
 
 impl RelativeOid {
@@ -30,7 +31,10 @@ impl RelativeOid {
         if ber.len() > 255 {
             return Err(MtcError::Dynamic("invalid relative OID".into()));
         }
-        Ok(Self { ber })
+        Ok(Self {
+            ber,
+            arcs: arcs.to_vec(),
+        })
     }
 
     /// Returns the DER-encoded content bytes.
@@ -39,6 +43,15 @@ impl RelativeOid {
     }
 }
 
+impl std::fmt::Display for RelativeOid {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        for arc in self.arcs.iter().take(self.arcs.len() - 1) {
+            write!(f, "{}.", arc)?;
+        }
+        write!(f, "{}", self.arcs[self.arcs.len() - 1])
+    }
+}
+
 impl FromStr for RelativeOid {
     type Err = MtcError;
     /// Parse the [`RelativeOid`] from a decimal-dotted string.
@@ -61,9 +74,15 @@ mod tests {
     use super::*;
 
     #[test]
-    fn encode_decode() {
+    fn encode_tagged() {
         let relative_oid = RelativeOid::from_str("13335.2").unwrap();
         let any = Any::new(Tag::RelativeOid, relative_oid.as_bytes()).unwrap();
         assert_eq!(any.to_der().unwrap(), b"\x0d\x03\xe8\x17\x02");
     }
+
+    #[test]
+    fn encode_string() {
+        let relative_oid = RelativeOid::from_str("13335.2").unwrap();
+        assert_eq!(relative_oid.to_string(), "13335.2");
+    }
 }