[git-object] Encode empty tags like git does

A tag with an empty commit message is now displayed with an extra
newline, consitently with the output of `git cat-file`.

The test fixture for `empty.txt` was updated, which means that the
`round_trip` test now expects a correct round trip with the new format.

Note that the changes to tests in `immutable` reflect the new state of
the code:
* `TagRefIter` now parses this body with an empty message.
* Where the length of the data was divided by 2, the point was to create
  an inconsistent dataset leading to an error. Unfortunately, with the
  extra character, it lines up so that the message can be parsed, so I
  had to change the magic number to keep the previous property.

Fixes #603

Author: pierrechevalier83

git-object/src/tag/decode.rs

@@ -53,7 +53,8 @@ pub fn message<'a, E: ParseError<&'a [u8]>>(i: &'a [u8]) -> IResult<&'a [u8], (&
     let (i, _) = tag(NL)(i)?;
     fn all_to_end<'a, E: ParseError<&'a [u8]>>(i: &'a [u8]) -> IResult<&'a [u8], (&'a [u8], &'a [u8]), E> {
         if i.is_empty() {
-            return Err(nom::Err::Error(E::from_error_kind(i, nom::error::ErrorKind::Eof)));
+            // Empty message. That's OK.
+            return Ok((&[], (&[], &[])));
         }
         // an empty signature message signals that there is none - the function signature is needed
         // to work with 'alt(…)'. PGP signatures are never empty

git-object/src/tag/write.rs

@@ -29,8 +29,8 @@ impl crate::WriteTo for Tag {
             encode::trusted_header_signature(b"tagger", &tagger.to_ref(), &mut out)?;
         }
 
+        out.write_all(NL)?;
         if !self.message.is_empty() {
-            out.write_all(NL)?;
             out.write_all(&self.message)?;
         }
         if let Some(ref message) = self.pgp_signature {
@@ -49,11 +49,7 @@ impl crate::WriteTo for Tag {
                 .as_ref()
                 .map(|t| b"tagger".len() + 1 /* space */ + t.size() + 1 /* nl */)
                 .unwrap_or(0)
-            + if self.message.is_empty() {
-                0
-            } else {
-                1 /* nl */ + self.message.len()
-            }
+            + 1 /* nl */ + self.message.len()
             + self.pgp_signature.as_ref().map(|m| 1 /* nl */ + m.len() ).unwrap_or(0)
     }
 
@@ -71,8 +67,8 @@ impl<'a> crate::WriteTo for TagRef<'a> {
             encode::trusted_header_signature(b"tagger", tagger, &mut out)?;
         }
 
+        out.write_all(NL)?;
         if !self.message.is_empty() {
-            out.write_all(NL)?;
             out.write_all(self.message)?;
         }
         if let Some(message) = self.pgp_signature {
@@ -91,11 +87,7 @@ impl<'a> crate::WriteTo for TagRef<'a> {
                 .as_ref()
                 .map(|t| b"tagger".len() + 1 /* space */ + t.size() + 1 /* nl */)
                 .unwrap_or(0)
-            + if self.message.is_empty() {
-                0
-            } else {
-                1 /* nl */ + self.message.len()
-            }
+            + 1 /* nl */ + self.message.len()
             + self.pgp_signature.as_ref().map(|m| 1 /* nl */ + m.len()).unwrap_or(0)
     }
 

git-object/tests/fixtures/tag/empty.txt

@@ -2,3 +2,4 @@ object 01dd4e2a978a9f5bd773dae6da7aa4a5ac1cdbbc
 type commit
 tag empty
 tagger Sebastian Thiel <sebastian.thiel@icloud.com> 1592381636 +0800
+

git-object/tests/immutable/tag.rs

@@ -37,6 +37,10 @@ mod iter {
                 Token::TargetKind(Kind::Commit),
                 Token::Name(b"empty".as_bstr()),
                 Token::Tagger(tagger),
+                Token::Body {
+                    message: b"".as_bstr(),
+                    pgp_signature: None,
+                }
             ]
         );
         assert_eq!(tag_iter.target_id()?, target_id);
@@ -103,7 +107,7 @@ KLMHist5yj0sw1E4hDTyQa0=
     #[test]
     fn error_handling() -> crate::Result {
         let data = fixture_bytes("tag", "empty.txt");
-        let iter = TagRefIter::from_bytes(&data[..data.len() / 2]);
+        let iter = TagRefIter::from_bytes(&data[..data.len() / 3]);
         let tokens = iter.collect::<Vec<_>>();
         assert!(
             tokens.last().expect("at least the errored token").is_err(),