Fix tree-extension loading for empty trees (#293)

This also fixes an issue with the node-id seemingly being optional,
even though it is not.

Author: Byron

git-index/src/extension/mod.rs

@@ -17,7 +17,7 @@ pub struct Iter<'a> {
 pub struct Tree {
     pub name: SmallVec<[u8; 23]>,
     /// Only set if there are any entries in the index we are associated with.
-    pub id: Option<tree::NodeId>,
+    pub id: tree::NodeId,
     pub children: Vec<Tree>,
 }
 

git-index/src/extension/tree.rs

@@ -1,5 +1,6 @@
 use git_hash::ObjectId;
 
+use crate::util::split_at_pos;
 use crate::{
     extension::{Signature, Tree},
     util::split_at_byte_exclusive,
@@ -31,19 +32,14 @@ pub fn one_recursive(data: &[u8], hash_len: usize) -> Option<(Tree, &[u8])> {
     let (entry_count, data) = split_at_byte_exclusive(data, b' ')?;
     let entry_count: u32 = atoi::atoi(entry_count)?;
 
-    let (subtree_count, mut data) = split_at_byte_exclusive(data, b'\n')?;
+    let (subtree_count, data) = split_at_byte_exclusive(data, b'\n')?;
     let subtree_count: usize = atoi::atoi(subtree_count)?;
 
-    let node_id = (entry_count != 0)
-        .then(|| {
-            (data.len() >= hash_len).then(|| {
-                let (hash, rest) = data.split_at(hash_len);
-                data = rest;
-                ObjectId::from(hash)
-            })
-        })
-        .flatten()
-        .map(|id| NodeId { id, entry_count });
+    let (hash, mut data) = split_at_pos(data, hash_len)?;
+    let node_id = NodeId {
+        id: ObjectId::from(hash),
+        entry_count,
+    };
 
     let mut subtrees = Vec::with_capacity(subtree_count);
     for _ in 0..subtree_count {
@@ -68,6 +64,6 @@ mod tests {
 
     #[test]
     fn size_of_tree() {
-        assert_eq!(std::mem::size_of::<Tree>(), 88);
+        assert_eq!(std::mem::size_of::<Tree>(), 80);
     }
 }

git-index/tests/fixtures/make_index/V2_empty.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+set -eu -o pipefail
+
+git init -q
+git read-tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904

git-index/tests/index/file/mod.rs

@@ -36,6 +36,12 @@ mod init {
             assert_eq!(entry.path(&file.state), "a");
         }
     }
+    #[test]
+    fn read_v2_empty() {
+        let file = file("v2_empty");
+        assert_eq!(file.version(), Version::V2);
+        assert_eq!(file.entries().len(), 0);
+    }
 
     #[test]
     fn read_v2_with_multiple_entries_without_eoie_ext() {

gitoxide-core/src/index/information.rs

@@ -12,7 +12,7 @@ mod serde_only {
         pub(crate) struct Tree {
             name: String,
             /// Only set if there are any entries in the index we are associated with.
-            id: Option<tree::NodeId>,
+            id: tree::NodeId,
             children: Vec<Tree>,
         }
 
@@ -24,10 +24,10 @@ mod serde_only {
                 fn from(t: &'a git_repository::index::extension::Tree) -> Self {
                     super::Tree {
                         name: t.name.as_bstr().to_string(),
-                        id: t.id.as_ref().map(|id| NodeId {
-                            entry_count: id.entry_count,
-                            id: id.id.to_hex().to_string(),
-                        }),
+                        id: NodeId {
+                            entry_count: t.id.entry_count,
+                            id: t.id.id.to_hex().to_string(),
+                        },
                         children: t.children.iter().map(|t| t.into()).collect(),
                     }
                 }