feat: Provide information about whether or not EOIE and IEOT extensions were available.

These are only relevant when reading, and have to be regenerated after writing.
During reading, they can speed up performance by allowing multi-threading.

Author: Byron

gix-index/src/access/mod.rs

@@ -557,6 +557,14 @@ impl State {
     pub fn fs_monitor(&self) -> Option<&extension::FsMonitor> {
         self.fs_monitor.as_ref()
     }
+    /// Return `true` if the end-of-index extension was present when decoding this index.
+    pub fn had_end_of_index_marker(&self) -> bool {
+        self.end_of_index_at_decode_time
+    }
+    /// Return `true` if the offset-table extension was present when decoding this index.
+    pub fn had_offset_table(&self) -> bool {
+        self.offset_table_at_decode_time
+    }
 }
 
 #[cfg(test)]

gix-index/src/decode/entries.rs

@@ -134,6 +134,8 @@ fn load_one<'a>(
             (path, skip_padding(data, first_byte_of_entry))
         };
 
+        // TODO(perf): for some reason, this causes tremendous `memmove` time even though the backing
+        //             has enough capacity most of the time.
         path_backing.extend_from_slice(path);
         data
     };

gix-index/src/decode/mod.rs

@@ -236,6 +236,8 @@ impl State {
             untracked,
             fs_monitor,
             is_sparse: is_sparse_from_ext, // a marker is needed in case there are no directories
+            end_of_index,
+            offset_table,
         } = ext;
         is_sparse |= is_sparse_from_ext;
 
@@ -248,6 +250,8 @@ impl State {
                 path_backing,
                 is_sparse,
 
+                end_of_index_at_decode_time: end_of_index,
+                offset_table_at_decode_time: offset_table,
                 tree,
                 link,
                 resolve_undo,

gix-index/src/extension/decode.rs

@@ -50,8 +50,12 @@ pub(crate) fn all(
             extension::fs_monitor::SIGNATURE => {
                 ext.fs_monitor = extension::fs_monitor::decode(ext_data);
             }
-            extension::end_of_index_entry::SIGNATURE => {} // skip already done
-            extension::index_entry_offset_table::SIGNATURE => {} // not relevant/obtained already
+            extension::end_of_index_entry::SIGNATURE => {
+                ext.end_of_index = true;
+            } // skip already done
+            extension::index_entry_offset_table::SIGNATURE => {
+                ext.offset_table = true;
+            } // not relevant/obtained already
             mandatory if mandatory[0].is_ascii_lowercase() => match mandatory {
                 extension::link::SIGNATURE => ext.link = extension::link::decode(ext_data, object_hash)?.into(),
                 extension::sparse::SIGNATURE => {
@@ -77,4 +81,6 @@ pub(crate) struct Outcome {
     pub untracked: Option<extension::UntrackedCache>,
     pub fs_monitor: Option<extension::FsMonitor>,
     pub is_sparse: bool,
+    pub offset_table: bool,
+    pub end_of_index: bool,
 }

gix-index/src/init.rs

@@ -26,6 +26,8 @@ mod from_tree {
                 resolve_undo: None,
                 untracked: None,
                 fs_monitor: None,
+                offset_table_at_decode_time: false,
+                end_of_index_at_decode_time: false,
             }
         }
         /// Create an index [`State`] by traversing `tree` recursively, accessing sub-trees
@@ -63,6 +65,8 @@ mod from_tree {
                 resolve_undo: None,
                 untracked: None,
                 fs_monitor: None,
+                offset_table_at_decode_time: false,
+                end_of_index_at_decode_time: false,
             })
         }
     }

gix-index/src/lib.rs

@@ -128,6 +128,8 @@ pub struct State {
     is_sparse: bool,
 
     // Extensions
+    end_of_index_at_decode_time: bool,
+    offset_table_at_decode_time: bool,
     tree: Option<extension::Tree>,
     link: Option<extension::Link>,
     resolve_undo: Option<extension::resolve_undo::Paths>,

gix-index/tests/index/file/read.rs

@@ -227,6 +227,8 @@ fn file_with_conflicts() {
 fn v4_with_delta_paths_and_ieot_ext() {
     let file = file("v4_more_files_IEOT");
     assert_eq!(file.version(), Version::V4);
+    assert!(file.had_end_of_index_marker());
+    assert!(file.had_offset_table());
 
     assert_eq!(file.entries().len(), 10);
     for (idx, path) in [