fix: refspec for shallow clones uses a single-branch (#2227)

When doing shallow clones (depth != NoChange), it now uses a single-branch
refspec instead of fetching all branches. This matches Git's behavior
and significantly reduces the repository size for shallow clones.

For shallow clones:
- If ref_name is specified: uses that branch
- Otherwise: attempts to detect from Protocol V1 handshake or falls
  back to init.defaultBranch config or "main"

This addresses issue #2227 where `gix clone --depth 1` was creating
repositories ~130MB vs Git's ~70MB due to fetching all branches.

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>

Author: Copilot

gix/src/clone/fetch/mod.rs

@@ -47,6 +47,8 @@ pub enum Error {
     },
     #[error(transparent)]
     CommitterOrFallback(#[from] crate::config::time::Error),
+    #[error(transparent)]
+    RefMap(#[from] crate::remote::ref_map::Error),
 }
 
 /// Modification
@@ -101,14 +103,81 @@ impl PrepareFetch {
         };
 
         let mut remote = repo.remote_at(self.url.clone())?;
+        
+        // For shallow clones without custom configuration, we'll use a single-branch refspec
+        // to match git's behavior (matching git's single-branch behavior for shallow clones).
+        let use_single_branch_for_shallow = self.shallow != remote::fetch::Shallow::NoChange 
+            && self.configure_remote.is_none() 
+            && remote.fetch_specs.is_empty();
+        
+        let target_ref = if use_single_branch_for_shallow {
+            // Determine target branch from user-specified ref_name or default branch
+            if let Some(ref_name) = &self.ref_name {
+                // User specified a branch, use that
+                Some(format!("refs/heads/{}", ref_name.as_ref().as_bstr()))
+            } else {
+                // For shallow clones without a specified ref, we need to determine the default branch.
+                // We'll connect to get HEAD information. For Protocol V2, we need to explicitly list refs.
+                let mut connection = remote.connect(remote::Direction::Fetch).await?;
+                
+                // Perform handshake and try to get HEAD from it (works for Protocol V1)
+                let _ = connection.ref_map_by_ref(&mut progress, Default::default()).await?;
+                
+                let target = if let Some(handshake) = &connection.handshake {
+                    // Protocol V1: refs are in handshake
+                    handshake.refs.as_ref().and_then(|refs| {
+                        refs.iter().find_map(|r| match r {
+                            gix_protocol::handshake::Ref::Symbolic {
+                                full_ref_name,
+                                target,
+                                ..
+                            } if full_ref_name == "HEAD" => Some(target.to_string()),
+                            _ => None,
+                        })
+                    })
+                } else {
+                    None
+                };
+                
+                // For Protocol V2 or if we couldn't determine HEAD, use the configured default branch
+                let fallback_branch = target.or_else(|| {
+                    repo.config
+                        .resolved
+                        .string(crate::config::tree::Init::DEFAULT_BRANCH)
+                        .and_then(|name| name.to_str().ok().map(|s| format!("refs/heads/{}", s)))
+                }).unwrap_or_else(|| "refs/heads/main".to_string());
+                
+                // Drop the connection explicitly to release the borrow on remote
+                drop(connection);
+                
+                Some(fallback_branch)
+            }
+        } else {
+            None
+        };
+        
+        // Set up refspec based on whether we're doing a single-branch shallow clone
         if remote.fetch_specs.is_empty() {
-            remote = remote
-                .with_refspecs(
-                    Some(format!("+refs/heads/*:refs/remotes/{remote_name}/*").as_str()),
-                    remote::Direction::Fetch,
-                )
-                .expect("valid static spec");
+            if let Some(target_ref) = &target_ref {
+                // Single-branch refspec for shallow clones
+                let short_name = target_ref.strip_prefix("refs/heads/").unwrap_or(target_ref.as_str());
+                remote = remote
+                    .with_refspecs(
+                        Some(format!("+{target_ref}:refs/remotes/{remote_name}/{short_name}").as_str()),
+                        remote::Direction::Fetch,
+                    )
+                    .expect("valid refspec");
+            } else {
+                // Wildcard refspec for non-shallow clones or when target couldn't be determined
+                remote = remote
+                    .with_refspecs(
+                        Some(format!("+refs/heads/*:refs/remotes/{remote_name}/*").as_str()),
+                        remote::Direction::Fetch,
+                    )
+                    .expect("valid static spec");
+            }
         }
+        
         let mut clone_fetch_tags = None;
         if let Some(f) = self.configure_remote.as_mut() {
             remote = f(remote).map_err(Error::RemoteConfiguration)?;
@@ -133,6 +202,7 @@ impl PrepareFetch {
         .expect("valid")
         .to_owned();
         let pending_pack: remote::fetch::Prepare<'_, '_, _> = {
+            // For shallow clones, we already connected once, so we need to connect again
             let mut connection = remote.connect(remote::Direction::Fetch).await?;
             if let Some(f) = self.configure_connection.as_mut() {
                 f(&mut connection).map_err(Error::RemoteConnection)?;

gix/tests/gix/clone.rs

@@ -83,6 +83,45 @@ mod blocking_io {
         Ok(())
     }
 
+    #[test]
+    fn shallow_clone_uses_single_branch_refspec() -> crate::Result {
+        let tmp = gix_testtools::tempfile::TempDir::new()?;
+        let (repo, _out) = gix::prepare_clone_bare(remote::repo("base").path(), tmp.path())?
+            .with_shallow(Shallow::DepthAtRemote(1.try_into()?))
+            .fetch_only(gix::progress::Discard, &std::sync::atomic::AtomicBool::default())?;
+        
+        assert!(repo.is_shallow(), "repository should be shallow");
+        
+        // Verify that only a single-branch refspec was configured
+        let remote = repo.find_remote("origin")?;
+        let refspecs: Vec<_> = remote
+            .refspecs(Direction::Fetch)
+            .iter()
+            .map(|spec| spec.to_ref().to_bstring())
+            .collect();
+        
+        assert_eq!(
+            refspecs.len(),
+            1,
+            "shallow clone should have only one fetch refspec"
+        );
+        
+        // The refspec should be for a single branch (main), not a wildcard
+        let refspec_str = refspecs[0].to_str().expect("valid utf8");
+        assert!(
+            !refspec_str.contains("*"),
+            "shallow clone refspec should not use wildcard: {}",
+            refspec_str
+        );
+        assert!(
+            refspec_str.contains("refs/heads/main"),
+            "shallow clone refspec should reference the main branch: {}",
+            refspec_str
+        );
+        
+        Ok(())
+    }
+
     #[test]
     fn from_shallow_prohibited_with_option() -> crate::Result {
         let tmp = gix_testtools::tempfile::TempDir::new()?;