1 files changed, 126 insertions, 0 deletions

diff --git a/src/workspace/git/discovery.rs b/src/workspace/git/discovery.rs
index de4b090c..7e973bb9 100644
--- a/src/workspace/git/discovery.rs
+++ b/src/workspace/git/discovery.rs
@@ -111,6 +111,11 @@ fn git_common_dir_for_git_dir(git_dir: &Path) -> PathBuf {
 pub fn git_branch(cwd: &Path) -> Option<String> {
     let repo_root = git_repo_root(cwd)?;
     let git_dir = git_dir_for_repo_root(&repo_root)?;
+    let git_common_dir = git_common_dir_for_git_dir(&git_dir);
+    if git_ref_storage_is_reftable(&git_common_dir) {
+        return git_symbolic_head_short(&repo_root);
+    }
+
     let head = std::fs::read_to_string(git_dir.join("HEAD")).ok()?;
     parse_git_head_branch(&head)
 }
@@ -131,11 +136,86 @@ pub(super) fn git_dir_for_repo_root(repo_root: &Path) -> Option<PathBuf> {
     })
 }
 
+pub(super) fn git_symbolic_head_full(repo_root: &Path) -> Option<String> {
+    git_trimmed_stdout(repo_root, &["symbolic-ref", "--quiet", "HEAD"])
+}
+
+fn git_symbolic_head_short(repo_root: &Path) -> Option<String> {
+    git_trimmed_stdout(repo_root, &["symbolic-ref", "--quiet", "--short", "HEAD"])
+}
+
+pub(super) fn git_rev_parse_verify(repo_root: &Path, revision: &str) -> Option<String> {
+    git_trimmed_stdout(repo_root, &["rev-parse", "--verify", revision])
+}
+
+pub(super) fn git_ref_storage_is_reftable(git_common_dir: &Path) -> bool {
+    read_git_config_value(&git_common_dir.join("config"), "extensions", "refstorage")
+        .is_some_and(|value| value.eq_ignore_ascii_case("reftable"))
+}
+
 fn parse_git_head_branch(head: &str) -> Option<String> {
     let branch = head.trim().strip_prefix("ref: refs/heads/")?;
     (!branch.is_empty()).then(|| branch.to_string())
 }
 
+fn read_git_config_value(path: &Path, section: &str, key: &str) -> Option<String> {
+    let contents = std::fs::read_to_string(path).ok()?;
+    let mut in_section = false;
+    for raw_line in contents.lines() {
+        let line = raw_line.trim();
+        if line.is_empty() || line.starts_with('#') || line.starts_with(';') {
+            continue;
+        }
+        if let Some(section_name) = simple_git_config_section(line) {
+            in_section = section_name.eq_ignore_ascii_case(section);
+            continue;
+        }
+        if !in_section {
+            continue;
+        }
+        let Some((name, value)) = line.split_once('=') else {
+            continue;
+        };
+        if name.trim().eq_ignore_ascii_case(key) {
+            return Some(strip_git_config_comment(value).trim().to_string());
+        }
+    }
+    None
+}
+
+fn simple_git_config_section(line: &str) -> Option<&str> {
+    let section = line.strip_prefix('[')?.split_once(']')?.0.trim();
+    (!section.contains('"')).then_some(section)
+}
+
+fn strip_git_config_comment(value: &str) -> &str {
+    let value = value.trim();
+    for marker in ['#', ';'] {
+        if let Some((prefix, _)) = value.split_once(marker) {
+            if prefix.chars().next_back().is_some_and(char::is_whitespace) {
+                return prefix;
+            }
+        }
+    }
+    value
+}
+
+fn git_trimmed_stdout(repo_root: &Path, args: &[&str]) -> Option<String> {
+    let output = std::process::Command::new("git")
+        .arg("-C")
+        .arg(repo_root)
+        .args(args)
+        .output()
+        .ok()?;
+    if !output.status.success() {
+        return None;
+    }
+
+    let stdout = String::from_utf8(output.stdout).ok()?;
+    let stdout = stdout.trim();
+    (!stdout.is_empty()).then(|| stdout.to_string())
+}
+
 pub(super) fn git_repo_root(start: &Path) -> Option<PathBuf> {
     let mut current = if start.is_dir() {
         start.to_path_buf()
@@ -187,6 +267,7 @@ mod tests {
     use std::time::{SystemTime, UNIX_EPOCH};
 
     use super::*;
+    use crate::workspace::git::test_support::run_git;
 
     fn temp_test_dir(name: &str) -> PathBuf {
         let unique = format!(
@@ -239,6 +320,24 @@ mod tests {
     }
 
     #[test]
+    fn git_branch_reads_symbolic_head_from_reftable_repo() {
+        let root = temp_test_dir("reftable-branch");
+        let root_arg = root.to_string_lossy().to_string();
+        let output = std::process::Command::new("git")
+            .args(["init", "--ref-format=reftable", "-b", "main", &root_arg])
+            .output()
+            .unwrap();
+        if !output.status.success() {
+            std::fs::remove_dir_all(root).unwrap();
+            return;
+        }
+
+        assert_eq!(git_branch(&root).as_deref(), Some("main"));
+
+        std::fs::remove_dir_all(root).unwrap();
+    }
+
+    #[test]
     fn git_repo_root_ignores_invalid_git_marker() {
         let base = temp_test_dir("invalid-git-root");
         let cwd = base.join("workspace");
@@ -275,4 +374,31 @@ mod tests {
 
         std::fs::remove_dir_all(root).unwrap();
     }
+
+    #[test]
+    fn git_rev_parse_verify_reads_reftable_refs() {
+        let root = temp_test_dir("reftable-ref-oid");
+        let root_arg = root.to_string_lossy().to_string();
+        let output = std::process::Command::new("git")
+            .args(["init", "--ref-format=reftable", "-b", "main", &root_arg])
+            .output()
+            .unwrap();
+        if !output.status.success() {
+            std::fs::remove_dir_all(root).unwrap();
+            return;
+        }
+
+        run_git(&root, &["config", "user.email", "herdr@example.invalid"]);
+        run_git(&root, &["config", "user.name", "Herdr Test"]);
+        run_git(&root, &["commit", "--allow-empty", "-m", "initial"]);
+
+        let head_oid = git_rev_parse_verify(&root, "HEAD").unwrap();
+
+        assert_eq!(
+            git_rev_parse_verify(&root, "refs/heads/main").as_deref(),
+            Some(head_oid.as_str())
+        );
+
+        std::fs::remove_dir_all(root).unwrap();
+    }
 }