Recover branch during rebase

packages/server/src/utils/checkout-git.test.ts

@@ -194,6 +194,24 @@ describe("checkout git utilities", () => {
     expect(branch).toBeNull();
   });
 
+  it("returns the branch being rebased when HEAD is detached during a rebase", async () => {
+    execSync("git checkout -b feature/rebase-test", { cwd: repoDir });
+    writeFileSync(join(repoDir, "file.txt"), "feature\n");
+    execSync("git add file.txt", { cwd: repoDir });
+    execSync("git -c commit.gpgsign=false commit -m 'feature change'", { cwd: repoDir });
+
+    execSync("git checkout main", { cwd: repoDir });
+    writeFileSync(join(repoDir, "file.txt"), "main\n");
+    execSync("git add file.txt", { cwd: repoDir });
+    execSync("git -c commit.gpgsign=false commit -m 'main change'", { cwd: repoDir });
+
+    execSync("git checkout feature/rebase-test", { cwd: repoDir });
+    expect(() => execSync("git rebase main", { cwd: repoDir, stdio: "pipe" })).toThrow();
+
+    const branch = await getCurrentBranch(repoDir);
+    expect(branch).toBe("feature/rebase-test");
+  });
+
   it("handles status/diff/commit in a normal repo", async () => {
     writeFileSync(join(repoDir, "file.txt"), "updated\n");
 

packages/server/src/utils/checkout-git.ts

@@ -1,6 +1,6 @@
 import { resolve, dirname, basename } from "path";
 import { existsSync, realpathSync } from "fs";
-import { open as openFile, stat as statFile } from "fs/promises";
+import { open as openFile, readFile, stat as statFile } from "fs/promises";
 import { TTLCache } from "@isaacs/ttlcache";
 import type { ParsedDiffFile } from "../server/utils/diff-highlighter.js";
 import { parseAndHighlightDiff } from "../server/utils/diff-highlighter.js";
@@ -701,12 +701,34 @@ export async function getCurrentBranch(cwd: string): Promise<string | null> {
       env: READ_ONLY_GIT_ENV,
     });
     const branch = stdout.trim();
+    if (branch === "HEAD") {
+      return await getRebaseHeadBranch(cwd);
+    }
     return branch.length > 0 ? branch : null;
   } catch {
     return null;
   }
 }
 
+async function getRebaseHeadBranch(cwd: string): Promise<string | null> {
+  for (const path of ["rebase-merge/head-name", "rebase-apply/head-name"]) {
+    try {
+      const { stdout } = await runGitCommand(["rev-parse", "--git-path", path], {
+        cwd,
+        env: READ_ONLY_GIT_ENV,
+      });
+      const headName = (await readFile(resolve(cwd, stdout.trim()), "utf8")).trim();
+      if (headName.startsWith("refs/heads/")) {
+        return headName.slice("refs/heads/".length) || null;
+      }
+      return headName || null;
+    } catch {
+      // Try the other rebase backend.
+    }
+  }
+  return null;
+}
+
 async function getWorktreeRoot(cwd: string): Promise<string | null> {
   try {
     const { stdout } = await runGitCommand(["rev-parse", "--show-toplevel"], {