fix: handle detached head error

__tests__/git.test.ts

@@ -285,6 +285,33 @@ describe('ref', () => {
     expect(ref).toEqual('refs/heads/main');
   });
 
+  it('infers ref from local branch when detached HEAD returns only "grafted, HEAD"', async () => {
+    jest.spyOn(Exec, 'getExecOutput').mockImplementation((cmd, args): Promise<ExecOutput> => {
+      const fullCmd = `${cmd} ${args?.join(' ')}`;
+      let result = '';
+      switch (fullCmd) {
+        case 'git branch --show-current':
+          result = '';
+          break;
+        case 'git show -s --pretty=%D':
+          result = 'grafted, HEAD';
+          break;
+        case 'git for-each-ref --format=%(refname) --contains HEAD --sort=-committerdate refs/heads/':
+          result = 'refs/heads/main\nrefs/heads/develop';
+          break;
+      }
+      return Promise.resolve({
+        stdout: result,
+        stderr: '',
+        exitCode: 0
+      });
+    });
+
+    const ref = await Git.ref();
+
+    expect(ref).toEqual('refs/heads/main');
+  });
+
   it('infers ref from remote branch when no local branch contains HEAD', async () => {
     jest.spyOn(Exec, 'getExecOutput').mockImplementation((cmd, args): Promise<ExecOutput> => {
       const fullCmd = `${cmd} ${args?.join(' ')}`;

src/git.ts

@@ -124,18 +124,20 @@ export class Git {
     const res = await Git.exec(['show', '-s', '--pretty=%D']);
     core.debug(`detached HEAD ref: ${res}`);
 
-    if (res === 'HEAD') {
+    const normalizedRef = res.replace(/^grafted, /, '').trim();
+
+    if (normalizedRef === 'HEAD') {
       return await Git.inferRefFromHead();
     }
 
     // Can be "HEAD, <tagname>" or "grafted, HEAD, <tagname>"
-    const refMatch = res.match(/^(grafted, )?HEAD, (.*)$/);
+    const refMatch = normalizedRef.match(/^HEAD, (.*)$/);
 
-    if (!refMatch || !refMatch[2]) {
+    if (!refMatch || !refMatch[1]) {
       throw new Error(`Cannot find detached HEAD ref in "${res}"`);
     }
 
-    const ref = refMatch[2].trim();
+    const ref = refMatch[1].trim();
 
     // Tag refs are formatted as "tag: <tagname>"
     if (ref.startsWith('tag: ')) {