fix: ensure symlinked git directories work

__test__/git-auth-helper.test.ts

@@ -238,6 +238,79 @@ describe('git-auth-helper tests', () => {
     expect(setSecretSpy).toHaveBeenCalledWith(expectedSecret)
   })
 
+  const configureAuth_resolvesSymlinksInIncludeIfGitdir =
+    'configureAuth resolves symlinks in includeIf gitdir'
+  it(configureAuth_resolvesSymlinksInIncludeIfGitdir, async () => {
+    if (isWindows) {
+      process.stdout.write(
+        `Skipped test "${configureAuth_resolvesSymlinksInIncludeIfGitdir}". Symlink creation requires admin privileges on Windows.\n`
+      )
+      return
+    }
+
+    // Arrange
+    await setup(configureAuth_resolvesSymlinksInIncludeIfGitdir)
+
+    // Create a symlink pointing to the real workspace directory
+    const symlinkPath = path.join(path.dirname(workspace), 'workspace-symlink')
+    await fs.promises.symlink(workspace, symlinkPath)
+
+    // Make git appear to be operating from the symlink path
+    ;(git.getWorkingDirectory as jest.Mock).mockReturnValue(symlinkPath)
+    process.env['GITHUB_WORKSPACE'] = symlinkPath
+
+    const authHelper = gitAuthHelper.createAuthHelper(git, settings)
+
+    // Act
+    await authHelper.configureAuth()
+
+    // Assert the host includeIf uses the real resolved path, not the symlink path
+    const localConfigContent = (
+      await fs.promises.readFile(localGitConfigPath)
+    ).toString()
+    const realGitDir = fs
+      .realpathSync(path.join(symlinkPath, '.git'))
+      .replace(/\\/g, '/')
+    const symlinkGitDir = path.join(symlinkPath, '.git').replace(/\\/g, '/')
+
+    expect(realGitDir).not.toBe(symlinkGitDir) // sanity check: paths differ
+    expect(
+      localConfigContent.indexOf(`includeIf.gitdir:${realGitDir}.path`)
+    ).toBeGreaterThanOrEqual(0)
+    expect(localConfigContent.indexOf(symlinkGitDir)).toBeLessThan(0)
+
+    // Clean up symlink
+    await fs.promises.unlink(symlinkPath)
+  })
+
+  const configureAuth_fallsBackWhenRealpathSyncFails =
+    'configureAuth falls back to constructed path when realpathSync fails'
+  it(configureAuth_fallsBackWhenRealpathSyncFails, async () => {
+    // Arrange
+    await setup(configureAuth_fallsBackWhenRealpathSyncFails)
+
+    // Use a non-existent path so realpathSync throws ENOENT naturally,
+    // exercising the catch fallback in configureToken()
+    const nonexistentPath = path.join(runnerTemp, 'does-not-exist')
+    ;(git.getWorkingDirectory as jest.Mock).mockReturnValue(nonexistentPath)
+
+    const authHelper = gitAuthHelper.createAuthHelper(git, settings)
+
+    // Act - should not throw despite realpathSync failure
+    await authHelper.configureAuth()
+
+    // Assert the fallback constructed path is used in the includeIf entry
+    const localConfigContent = (
+      await fs.promises.readFile(localGitConfigPath)
+    ).toString()
+    const fallbackGitDir = path
+      .join(nonexistentPath, '.git')
+      .replace(/\\/g, '/')
+    expect(
+      localConfigContent.indexOf(`includeIf.gitdir:${fallbackGitDir}.path`)
+    ).toBeGreaterThanOrEqual(0)
+  })
+
   const setsSshCommandEnvVarWhenPersistCredentialsFalse =
     'sets SSH command env var when persist-credentials false'
   it(setsSshCommandEnvVarWhenPersistCredentialsFalse, async () => {

dist/index.js

@@ -406,9 +406,18 @@ class GitAuthHelper {
                 );
             }
             else {
-                // Host git directory
-                let gitDir = path.join(this.git.getWorkingDirectory(), '.git');
-                gitDir = gitDir.replace(/\\/g, '/'); // Use forward slashes, even on Windows
+                // Host git directory - resolve symlinks so includeIf gitdir matching works
+                // on self-hosted runners where _work is a symlink to an external volume.
+                let gitDir;
+                try {
+                    const constructed = path.join(this.git.getWorkingDirectory(), '.git');
+                    gitDir = fs.realpathSync(constructed).replace(/\\/g, '/');
+                }
+                catch (_a) {
+                    // Fall back to constructed path if realpathSync fails
+                    gitDir = path.join(this.git.getWorkingDirectory(), '.git');
+                    gitDir = gitDir.replace(/\\/g, '/');
+                }
                 // Configure host includeIf
                 const hostIncludeKey = `includeIf.gitdir:${gitDir}.path`;
                 yield this.git.config(hostIncludeKey, credentialsConfigPath);

src/git-auth-helper.ts

@@ -8,9 +8,9 @@ import * as path from 'path'
 import * as regexpHelper from './regexp-helper'
 import * as stateHelper from './state-helper'
 import * as urlHelper from './url-helper'
-import {v4 as uuid} from 'uuid'
-import {IGitCommandManager} from './git-command-manager'
-import {IGitSourceSettings} from './git-source-settings'
+import { v4 as uuid } from 'uuid'
+import { IGitCommandManager } from './git-command-manager'
+import { IGitSourceSettings } from './git-source-settings'
 
 const IS_WINDOWS = process.platform === 'win32'
 const SSH_COMMAND_KEY = 'core.sshCommand'
@@ -92,7 +92,7 @@ class GitAuthHelper {
     assert.ok(runnerTemp, 'RUNNER_TEMP is not defined')
     const uniqueId = uuid()
     this.temporaryHomePath = path.join(runnerTemp, uniqueId)
-    await fs.promises.mkdir(this.temporaryHomePath, {recursive: true})
+    await fs.promises.mkdir(this.temporaryHomePath, { recursive: true })
 
     // Copy the global git config
     const gitConfigPath = path.join(
@@ -258,11 +258,11 @@ class GitAuthHelper {
     const uniqueId = uuid()
     this.sshKeyPath = path.join(runnerTemp, uniqueId)
     stateHelper.setSshKeyPath(this.sshKeyPath)
-    await fs.promises.mkdir(runnerTemp, {recursive: true})
+    await fs.promises.mkdir(runnerTemp, { recursive: true })
     await fs.promises.writeFile(
       this.sshKeyPath,
       this.settings.sshKey.trim() + '\n',
-      {mode: 0o600}
+      { mode: 0o600 }
     )
 
     // Remove inherited permissions on Windows
@@ -366,9 +366,17 @@ class GitAuthHelper {
         true // globalConfig?
       )
     } else {
-      // Host git directory
-      let gitDir = path.join(this.git.getWorkingDirectory(), '.git')
-      gitDir = gitDir.replace(/\\/g, '/') // Use forward slashes, even on Windows
+      // Host git directory - resolve symlinks so includeIf gitdir matching works
+      // on self-hosted runners where _work is a symlink to an external volume.
+      let gitDir: string
+      try {
+        const constructed = path.join(this.git.getWorkingDirectory(), '.git')
+        gitDir = fs.realpathSync(constructed).replace(/\\/g, '/')
+      } catch {
+        // Fall back to constructed path if realpathSync fails
+        gitDir = path.join(this.git.getWorkingDirectory(), '.git')
+        gitDir = gitDir.replace(/\\/g, '/')
+      }
 
       // Configure host includeIf
       const hostIncludeKey = `includeIf.gitdir:${gitDir}.path`