feat: support checkout on a commit in addition to a ref

src/create-or-update-branch.ts

@@ -5,6 +5,28 @@ import {v4 as uuidv4} from 'uuid'
 const CHERRYPICK_EMPTY =
   'The previous cherry-pick is now empty, possibly due to conflict resolution.'
 
+export enum WorkingBaseType {
+  Branch = 'branch',
+  Commit = 'commit'
+}
+
+export async function getWorkingBaseAndType(
+  git: GitCommandManager
+): Promise<[string, WorkingBaseType]> {
+  const symbolicRefResult = await git.exec(
+    ['symbolic-ref', 'HEAD', '--short'],
+    true
+  )
+  if (symbolicRefResult.exitCode == 0) {
+    // A ref is checked out
+    return [symbolicRefResult.stdout.trim(), WorkingBaseType.Branch]
+  } else {
+    // A commit is checked out (detached HEAD)
+    const headSha = await git.revParse('HEAD')
+    return [headSha, WorkingBaseType.Commit]
+  }
+}
+
 export async function tryFetch(
   git: GitCommandManager,
   remote: string,
@@ -80,8 +102,15 @@ export async function createOrUpdateBranch(
   branchRemoteName: string,
   signoff: boolean
 ): Promise<CreateOrUpdateBranchResult> {
-  // Get the working base. This may or may not be the actual base.
-  const workingBase = await git.symbolicRef('HEAD', ['--short'])
+  // Get the working base.
+  // When a ref, it may or may not be the actual base.
+  // When a commit, we must rebase onto the actual base.
+  const [workingBase, workingBaseType] = await getWorkingBaseAndType(git)
+  core.info(`Working base is ${workingBaseType} '${workingBase}'`)
+  if (workingBaseType == WorkingBaseType.Commit && !base) {
+    throw new Error(`When in 'detached HEAD' state, 'base' must be supplied.`)
+  }
+
   // If the base is not specified it is assumed to be the working base.
   base = base ? base : workingBase
   const baseRemote = 'origin'
@@ -109,12 +138,16 @@ export async function createOrUpdateBranch(
 
   // Perform fetch and reset the working base
   // Commits made during the workflow will be removed
-  await git.fetch([`${workingBase}:${workingBase}`], baseRemote, ['--force'])
+  if (workingBaseType == WorkingBaseType.Branch) {
+    core.info(`Resetting working base branch '${workingBase}' to its remote`)
+    await git.fetch([`${workingBase}:${workingBase}`], baseRemote, ['--force'])
+  }
 
   // If the working base is not the base, rebase the temp branch commits
+  // This will also be true if the working base type is a commit
   if (workingBase != base) {
     core.info(
-      `Rebasing commits made to branch '${workingBase}' on to base branch '${base}'`
+      `Rebasing commits made to ${workingBaseType} '${workingBase}' on to base branch '${base}'`
     )
     // Checkout the actual base
     await git.fetch([`${base}:${base}`], baseRemote, ['--force'])

src/create-pull-request.ts

@@ -1,5 +1,9 @@
 import * as core from '@actions/core'
-import {createOrUpdateBranch} from './create-or-update-branch'
+import {
+  createOrUpdateBranch,
+  getWorkingBaseAndType,
+  WorkingBaseType
+} from './create-or-update-branch'
 import {GitHubHelper} from './github-helper'
 import {GitCommandManager} from './git-command-manager'
 import {GitAuthHelper} from './git-auth-helper'
@@ -81,24 +85,16 @@ export async function createPullRequest(inputs: Inputs): Promise<void> {
       core.endGroup()
     }
 
-    // Determine if the checked out ref is a valid base for a pull request
-    // The action needs the checked out HEAD ref to be a branch
-    // This check will fail in the following cases:
-    // - HEAD is detached
-    // - HEAD is a merge commit (pull_request events)
-    // - HEAD is a tag
-    core.startGroup('Checking the checked out ref')
-    const symbolicRefResult = await git.exec(
-      ['symbolic-ref', 'HEAD', '--short'],
-      true
-    )
-    if (symbolicRefResult.exitCode != 0) {
-      core.debug(`${symbolicRefResult.stderr}`)
+    core.startGroup('Checking the base repository state')
+    const [workingBase, workingBaseType] = await getWorkingBaseAndType(git)
+    core.info(`Working base is ${workingBaseType} '${workingBase}'`)
+    // When in detached HEAD state (checked out on a commit), we need to
+    // know the 'base' branch in order to rebase changes.
+    if (workingBaseType == WorkingBaseType.Commit && !inputs.base) {
       throw new Error(
-        'The checked out ref is not a valid base for a pull request. Unable to continue.'
+        `When the repository is checked out on a commit instead of a branch, the 'base' input must be supplied.`
       )
     }
-    const workingBase = symbolicRefResult.stdout.trim()
     // If the base is not specified it is assumed to be the working base.
     const base = inputs.base ? inputs.base : workingBase
     // Throw an error if the base and branch are not different branches