Improve change detection for feature branches (#16)

* Detect changes against configured base branch

* Update README and action.yml

* Add job.outputs example

* Update CHANGELOG

src/main.ts

@@ -15,9 +15,17 @@ async function run(): Promise<void> {
     const filter = new Filter(filtersYaml)
     const files = await getChangedFiles(token)
 
-    const result = filter.match(files)
-    for (const key in result) {
-      core.setOutput(key, String(result[key]))
+    if (files === null) {
+      // Change detection was not possible
+      // Set all filter keys to true (i.e. changed)
+      for (const key in filter.rules) {
+        core.setOutput(key, String(true))
+      }
+    } else {
+      const result = filter.match(files)
+      for (const key in result) {
+        core.setOutput(key, String(result[key]))
+      }
     }
   } catch (error) {
     core.setFailed(error.message)
@@ -40,23 +48,45 @@ function getConfigFileContent(configPath: string): string {
   return fs.readFileSync(configPath, {encoding: 'utf8'})
 }
 
-async function getChangedFiles(token: string): Promise<string[]> {
+async function getChangedFiles(token: string): Promise<string[] | null> {
   if (github.context.eventName === 'pull_request') {
     const pr = github.context.payload.pull_request as Webhooks.WebhookPayloadPullRequestPullRequest
     return token ? await getChangedFilesFromApi(token, pr) : await getChangedFilesFromGit(pr.base.sha)
   } else if (github.context.eventName === 'push') {
-    const push = github.context.payload as Webhooks.WebhookPayloadPush
-    return await getChangedFilesFromGit(push.before)
+    return getChangedFilesFromPush()
   } else {
     throw new Error('This action can be triggered only by pull_request or push event')
   }
 }
 
+async function getChangedFilesFromPush(): Promise<string[] | null> {
+  const push = github.context.payload as Webhooks.WebhookPayloadPush
+
+  // No change detection for pushed tags
+  if (git.isTagRef(push.ref)) return null
+
+  // Get base from input or use repo default branch.
+  // It it starts with 'refs/', it will be trimmed (git fetch refs/heads/<NAME> doesn't work)
+  const baseInput = git.trimRefs(core.getInput('base', {required: false}) || push.repository.default_branch)
+
+  // If base references same branch it was pushed to, we will do comparison against the previously pushed commit.
+  // Otherwise changes are detected against the base reference
+  const base = git.trimRefsHeads(baseInput) === git.trimRefsHeads(push.ref) ? push.before : baseInput
+
+  // There is no previous commit for comparison
+  // e.g. change detection against previous commit of just pushed new branch
+  if (base === git.NULL_SHA) return null
+
+  return await getChangedFilesFromGit(base)
+}
+
 // Fetch base branch and use `git diff` to determine changed files
-async function getChangedFilesFromGit(sha: string): Promise<string[]> {
+async function getChangedFilesFromGit(ref: string): Promise<string[]> {
   core.debug('Fetching base branch and using `git diff-index` to determine changed files')
-  await git.fetchCommit(sha)
-  return await git.getChangedFiles(sha)
+  await git.fetchCommit(ref)
+  // FETCH_HEAD will always point to the just fetched commit
+  // No matter if ref is SHA, branch or tag name or full git ref
+  return await git.getChangedFiles(git.FETCH_HEAD)
 }
 
 // Uses github REST api to get list of files changed in PR