Find merge base by progressively deepening history

README.md

@@ -27,10 +27,16 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      # Check out a copy of this repository so that we can compute a diff to
-      # evaluate and load the Action's configuration.
+      # Check out a copy of this repository so that we can retrieve a diff to
+      # evaluate and load the `sizeup` configuration file.
+      #
+      # This step must fetch the pull request's base branch using the `ref`
+      # and `filter` arguments as shown below.
       - name: Checkout this repository
         uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.base.ref }}
+          filter: tree:0
 
       # Run the estimation tool
       - name: Run sizeup
@@ -49,13 +55,13 @@ jobs:
           # This input is required.
           token: "${{ secrets.GITHUB_TOKEN }}"
 
-          # Optional arguments that will be forwarded to `git diff` when
-          # computing the diff to evaluate with this Action.
+          # Options that will be forwarded to `git diff` when computing the
+          # diff to evaluate with this Action.
           #
           # Defaults to "--ignore-space-change", which ignores lines of the
           # diff in which the only change is to the amount of whitespace on the
           # line.
-          git-diff-args: ""
+          git-diff-options: ""
 
           # Path to a YAML configuration file for this Action that is stored in
           # this repository.

action.yml

@@ -6,9 +6,9 @@ inputs:
   token:
     description: 'GitHub API token with read permissions for pull requests in this repository.'
     required: true
-  git-diff-args:
+  git-diff-options:
     description: |
-      Optional custom arguments to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
+      Custom options to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
       The result of that `git diff` command will be used for evaluation with sizeup.
     default: '--ignore-space-change'
   configuration-file-path:

src/initializer.ts

@@ -26,60 +26,6 @@ export function loadConfiguration(): Configuration {
   return YAML.parse(fs.readFileSync(configFile, 'utf8')) as Configuration
 }
 
-/**
- * Retrieves the diff of the pull request that triggered this workflow which we
- * will use for evaluation.
- *
- * @param pull The pull request that triggered this workflow.
- * @returns The diff of the given pull request.
- */
-export async function fetchDiff(pull: PullRequest): Promise<string> {
-  const git = simpleGit('.', { trimmed: true })
-  const diffArgs = ['--merge-base', `origin/${pull.base.ref}`].concat(
-    core.getInput('git-diff-args').split(/\s+/)
-  )
-
-  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
-
-  // Fetch all commits for the head branch back to where it diverged from the base.
-  core.debug(`Fetching ${pull.commits + 1} commits for ${pull.head.ref}`)
-  await git.fetch([
-    'origin',
-    `+${pull.head.ref}:${pull.head.ref}`,
-    `--depth=${pull.commits + 1}`,
-    '--no-tags',
-    '--prune',
-    '--no-recurse-submodules'
-  ])
-
-  core.debug(`Switching to branch ${pull.head.ref}`)
-  await git.raw('switch', pull.head.ref)
-
-  // Fetch commits for the base branch that were made since the head diverged from it.
-  const divergedFrom = await git.raw('rev-list', '--max-parents=0', 'HEAD')
-  const divergedAt = await git.show([
-    '--quiet',
-    '--date=unix',
-    '--format=%cd',
-    divergedFrom
-  ])
-  core.debug(
-    `Retrieving history for origin/${pull.base.ref} since ${divergedFrom} which was committed at ${divergedAt}`
-  )
-  await git.fetch([
-    'origin',
-    `+${pull.base.ref}:${pull.base.ref}`,
-    `--shallow-since=${divergedAt}`,
-    '--no-tags',
-    '--prune',
-    '--no-recurse-submodules'
-  ])
-
-  // Now we have all relevant history from both base and head branches, so we
-  // can compute an accurate diff relative to the base branch.
-  return git.diff(diffArgs)
-}
-
 export function workflowTriggeredForUnsupportedEvent(): boolean {
   if (github.context.eventName !== 'pull_request') {
     core.setFailed(
@@ -113,3 +59,55 @@ export function pullRequestAuthorHasNotOptedIn(
 
   return false
 }
+
+/**
+ * Retrieves the diff of the pull request that triggered this workflow which we
+ * will use for evaluation.
+ *
+ * @param pull The pull request that triggered this workflow.
+ * @returns The diff of the given pull request or `undefined` if we failed to retrieve it.
+ */
+export async function fetchDiff(
+  pull: PullRequest
+): Promise<string | undefined> {
+  const git = simpleGit('.', { trimmed: true })
+
+  let baseRefExists = false
+  try {
+    baseRefExists = !!(await git.raw('rev-parse', '--verify', pull.base.ref))
+  } catch (e) {
+    core.error(
+      `Error from 'git rev-parse --verfy ${pull.base.ref}': ${
+        (e as Error).message
+      }`
+    )
+  }
+
+  if (!baseRefExists) {
+    core.setFailed(
+      `Could not find pull request base branch ${pull.base.ref}. ` +
+        `Please make sure actions/checkout was used beforehand to fetch ${pull.base.ref}.`
+    )
+    return
+  }
+
+  core.debug(`Fetching head ref "${pull.head.ref}"`)
+
+  await git.fetch([
+    'origin',
+    `+${pull.head.ref}:${pull.head.ref}`,
+    `--filter=tree:0`,
+    '--no-tags',
+    '--prune',
+    '--no-recurse-submodules'
+  ])
+
+  core.debug(`Switching to head ref "${pull.head.ref}"`)
+  await git.raw('switch', pull.head.ref)
+
+  const diffArgs = ['--merge-base', pull.base.ref].concat(
+    core.getInput('git-diff-options').split(/\s+/)
+  )
+  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
+  return git.diff(diffArgs)
+}

src/main.ts

@@ -37,7 +37,10 @@ export async function run(): Promise<void> {
 
     if (pullRequestAuthorHasNotOptedIn(config, pullRequest)) return
 
-    const score = await evaluatePullRequest(pullRequest, config)
+    const diff = await fetchDiff(pullRequest)
+    if (!diff) return
+
+    const score = await evaluatePullRequest(pullRequest, diff, config)
 
     await applyCategoryLabel(pullRequest, score, config)
     await addScoreThresholdExceededComment(pullRequest, score, config)
@@ -58,6 +61,7 @@ export async function run(): Promise<void> {
  */
 async function evaluatePullRequest(
   pull: PullRequest,
+  diff: string,
   config: Configuration
 ): Promise<Score> {
   const pullRequestNickname = `${pull.base.repo.owner.login}/${pull.base.repo.name}#${pull.number}`
@@ -70,7 +74,6 @@ async function evaluatePullRequest(
     fs.writeFileSync(sizeupConfigFile, YAML.stringify(config.sizeup))
   }
 
-  const diff = await fetchDiff(pull)
   const score = SizeUp.evaluate(diff, sizeupConfigFile)
 
   if (sizeupConfigFile) {