Find merge base by progressively deepening history

action.yml

@@ -6,9 +6,14 @@ inputs:
   token:
     description: 'GitHub API token with read permissions for pull requests in this repository.'
     required: true
+  fetch-depth:
+    description: |
+      Number of commits to fetch each time we need to deepen git history in order to find the
+      merge base between the head ref and the base ref of the pull request
+    default: 10
   git-diff-args:
     description: |
-      Optional custom arguments to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
+      Custom arguments 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

@@ -35,48 +35,65 @@ export function loadConfiguration(): Configuration {
  */
 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+/)
-  )
+  const fetchDepth = parseInt(core.getInput('fetch-depth'))
 
-  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
+  core.debug(
+    `Fetching base ref "${pull.base.ref}" and head ref "${pull.head.ref}" with depth ${fetchDepth}`
+  )
 
-  // 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.base.ref}:${pull.base.ref}`,
     `+${pull.head.ref}:${pull.head.ref}`,
-    `--depth=${pull.commits + 1}`,
+    `--depth=${fetchDepth}`,
     '--no-tags',
     '--prune',
     '--no-recurse-submodules'
   ])
 
-  core.debug(`Switching to branch ${pull.head.ref}`)
+  core.debug(`Switching to head ref "${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.info(
+    `Searching for merge base between "${pull.base.ref}" and "${pull.head.ref}"`
+  )
+  let mergeBase = ''
+  let fetches = 0
+  while (mergeBase) {
+    try {
+      mergeBase = await git.raw('merge-base', pull.base.ref, pull.head.ref)
+      core.info(`Found merge base: ${mergeBase}`)
+    } catch (e) {
+      if (fetches === 0) {
+        core.debug(`git merge-base error: ${(e as Error).message}`)
+        core.info('Deepening git history in search of merge base')
+      } else if (fetches % 10 === 0) {
+        core.debug(`git merge-base error: ${(e as Error).message}`)
+        core.info(`Still searching for merge base after ${fetches} fetches`)
+      }
+      fetches++
+      await git.fetch([
+        'origin',
+        pull.base.ref,
+        pull.head.ref,
+        `--deepen=${fetchDepth}`,
+        '--no-tags',
+        '--prune',
+        '--no-recurse-modules'
+      ])
+    }
+  }
+
   core.debug(
-    `Retrieving history for origin/${pull.base.ref} since ${divergedFrom} which was committed at ${divergedAt}`
+    `Performed ${fetches} fetch${
+      fetches === 1 ? '' : 'es'
+    } to find the merge base`
   )
-  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.
+  const diffArgs = ['--merge-base', pull.base.ref].concat(
+    core.getInput('git-diff-args').split(/\s+/)
+  )
+  core.info(`Retrieving diff with \`git diff ${diffArgs.join(' ')}\``)
   return git.diff(diffArgs)
 }