feat: enable git rerere

script/upstream/merge.ts

@@ -141,6 +141,22 @@ async function main() {
   const currentBranch = await git.getCurrentBranch()
   logger.info(`Current branch: ${currentBranch}`)
 
+  // Enable git rerere so conflict resolutions are recorded and reused across merges
+  if (!options.dryRun) {
+    await git.ensureRerere()
+    logger.info("git rerere enabled (resolutions will be recorded and reused automatically)")
+
+    // Train rerere from past upstream merge commits so the cache is populated
+    // even on a fresh clone. This replays past merges to learn their resolutions.
+    logger.info("Training rerere cache from past merge history...")
+    const learned = await git.trainRerere("merge: upstream\\|Resolve merge conflict")
+    if (learned > 0) {
+      logger.success(`Learned ${learned} conflict resolution(s) from history`)
+    } else {
+      logger.info("No new resolutions to learn from history (cache already up to date)")
+    }
+  }
+
   // Step 2: Fetch upstream
   logger.step(2, 8, "Fetching upstream...")
 
@@ -385,6 +401,14 @@ async function main() {
     logger.info("Conflicted files:")
     logger.list(mergeResult.conflicts)
 
+    // Check if git rerere already auto-resolved any conflicts from recorded history.
+    // rerere.autoupdate stages them automatically; we just log how many were handled.
+    const rerereResolved = await git.getRerereResolved()
+    if (rerereResolved.length > 0) {
+      logger.success(`git rerere auto-resolved ${rerereResolved.length} conflict(s) from recorded history:`)
+      logger.list(rerereResolved)
+    }
+
     // Since we applied all branding transforms pre-merge, remaining conflicts should be minimal.
     // These are likely files with kilocode_change markers or actual logic differences.
 

script/upstream/utils/git.ts

@@ -236,3 +236,105 @@ export async function oursHasKilocodeChanges(file: string): Promise<boolean> {
   if (result.exitCode !== 0) return false
   return result.stdout.toString().includes("kilocode_change")
 }
+
+/**
+ * Enable git rerere (REuse REcorded REsolution) in the local repo config.
+ * Also enables autoupdate so resolved files are automatically staged.
+ */
+export async function ensureRerere(): Promise<void> {
+  await $`git config rerere.enabled true`.quiet()
+  await $`git config rerere.autoupdate true`.quiet()
+}
+
+/**
+ * Train the rerere cache from past merge commits in the repo history.
+ * Implements the same logic as git's contrib/rerere-train.sh:
+ *   For each merge commit in the range, replay the merge to let rerere
+ *   record the pre-image, then check out the resolved tree so rerere
+ *   records the post-image (the resolution).
+ *
+ * Returns the number of resolutions learned.
+ */
+export async function trainRerere(grep: string): Promise<number> {
+  // Save the current HEAD so we can restore it afterwards
+  const headResult = await $`git symbolic-ref -q HEAD`.quiet().nothrow()
+  const branch = headResult.exitCode === 0 ? headResult.stdout.toString().trim() : null
+  const originalHead = branch ?? (await $`git rev-parse --verify HEAD`.text()).trim()
+
+  let learned = 0
+
+  try {
+    // Find all merge commits matching the grep pattern (merges have multiple parents)
+    const revList = await $`git rev-list --parents --all --grep=${grep}`.quiet().nothrow()
+    if (revList.exitCode !== 0 || !revList.stdout.toString().trim()) return 0
+
+    const lines = revList.stdout
+      .toString()
+      .trim()
+      .split("\n")
+      .filter((l) => l.trim())
+
+    for (const line of lines) {
+      const parts = line.trim().split(/\s+/)
+      if (parts.length < 3) continue // skip non-merges (need commit + at least 2 parents)
+
+      const [commit, parent1, ...otherParents] = parts
+
+      // Checkout the first parent
+      const coResult = await $`git checkout -q ${parent1}`.quiet().nothrow()
+      if (coResult.exitCode !== 0) continue
+
+      // Attempt the merge - we expect it to fail with conflicts
+      const mergeResult = await $`git merge --no-gpg-sign ${otherParents}`.quiet().nothrow()
+      if (mergeResult.exitCode === 0) {
+        // Cleanly merged — no conflicts to learn from, reset and skip
+        await $`git reset -q --hard`.quiet().nothrow()
+        continue
+      }
+
+      // Check if rerere recorded a pre-image (MERGE_RR exists and is non-empty)
+      const mergeRR = Bun.file(`${process.env.GIT_DIR || ".git"}/MERGE_RR`)
+      const hasMergeRR = await mergeRR.exists().catch(() => false)
+      if (!hasMergeRR) {
+        await $`git reset -q --hard`.quiet().nothrow()
+        continue
+      }
+
+      // Record the conflict pre-image
+      await $`git rerere`.quiet().nothrow()
+
+      // Apply the actual resolution by checking out the merge commit's tree
+      await $`git checkout -q ${commit} -- .`.quiet().nothrow()
+
+      // Record the resolution post-image
+      await $`git rerere`.quiet().nothrow()
+
+      learned++
+      await $`git reset -q --hard`.quiet().nothrow()
+    }
+  } finally {
+    // Always restore original branch
+    if (branch) {
+      await $`git checkout ${branch.replace("refs/heads/", "")}`.quiet().nothrow()
+    } else {
+      await $`git checkout ${originalHead}`.quiet().nothrow()
+    }
+  }
+
+  return learned
+}
+
+/**
+ * Return files that git rerere has already auto-resolved.
+ * These files no longer have conflict markers but haven't been staged yet
+ * (unless rerere.autoupdate is true, in which case they're already staged).
+ */
+export async function getRerereResolved(): Promise<string[]> {
+  const result = await $`git rerere status`.quiet().nothrow()
+  if (result.exitCode !== 0 || !result.stdout.toString().trim()) return []
+  return result.stdout
+    .toString()
+    .trim()
+    .split("\n")
+    .filter((f) => f.length > 0)
+}