Avoid unnecessary localization branch rebases

tools/pipelines/localization-update.yml

@@ -26,7 +26,7 @@ jobs:
     steps:
       - checkout: self
         clean: true
-        fetchDepth: 1
+        fetchDepth: 0
         fetchTags: false
         submodules: false
 
@@ -207,27 +207,37 @@ jobs:
                 git config --local http.https://github.com/.extraheader "AUTHORIZATION: basic $basicAuth"
                 git remote set-url origin https://github.com/microsoft/TypeScript.git
 
+                $existingPr = gh pr list --repo microsoft/TypeScript --base main --head $branch --state open --json url --jq '.[0].url // empty'
                 $remoteBranch = git ls-remote --heads origin $branch
                 $branchExists = [bool] $remoteBranch
+                $remoteCommit = ""
                 if ($branchExists) {
-                    git fetch --depth=1 --no-tags --progress origin "refs/heads/${branch}:refs/remotes/origin/${branch}"
+                    git fetch --no-tags --progress origin "refs/heads/${branch}:refs/remotes/origin/${branch}"
+                    $remoteCommit = git rev-parse "origin/${branch}"
+                }
+
+                $candidate = git rev-parse HEAD
+                if ($existingPr) {
+                    if (!$branchExists) {
+                        throw "The open localization pull request no longer has a remote branch."
+                    }
+                    $candidate = node tools/scripts/preserve-localization-base.mjs $remoteCommit
                 }
 
                 $shouldPush = $true
                 if ($branchExists) {
-                    $localTree = git rev-parse 'HEAD^{tree}'
-                    $remoteTree = git rev-parse "origin/${branch}^{tree}"
+                    $localTree = git rev-parse "${candidate}^{tree}"
+                    $remoteTree = git rev-parse "${remoteCommit}^{tree}"
                     if ($localTree -eq $remoteTree) {
                         Write-Host "Remote branch already contains the same files."
                         $shouldPush = $false
                     }
                 }
 
                 if ($shouldPush) {
-                    git push --force-with-lease --set-upstream origin $branch
+                    git push "--force-with-lease=refs/heads/${branch}:${remoteCommit}" origin "${candidate}:refs/heads/${branch}"
                 }
 
-                $existingPr = gh pr list --repo microsoft/TypeScript --base main --head $branch --state open --json url --jq '.[0].url // empty'
                 if ($existingPr) {
                     Write-Host "Updated existing pull request: $existingPr"
                 }

tools/scripts/preserve-localization-base.mjs

@@ -0,0 +1,48 @@
+import {
+    execFileSync,
+    spawnSync,
+} from "node:child_process";
+
+const [previous] = process.argv.slice(2);
+if (!previous || process.argv.length !== 3) {
+    throw new Error("Usage: node tools/scripts/preserve-localization-base.mjs <previous-localization-commit>");
+}
+
+console.log(prepareUpdate());
+
+function prepareUpdate() {
+    const generated = git("rev-parse", "HEAD");
+    const main = git("rev-parse", `${generated}^`);
+    const previousHead = git("rev-parse", "--verify", `${previous}^{commit}`);
+    const base = git("merge-base", main, previousHead);
+    if (base === main) return generated;
+
+    // Apply only the generated delta to the old base, without changing the checkout.
+    const tree = mergeTree(`--merge-base=${main}`, base, generated);
+    if (!tree) {
+        console.error("Localization changes conflict with the existing PR base; using current main.");
+        return generated;
+    }
+
+    const candidate = git("commit-tree", tree, "-p", base, "-m", "Update localization files");
+    if (mergeTree(main, candidate) !== git("rev-parse", `${generated}^{tree}`)) {
+        console.error("Preserving the PR base would conflict or change the generated merge result; using current main.");
+        return generated;
+    }
+
+    return tree === git("rev-parse", `${previousHead}^{tree}`) ? previousHead : candidate;
+}
+
+function git(...args) {
+    return execFileSync("git", args, { encoding: "utf8", stdio: ["ignore", "pipe", "pipe"] }).trim();
+}
+
+function mergeTree(...args) {
+    const result = spawnSync("git", ["merge-tree", "--write-tree", ...args], { encoding: "utf8" });
+    if (result.error) throw result.error;
+    if (result.status === 1) return undefined;
+    if (result.status !== 0) {
+        throw new Error(`git merge-tree failed (${result.status}): ${result.stderr}`);
+    }
+    return result.stdout.trim().split(/\r?\n/, 1)[0];
+}