feat(web): resolve walkthrough diffs for GitLab merge requests

packages/web/server/lib/walkthrough/pull-request.js

@@ -1,14 +1,20 @@
 import { getOctokitOrNull } from '../github/octokit.js';
 import { resolveGitHubRepoFromDirectory } from '../github/repo/index.js';
+import { getGitLabClientOrNull } from '../gitlab/client.js';
+import { resolveGitLabRepoFromDirectory } from '../gitlab/repo.js';
+
+// GitLab diff pagination cap: never loop more than 10 pages of 100 files,
+// mirroring gitlab/routes.js.
+const GITLAB_DIFFS_MAX_PAGES = 10;
 
 /**
- * Raw unified diff for a pull request.
+ * Raw unified diff for a GitHub pull request.
  *
  * GitHub already returns the merge-base diff for a PR, so this matches the
  * three-dot semantics used for local branch reviews: work merged in from the
  * base branch is not part of it.
  */
-export async function getPullRequestDiff(directory, number) {
+async function getGitHubPullRequestDiff(directory, number) {
   const octokit = getOctokitOrNull();
   if (!octokit) {
     throw Object.assign(new Error('Connect a GitHub account to review pull requests'), {
@@ -44,3 +50,75 @@ export async function getPullRequestDiff(directory, number) {
 
   return { patch, meta: { owner: repo.owner, repo: repo.repo, number } };
 }
+
+/**
+ * Raw unified diff for a GitLab merge request.
+ *
+ * GitLab's merge request diffs endpoint returns one entry per file, so the
+ * pages are concatenated into a single patch. `repo` comes from the
+ * dispatcher's `resolveGitLabRepoFromDirectory` call and is never re-resolved
+ * here.
+ */
+async function getGitLabMergeRequestDiff(repo, number) {
+  const client = getGitLabClientOrNull();
+  if (!client) {
+    throw Object.assign(new Error('Connect a GitLab account to review merge requests'), {
+      statusCode: 401,
+      code: 'gitlab-not-connected',
+    });
+  }
+
+  // The parser always populates both fields; this guards a malformed repo so
+  // the failure is explicit rather than a downstream TypeError.
+  if (!repo?.namespace || !repo?.project) {
+    throw Object.assign(new Error('This directory has no GitLab remote'), {
+      statusCode: 400,
+      code: 'no-gitlab-remote',
+    });
+  }
+
+  // The client URL-encodes the path internally; never pre-encode it.
+  const projectPath = `${repo.namespace}/${repo.project}`;
+
+  const diffs = [];
+  for (let page = 1; page <= GITLAB_DIFFS_MAX_PAGES; page += 1) {
+    const response = await client.mergeRequestDiffs(projectPath, number, { per_page: 100, page });
+    if (response.status !== 200 || !Array.isArray(response.data)) {
+      break;
+    }
+    diffs.push(...response.data);
+    // The page object is the authoritative signal; the 10-page cap above is
+    // what stops a server that lies about hasMore from looping forever.
+    if (!response.page?.hasMore) {
+      break;
+    }
+  }
+
+  const patch = diffs
+    .map((item) => (typeof item?.diff === 'string' ? item.diff : ''))
+    .filter(Boolean)
+    .join('\n');
+  if (!patch.trim()) {
+    throw Object.assign(new Error(`Merge request #${number} has no diff`), {
+      statusCode: 404,
+      code: 'empty-diff',
+    });
+  }
+
+  return { patch, meta: { namespace: repo.namespace, project: repo.project, number } };
+}
+
+/**
+ * Raw unified diff for a pull request or merge request.
+ *
+ * The provider is chosen by the repository's git remote: a GitLab remote uses
+ * the GitLab merge request API, anything else falls back to the GitHub pull
+ * request API.
+ */
+export async function getPullRequestDiff(directory, number) {
+  const { repo } = await resolveGitLabRepoFromDirectory(directory);
+  if (repo) {
+    return getGitLabMergeRequestDiff(repo, number);
+  }
+  return getGitHubPullRequestDiff(directory, number);
+}