chore: add Action for generating diff tour links

.gitignore

@@ -0,0 +1 @@
+.DS_Store

README.md

@@ -7,6 +7,7 @@ Reusable GitHub Actions for Sourcegraph repositories.
 | Action | Description |
 |--------|-------------|
 | [go-setup](./go-setup) | Setup Go with private Sourcegraph repository access |
+| [diff-tour](./diff-tour) | Generate a link to the Diff Tour for a PR |
 
 ## Usage
 

diff-tour/README.md

@@ -0,0 +1,35 @@
+# `diff-tour`
+
+Generates a link to the Diff Tour for a PR and posts it as a PR comment.
+
+## Usage
+
+```yaml
+on:
+  pull_request:
+    # `closed` fires on merge, so the comment is updated to link the merge commit
+    types: [opened, synchronize, reopened, closed]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  diff-tour:
+    runs-on: ubuntu-latest
+    # Diff Tour resolves branch names against the base repo, so skip fork PRs.
+    # Closed PRs only need an update when they were merged.
+    if: >
+      github.event.pull_request.head.repo.full_name == github.repository
+      && (github.event.action != 'closed' || github.event.pull_request.merged)
+    steps:
+      - uses: sourcegraph/actions/diff-tour@main
+```
+
+## Inputs
+
+This action has no inputs.
+
+## What it does
+
+1. Builds a Diff Tour URL for the PR — a commit link if the PR is merged, or a branch compare link if it's still open
+2. Creates a PR comment with the link, or updates the existing one if it has already posted a comment

diff-tour/action.yaml

@@ -0,0 +1,13 @@
+name: Diff Tour link
+description: Generate a link to the Diff Tour based on a PR.
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+    - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+      with:
+        script: |
+          const script = require(`${{ github.action_path }}/generateTourLink.cjs`)
+          const result = await script({ github, context })
+          console.log(result)

diff-tour/generateTourLink.cjs

@@ -0,0 +1,108 @@
+/**
+ * @file This file is written in plain JavaScript to remove an extra build step, but we should use the @ts-check
+ * directive and JSDoc comments to make sure the logic is still type-safe.
+ *
+ * @todo 2026-09-10 - This file recreates local versions of the `AsyncFunctionArguments` type from
+ * `@actions/github-script`.so that we can keep this file fully self-contained and type-safe without having to bring in
+ * typical JS/TS tooling. If this repo gets to the point where it needs multiple JS scripts, consider ripping out these
+ * local types in favor of bringing in the package straight from GitHub.
+ */
+//@ts-check
+
+/** @type {{log: (...args: unknown[]) => void}} */
+const console = /** @type {any} */ (globalThis).console
+
+/**
+ * @typedef {object} PullRequest
+ * @property {number} number
+ * @property {boolean} merged
+ * @property {string} merge_commit_sha
+ * @property {{ref: string}} base
+ * @property {{ref: string}} head
+ */
+
+/**
+ * @typedef {object} Context
+ * @property {{pull_request?: PullRequest}} payload
+ * @property {{owner: string, repo: string}} repo
+ */
+
+/**
+ * @typedef {object} Comment
+ * @property {number} id
+ * @property {string | null | undefined} body
+ */
+
+/**
+ * @typedef {object} Github
+ * @property {object} rest
+ * @property {object} rest.issues
+ * @property {(params: {owner: string, repo: string, issue_number: number}) => Promise<{data: Comment[]}>} rest.issues.listComments
+ * @property {(params: {owner: string, repo: string, comment_id: number, body: string}) => Promise<unknown>} rest.issues.updateComment
+ * @property {(params: {owner: string, repo: string, issue_number: number, body: string}) => Promise<unknown>} rest.issues.createComment
+ */
+
+/**
+ * @typedef {object} GenerateTourLinkArgs
+ * @property {Github} github
+ * @property {Context} context
+ */
+
+const instance = "https://sourcegraph.sourcegraph.com"
+const diffTourCommentMarker = "<!-- difftour-link -->"
+
+/** @param {GenerateTourLinkArgs} args */
+async function generateTourLink({ github, context }) {
+	const pullRequest = context.payload.pull_request
+	if (pullRequest === undefined) {
+		return
+	}
+
+	/** @type {string} */
+	let url
+	const { repo, owner } = context.repo
+	const repoLink = `${owner}/${repo}`
+	if (pullRequest.merged) {
+		const commit = encodeURIComponent(pullRequest.merge_commit_sha)
+		url = `${instance}/r/${repoLink}/-/commit/${commit}?mode=Tour`
+	} else {
+		const base = encodeURIComponent(pullRequest.base.ref)
+		const head = encodeURIComponent(pullRequest.head.ref)
+		url = `${instance}/r/${repoLink}/-/compare/${base}...${head}?mode=Tour`
+	}
+
+	const { data: comments } = await github.rest.issues.listComments({
+		owner,
+		repo,
+		issue_number: pullRequest.number,
+	})
+
+	const markedComments = comments.filter(
+		(c) => c.body?.includes(diffTourCommentMarker) ?? false,
+	)
+	if (markedComments.length > 1) {
+		console.log(
+			"Found multiple comments with the Diff Tour marker. Updating only the first",
+		)
+	}
+
+	const commentContent = `${diffTourCommentMarker}\n[Open the Diff Tour for this PR in Sourcegraph](${url})`
+	const first = markedComments[0]
+	if (first !== undefined) {
+		await github.rest.issues.updateComment({
+			owner,
+			repo,
+			comment_id: first.id,
+			body: commentContent,
+		})
+	} else {
+		await github.rest.issues.createComment({
+			owner,
+			repo,
+			issue_number: pullRequest.number,
+			body: commentContent,
+		})
+	}
+}
+
+module.exports = generateTourLink

go-setup/README.md

@@ -1,14 +1,8 @@
-# Sourcegraph Actions
-
-Reusable GitHub Actions for Sourcegraph repositories.
-
-## Available Actions
-
-### go-setup
+# `go-setup`
 
 Sets up Go with private Sourcegraph repository access.
 
-**Usage:**
+## Usage
 
 ```yaml
 steps:
@@ -18,14 +12,14 @@ steps:
       private-token: ${{ secrets.PRIVATE_SG_ACCESS_TOKEN }}
 ```
 
-**Inputs:**
+## Inputs
 
 | Input | Required | Default | Description |
 |-------|----------|---------|-------------|
 | `private-token` | Yes | - | Token for accessing private Sourcegraph repos |
 | `go-version-file` | No | `go.mod` | Path to go.mod or go.work file to determine Go version |
 
-**What it does:**
+## What it does
 
 1. Installs Go using the version from your `go.mod`
 2. Configures git to access private `github.com/sourcegraph/*` repos