CI: build without lockfile if dependencies changed

- Makes this job less noisy if dependencies weren't modified.
- Comment on the PR in case of failure to communicate that PR can still
  be merged.
- Add badge to readme to make failures on main or scheduled builds
  visible.

Author: sveitser

.github/workflows/build-without-lockfile.yml

@@ -11,6 +11,10 @@ on:
   schedule:
     - cron: "0 0 * * 1"
   pull_request:
+    # Only run on PRs if dependencies were modified.
+    paths:
+      - "Cargo.lock"
+      - "**/Cargo.toml"
   workflow_dispatch:
 
 concurrency:
@@ -33,3 +37,30 @@ jobs:
         run: |
           cargo generate-lockfile
           cargo check --all-targets
+
+      - name: Comment on PR
+        uses: actions/github-script@v7
+        # TODO uncomment
+        # if: failure()
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: `Unable to build without Cargo.lock file.
+
+              This means that after this change 3rd party projects may have
+              difficulties using crates in this repo as a dependency. If this
+              isn't easy to fix please open an issue so we can fix it later.
+
+              To reproduce locally run
+
+              \`\`\`
+              cargo generate-lockfile
+              cargo check --all-targets
+              \`\`\`
+
+              This PR can still be merged.`
+            })

README.md

@@ -7,6 +7,7 @@
 [![Lint](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/lint.yml/badge.svg)](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/lint.yml)
 [![Audit](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/audit.yml/badge.svg)](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/audit.yml)
 [![Ubuntu](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/ubuntu-install-without-nix.yml/badge.svg)](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/ubuntu-install-without-nix.yml)
+[![Build without lockfile](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/build-without-lockfile.yml/badge.svg)](https://github.com/EspressoSystems/espresso-sequencer/actions/workflows/build-without-lockfile.yml)
 [![Coverage Status](https://coveralls.io/repos/github/EspressoSystems/espresso-sequencer/badge.svg?branch=main)](https://coveralls.io/github/EspressoSystems/espresso-sequencer?branch=main)
 
 The Espresso Sequencer offers rollups credible neutrality and enhanced interoperability, without compromising on scale.