Add an action to run a linter over the code

Author: mhagger

.github/workflows/lint.yml

@@ -0,0 +1,55 @@
+name: Lint
+on:
+  push:
+    paths:
+      - "**.go"
+      - go.mod
+      - go.sum
+  pull_request:
+    paths:
+      - "**.go"
+      - go.mod
+      - go.sum
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Set up Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: 1.17
+
+      - name: Check out code
+        uses: actions/checkout@v2
+
+      - name: Verify dependencies
+        run: |
+          go mod verify
+          go mod download
+
+          LINT_VERSION=1.43.0
+          curl -fsSL https://github.com/golangci/golangci-lint/releases/download/v${LINT_VERSION}/golangci-lint-${LINT_VERSION}-linux-amd64.tar.gz | \
+            tar xz --strip-components 1 --wildcards \*/golangci-lint
+          mkdir -p bin && mv golangci-lint bin/
+
+      - name: Run checks
+        run: |
+          STATUS=0
+          assert-nothing-changed() {
+            local diff
+            "$@" >/dev/null || return 1
+            if ! diff="$(git diff -U1 --color --exit-code)"; then
+              printf '\e[31mError: running `\e[1m%s\e[22m` results in modifications that you must check into version control:\e[0m\n%s\n\n' "$*" "$diff" >&2
+              git checkout -- .
+              STATUS=1
+            fi
+          }
+
+          assert-nothing-changed go fmt ./...
+          assert-nothing-changed go mod tidy
+
+          bin/golangci-lint run --out-format=github-actions --timeout=3m || STATUS=$?
+
+          exit $STATUS