github: run tests against custom cockroach Go

Add a script and a nightly target that runs tests with the
custom Go runtime and the `cockroach_go` tag. The script is lifted
from crlib.

Author: RaduBerinde

.github/workflows/nightlies.yaml

@@ -49,3 +49,34 @@ jobs:
       sha: ${{ github.sha }}
       file_issue_branch: 'master'
       go_version: ${{ matrix.go }}
+
+  linux-cockroach-go:
+    runs-on: ubuntu-latest
+    env:
+      GO_BRANCH: cockroach-go1.23.12
+
+    steps:
+      - uses: actions/checkout@v4
+
+      # Step 1: Fetch the branch tip SHA for cache key
+      - name: Get cockroachdb/go commit hash
+        id: go-sha
+        run: |
+          SHA=$(git ls-remote https://github.com/cockroachdb/go.git refs/heads/$GO_BRANCH | cut -f1)
+          echo "GO_SHA=$SHA" >> $GITHUB_ENV
+
+      # Step 2: Restore cache (per branch + commit SHA)
+      - name: Cache custom Go toolchain
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/cockroachdb-go/${{ env.GO_SHA }}
+          key: cockroachdb-${{ env.GO_SHA }}
+
+      # Step 3: Install bootstrap Go (needed to build fork)
+      - name: Install bootstrap Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: "1.23.x"
+
+      # Step 4: Run tests with custom Go
+      - run: ./scripts/run-tests-with-custom-go.sh -tags invariants ./...

scripts/run-tests-with-custom-go.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+# run-tests-with-custom-go.sh
+#
+# Downloads and builds a custom Go toolchain from cockcroachdb/go (specific
+# branch), caches it in ~/.cache/cockcroachdb-go/<branch>/<sha>, and runs
+# `go test`.
+#
+# Works in GitHub Actions or when run manually.
+# In CI: pass GO_SHA to pin an exact commit.
+# Locally: if GO_SHA is unset, script fetches the latest branch tip.
+
+set -euo pipefail
+
+GO_REPO="https://github.com/cockroachdb/go.git"
+GO_SHA="${GO_SHA:-}"
+
+if [ -z "$GO_SHA" ]; then
+  GO_BRANCH="${GO_BRANCH:-cockroach-go1.23.12}"
+  echo "==> Resolving latest SHA for branch $GO_BRANCH..."
+  GO_SHA=$(git ls-remote "$GO_REPO" "refs/heads/$GO_BRANCH" | cut -f1)
+fi
+
+# Use GITHUB_WORKSPACE if present (GitHub Actions), else current dir
+REPO_ROOT="${GITHUB_WORKSPACE:-$(pwd)}"
+
+# Cache location (works locally and in CI)
+CACHE_BASE="${XDG_CACHE_HOME:-$HOME/.cache}/cockroachdb-go"
+CACHE_DIR="$CACHE_BASE/$GO_SHA"
+SRC_DIR="$CACHE_DIR/src"
+
+echo "==> Commit SHA: $GO_SHA"
+echo "==> Repository root: $REPO_ROOT"
+echo "==> Cache directory: $CACHE_DIR"
+
+if [ ! -x "$SRC_DIR/go/bin/go" ]; then
+  echo "==> Building new Go toolchain..."
+  mkdir -p "$SRC_DIR"
+  rm -rf "$SRC_DIR/go" # in case of partial/incomplete build
+
+  git clone "$GO_REPO" "$SRC_DIR/go"
+  cd "$SRC_DIR/go"
+  git checkout "$GO_SHA"
+
+  cd "$SRC_DIR/go/src"
+  ./make.bash
+else
+  echo "==> Reusing cached Go toolchain"
+fi
+
+# Point environment to new Go
+export GOROOT="$SRC_DIR/go"
+export PATH="$GOROOT/bin:$PATH"
+
+echo "==> Custom Go version:"
+go version
+
+echo "==> Running tests in $REPO_ROOT"
+cd "$REPO_ROOT"
+echo go test -tags cockroach_go "$@"
+go test -tags cockroach_go "$@"