Add parallel build support with -parallel and -jobs options

[arthurbdiniz]

This commit introduces parallel package building to ratt.

• New flag -parallel enables parallel builds.
• New flag -jobs controls the number of concurrent workers (default: number of CPU cores, must be > 0).
• Added buildPackagesSequential and buildPackagesParallel helpers.
• Refactored main build loop to support sequential or parallel execution.

This allows faster builds by utilizing multiple CPU cores.

---

Example:

~/go/bin/ratt -parallel golang-github-go-logr-zapr_1.3.0-1_amd64.changes
2025/09/12 17:09:43 Loading changes file "golang-github-go-logr-zapr_1.3.0-1_amd64.changes"
2025/09/12 17:09:43  - 1 binary packages: golang-github-go-logr-zapr-dev
2025/09/12 17:09:43 Corresponding .debs (will be injected when building):
2025/09/12 17:09:43     golang-github-go-logr-zapr-dev_1.3.0-1_all.deb
2025/09/12 17:09:43 Setting -dist=sid (from .changes file)
2025/09/12 17:09:43 Figuring out reverse build dependencies using dose-ceve(1). This might take a while
2025/09/12 17:10:14 Found 56 reverse build dependencies
2025/09/12 17:10:14 Setting -sbuild_dist=unstable (from .changes file)
2025/09/12 17:10:14 Building packages in parallel using 6 workers
2025/09/12 17:10:14 Building package 1 of 56: golang-github-containerd-stargz-snapshotter
2025/09/12 17:10:14 Building package 2 of 56: sigstore-go
2025/09/12 17:10:14 Building package 3 of 56: golang-github-containers-common
2025/09/12 17:10:14 Building package 4 of 56: golang-github-containerd-nydus-snapshotter
2025/09/12 17:10:14 Building package 4 of 56: golang-github-openshift-imagebuilder
2025/09/12 17:10:14 Building package 6 of 56: docker-compose

[stapelberg]

I think using an errgroup instead of a WaitGroup makes for clearer/simpler code in this case.

In particular, we could use https://pkg.go.dev/golang.org/x/sync/errgroup#Group.SetLimit to limit the number of active goroutines to numJobs and then iterate the same way as in buildPackagesSequential, but use eg.Go like so:

for src, versions := range rebuild {
	eg.Go(func() error {
		sort.Sort(sort.Reverse(version.Slice(versions)))
		newest := versions[0]
		log.Printf("Building package %d of %d: %s\n", cnt, len(rebuild), src)
		cnt++
		result := builder.build(src, &newest)
		if result.err != nil {
			log.Printf("building %s failed: %v\n", src, result.err)
		}
  		resultsMu.Lock()
  		defer resultsMu.Unlock()
  		results[src] = result
  		return nil
	})
}

[stapelberg]

Remove this line, capturing loop variables has not been necessary since Go 1.22 (https://go.dev/blog/loopvar-preview).

By the way, your editor should show you a warning about this and offer a code action to remove the line. If you don’t see this, maybe you use an old version of gopls, or something is wrong with your editor or IDE’s LSP setup…?

[stapelberg]

While currently, the function we run on the errgroup never returns an error, please add error handling for this eg.Wait() call regardless (to make future changes less surprising).

[stapelberg]

buildPackagesSequential(builder, rebuild) could be replaced with buildPackagesParallel(builder, rebuild, 1), right? Then, we can delete the entire buildPackagesSequential function and remove all the duplicate code.