depsync: add xk6-depsync command

Dockerfile

@@ -7,7 +7,8 @@ WORKDIR /build
 COPY . .
 
 ARG GOFLAGS="-ldflags=-w -ldflags=-s"
-RUN CGO_ENABLED=0 go build -o xk6 -trimpath ./cmd/xk6/main.go
+RUN CGO_ENABLED=0 go build -o xk6 -trimpath ./cmd/xk6/main.go && \
+    CGO_ENABLED=0 go build -o xk6-depsync -trimpath ./cmd/xk6-depsync/main.go
 
 
 FROM golang:${GO_VERSION}-${VARIANT}
@@ -24,7 +25,7 @@ RUN USER=xk6 && \
     mkdir -p /etc/fixuid && \
     printf "user: $USER\ngroup: $GROUP\n" > /etc/fixuid/config.yml
 
-COPY --from=builder /build/xk6 /usr/local/bin/
+COPY --from=builder /build/xk6 /build/xk6-depsync /usr/local/bin/
 
 COPY docker-entrypoint.sh /usr/local/bin/entrypoint.sh
 

cmd/xk6-depsync/main.go

@@ -0,0 +1,112 @@
+// main is the main package for xk6-depsync, a script that checks and provides commands to synchronize
+// common dependencies with k6 core.
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"sort"
+	"strings"
+)
+
+const (
+	k6Core     = "go.k6.io/k6"
+	k6GoModURL = "https://proxy.golang.org/" + k6Core + "/@v/%s.mod"
+)
+
+func main() {
+	gomod := "go.mod"
+	if len(os.Args) >= 2 {
+		gomod = os.Args[1]
+	}
+
+	file, err := os.Open(gomod)
+	if err != nil {
+		log.Fatalf("opening local go.mod: %v", err)
+	}
+
+	ownDeps, err := dependencies(file)
+	if err != nil {
+		log.Fatalf("reading dependencies: %v", err)
+	}
+
+	k6Version := ownDeps[k6Core]
+	if k6Version == "" {
+		log.Fatalf("K6 core %q not found in %q", k6Core, gomod)
+	}
+
+	log.Printf("detected k6 core version %s", k6Version)
+
+	k6CoreVersionedURL := fmt.Sprintf(k6GoModURL, k6Version)
+	//nolint:bodyclose // Single-run script.
+	response, err := http.Get(k6CoreVersionedURL)
+	if err != nil {
+		log.Fatalf("error fetching k6 go.mod: %v", err)
+	}
+
+	if response.StatusCode != http.StatusOK {
+		log.Fatalf("got HTTP status %d for %s", response.StatusCode, k6CoreVersionedURL)
+	}
+
+	coreDeps, err := dependencies(response.Body)
+	if err != nil {
+		log.Fatalf("reading k6 core dependencies: %v", err)
+	}
+
+	//nolint:prealloc // Number of mismatched deps cannot be accurately predicted.
+	var mismatched []string
+	for dep, version := range ownDeps {
+		coreVersion, inCore := coreDeps[dep]
+		if !inCore {
+			continue
+		}
+
+		if version == coreVersion {
+			continue
+		}
+
+		log.Printf("Mismatched versions for %s: %s (this package) -> %s (core)", dep, version, coreVersion)
+		mismatched = append(mismatched, fmt.Sprintf("%s@%s", dep, coreVersion))
+	}
+
+	if len(mismatched) == 0 {
+		log.Println("All deps are in sync, nothing to do.")
+		return
+	}
+
+	// TODO: Use slices.Sort when we move to a go version that has it on the stdlib.
+	sort.Strings(mismatched)
+
+	//nolint:forbidigo // We are willingly writing to stdout here.
+	fmt.Printf("go get %s\n", strings.Join(mismatched, " "))
+}
+
+// dependencies reads a go.mod file from an io.Reader and returns a map of dependency name to their specified versions.
+func dependencies(reader io.Reader) (map[string]string, error) {
+	buf := bufio.NewReader(reader)
+
+	deps := make(map[string]string)
+	for {
+		line, err := buf.ReadString('\n')
+		if errors.Is(err, io.EOF) {
+			return deps, nil
+		}
+
+		if err != nil {
+			return nil, fmt.Errorf("reading go.mod: %w", err)
+		}
+
+		if !strings.HasPrefix(line, "\t") {
+			continue
+		}
+
+		line = strings.Trim(line, "\t\n")
+		depVer := strings.Split(line, " ")
+		deps[depVer[0]] = depVer[1]
+	}
+}