func sandboxBuild()

in sandbox.go [407:525]
96 lines of code
25 McCabe index (conditional complexity)

func sandboxBuild(ctx context.Context, tmpDir string, in []byte, vet bool) (br *buildResult, err error) {
	start := time.Now()
	defer func() {
		status := "success"
		if err != nil {
			status = "error"
		}
		// Ignore error. The only error can be invalid tag key or value
		// length, which we know are safe.
		stats.RecordWithTags(ctx, []tag.Mutator{tag.Upsert(kGoBuildSuccess, status)},
			mGoBuildLatency.M(float64(time.Since(start))/float64(time.Millisecond)))
	}()

	files, err := splitFiles(in)
	if err != nil {
		return &buildResult{errorMessage: err.Error()}, nil
	}

	br = new(buildResult)
	defer br.cleanup()
	var buildPkgArg = "."
	if files.Num() == 1 && len(files.Data(progName)) > 0 {
		buildPkgArg = progName
		src := files.Data(progName)
		if code := getTestProg(src); code != nil {
			br.testParam = "-test.v"
			files.AddFile(progName, code)
		}
	}

	if !files.Contains("go.mod") {
		files.AddFile("go.mod", []byte("module play\n"))
	}

	for f, src := range files.m {
		// Before multi-file support we required that the
		// program be in package main, so continue to do that
		// for now. But permit anything in subdirectories to have other
		// packages.
		if !strings.Contains(f, "/") {
			fset := token.NewFileSet()
			f, err := parser.ParseFile(fset, f, src, parser.PackageClauseOnly)
			if err == nil && f.Name.Name != "main" {
				return &buildResult{errorMessage: "package name must be main"}, nil
			}
		}

		in := filepath.Join(tmpDir, f)
		if strings.Contains(f, "/") {
			if err := os.MkdirAll(filepath.Dir(in), 0755); err != nil {
				return nil, err
			}
		}
		if err := ioutil.WriteFile(in, src, 0644); err != nil {
			return nil, fmt.Errorf("error creating temp file %q: %v", in, err)
		}
	}

	br.exePath = filepath.Join(tmpDir, "a.out")
	goCache := filepath.Join(tmpDir, "gocache")

	cmd := exec.Command("/usr/local/go-faketime/bin/go", "build", "-o", br.exePath, "-tags=faketime")
	cmd.Dir = tmpDir
	cmd.Env = []string{"GOOS=linux", "GOARCH=amd64", "GOROOT=/usr/local/go-faketime"}
	cmd.Env = append(cmd.Env, "GOCACHE="+goCache)
	cmd.Env = append(cmd.Env, "CGO_ENABLED=0")
	// Create a GOPATH just for modules to be downloaded
	// into GOPATH/pkg/mod.
	cmd.Args = append(cmd.Args, "-modcacherw")
	cmd.Args = append(cmd.Args, "-mod=mod")
	br.goPath, err = ioutil.TempDir("", "gopath")
	if err != nil {
		log.Printf("error creating temp directory: %v", err)
		return nil, fmt.Errorf("error creating temp directory: %v", err)
	}
	cmd.Env = append(cmd.Env, "GO111MODULE=on", "GOPROXY="+playgroundGoproxy())
	cmd.Args = append(cmd.Args, buildPkgArg)
	cmd.Env = append(cmd.Env, "GOPATH="+br.goPath)
	out := &bytes.Buffer{}
	cmd.Stderr, cmd.Stdout = out, out

	if err := cmd.Start(); err != nil {
		return nil, fmt.Errorf("error starting go build: %v", err)
	}
	ctx, cancel := context.WithTimeout(ctx, maxBuildTime)
	defer cancel()
	if err := internal.WaitOrStop(ctx, cmd, os.Interrupt, 250*time.Millisecond); err != nil {
		if errors.Is(err, context.DeadlineExceeded) {
			br.errorMessage = fmt.Sprintln(goBuildTimeoutError)
		} else if ee := (*exec.ExitError)(nil); !errors.As(err, &ee) {
			log.Printf("error building program: %v", err)
			return nil, fmt.Errorf("error building go source: %v", err)
		}
		// Return compile errors to the user.
		// Rewrite compiler errors to strip the tmpDir name.
		br.errorMessage = br.errorMessage + strings.Replace(string(out.Bytes()), tmpDir+"/", "", -1)

		// "go build", invoked with a file name, puts this odd
		// message before any compile errors; strip it.
		br.errorMessage = strings.Replace(br.errorMessage, "# command-line-arguments\n", "", 1)

		return br, nil
	}
	const maxBinarySize = 100 << 20 // copied from sandbox backend; TODO: unify?
	if fi, err := os.Stat(br.exePath); err != nil || fi.Size() == 0 || fi.Size() > maxBinarySize {
		if err != nil {
			return nil, fmt.Errorf("failed to stat binary: %v", err)
		}
		return nil, fmt.Errorf("invalid binary size %d", fi.Size())
	}
	if vet {
		// TODO: do this concurrently with the execution to reduce latency.
		br.vetOut, err = vetCheckInDir(ctx, tmpDir, br.goPath)
		if err != nil {
			return nil, fmt.Errorf("running vet: %v", err)
		}
	}
	return br, nil
}