func buildFn()

in cmd/go/legacy_worker/main.go [66:128]
53 lines of code
13 McCabe index (conditional complexity)

func buildFn(ctx *gcp.Context) error {
	l, err := ctx.Layer(layerName, gcp.LaunchLayer)
	if err != nil {
		return fmt.Errorf("creating %v layer: %w", layerName, err)
	}
	if err := ctx.SetFunctionsEnvVars(l); err != nil {
		return err
	}
	ctx.AddWebProcess([]string{golang.OutBin})

	fnTarget := os.Getenv(env.FunctionTarget)

	// Move the function source code into a subdirectory in order to construct the app in the main application root.
	if err := ctx.RemoveAll(fnSourceDir); err != nil {
		return err
	}
	if err := ctx.MkdirAll(fnSourceDir, 0755); err != nil {
		return err
	}
	// mindepth=1 excludes '.', '+' collects all file names before running the command.
	// Exclude serverless_function_source_code and .google* dir e.g. .googlebuild, .googleconfig
	command := fmt.Sprintf("find . -mindepth 1 -not -name %[1]s -prune -not -name %[2]q -prune -exec mv -t %[1]s {} +", fnSourceDir, ".google*")
	if _, err := ctx.Exec([]string{"bash", "-c", command}, gcp.WithUserTimingAttribution); err != nil {
		return err
	}

	fnSource := filepath.Join(ctx.ApplicationRoot(), fnSourceDir)
	pkgName, err := extractPackageNameInDir(ctx, fnSource)
	if err != nil {
		return fmt.Errorf("extracting package name: %w", err)
	}
	fn := fnInfo{
		Source:  fnSource,
		Target:  fnTarget,
		Package: pkgName,
	}

	l.LaunchEnvironment.Default("X_GOOGLE_ENTRY_POINT", os.Getenv(env.FunctionTarget))
	triggerType := os.Getenv(env.FunctionSignatureType)
	if triggerType == "http" || triggerType == "" {
		triggerType = "HTTP_TRIGGER"
	}
	l.LaunchEnvironment.Default("X_GOOGLE_FUNCTION_TRIGGER_TYPE", triggerType)

	goMod := filepath.Join(fn.Source, "go.mod")
	goModExists, err := ctx.FileExists(goMod)
	if err != nil {
		return err
	}
	if !goModExists {
		return createMainVendored(ctx, fn)
	}
	isWriteable, err := ctx.IsWritable(goMod)
	if err != nil {
		return err
	}
	if !isWriteable {
		// Preempt an obscure failure mode: if go.mod is not writable then `go list -m` can fail saying:
		//     go: updates to go.sum needed, disabled by -mod=readonly
		return gcp.UserErrorf("go.mod exists but is not writable")
	}
	return createMainGoMod(ctx, fn)
}