Re: [RESEND PATCH 08/12] golang/xenlight: add functional options to configure Context

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> wrote:
> 
> Add a ContextOption type to support functional options in NewContext.
> Then, add a variadic ContextOption parameter to NewContext, which allows
> callers to specify 0 or more configuration options.
> 
> For now, just add the WithLogLevel option so that callers can set the
> log level of the Context's xentoollog_logger. Future configuration
> options can be created by adding an appropriate field to the
> contextOptions struct and creating a With<OptionName> function to return
> a ContextOption
> 
> Signed-off-by: Nick Rosbrook <rosbrookn@xxxxxxxxxxxx>
> ---
> tools/golang/xenlight/xenlight.go | 44 +++++++++++++++++++++++++++++--
> 1 file changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/golang/xenlight/xenlight.go 
> b/tools/golang/xenlight/xenlight.go
> index f68d7b6e97..65f93abe32 100644
> --- a/tools/golang/xenlight/xenlight.go
> +++ b/tools/golang/xenlight/xenlight.go
> @@ -136,7 +136,7 @@ func sigchldHandler(ctx *Context) {
> }
> 
> // NewContext returns a new Context.
> -func NewContext() (ctx *Context, err error) {
> +func NewContext(opts ...ContextOption) (ctx *Context, err error) {
>       ctx = &Context{}
> 
>       defer func() {
> @@ -146,8 +146,19 @@ func NewContext() (ctx *Context, err error) {
>               }
>       }()
> 
> +     // Set the default context options. These fields may
> +     // be modified by the provided opts.
> +     copts := &contextOptions{
> +             logLevel: LogLevelError,
> +     }
> +
> +     for _, opt := range opts {
> +             opt.apply(copts)
> +     }
> +
>       // Create a logger
> -     ctx.logger = C.xtl_createlogger_stdiostream(C.stderr, C.XTL_ERROR, 0)
> +     ctx.logger = C.xtl_createlogger_stdiostream(C.stderr,
> +             C.xentoollog_level(copts.logLevel), 0)
> 
>       // Allocate a context
>       ret := C.libxl_ctx_alloc(&ctx.ctx, C.LIBXL_VERSION, 0,
> @@ -201,6 +212,35 @@ func (ctx *Context) Close() error {
>       return nil
> }
> 
> +type contextOptions struct {
> +     logLevel LogLevel
> +}
> +
> +// ContextOption is used to configure options for a Context.
> +type ContextOption interface {
> +     apply(*contextOptions)
> +}
> +
> +type funcContextOption struct {
> +     f func(*contextOptions)
> +}
> +
> +func (fco *funcContextOption) apply(c *contextOptions) {
> +     fco.f(c)
> +}

Why all this convolution with interfaces and such, rather than just defining 
ContextOption as a function pointer?  Is it just to keep contextOptions out of 
the documentation page?

 -George