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

[Nick Rosbrook <rosbrookn@xxxxxxxxx>]

On Fri, Jun 18, 2021 at 04:18:44PM +0000, George Dunlap wrote:
> 
> 
> > On Jun 18, 2021, at 4:08 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> wrote:
> > 
> > On Fri, Jun 18, 2021 at 02:44:15PM +0000, George Dunlap wrote:
> >> 
> >> 
> >>> 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?
> > 
> > Part of the motivation for using functional options is to abstract the
> > "options" struct, yes. This allows internal defaults to be applied more
> > easily -- if you require e.g. a ContextOptions struct to be passed by
> > the caller, how do you know if they intended to override a default, or
> > if they just didn't set the field? Additionally, using the ContextOption
> > as an interface allows variadic arguments, which are just convenient for
> > API users -- the same NewContext function can be used whether you need
> > to pass 3 options or 0.
> > 
> > The reason we use ContextOption as an interface, rather than function
> > pointer of sorts is for flexibility in the signatures of ContextOption
> > implementations. E.g., we could have
> > 
> > func WithLogLevel(lvl LogLevel) ContextOption
> > func WithLogContext(s string) ContextOption
> > func WithFooAndBar(s string, n int) ContextOption
> > 
> > See [1] for more background on this pattern.
> > 
> > Thanks,
> > NR
> > 
> > [1] https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
> 
> Yes, I frequently use a pattern like the one described in that blog post 
> myself.  But that blog post doesn’t use interfaces — the final slide actually 
> has the “option function” type as an open-coded function pointer type.
> 
> So my question was, why not do something like this:
> 
> type ContextOption func(*contextOptions) error
> 
> func WithLogLevel(level LogLevel) ContextOption {
>   return func(co *contextOptions) {
>     co.logLevel = level
>   }
> }
> 
> ATM the only advantage I can see of defining ContextOption as an interface 
> rather than as a function pointer is that the godoc for ContextOption would 
> look like:
> 
> type ContextOption interface {
>    // contains filtered or unexported fields
> }
> 
> Rather than
> 
> type ContextOption func(*contextOptions) error
> 
> Which shows you the name of the unexported field.
> 
> Is there another reason I missed?

Technically it does allow more flexibility in implementing
ContextOption, e.g. you could do...

func (lvl LogLevel) apply(co *contextOptions) { co.logLevel = lvl }

...and then pass a LogLevel directly as a ContextOption. But generally
everyone implements these things as funcs.

I will admit that when it comes to my choice of using the interface
version instead of function pointers, I am just more familiar with the
former and encounter it more often in other Go packages I use.

Thanks,
NR