Re: [RESEND PATCH 07/12] golang/xenlight: add logging conveniences for within xenlight

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On May 24, 2021, at 9:36 PM, Nick Rosbrook <rosbrookn@xxxxxxxxx> wrote:
> 
> Add some logging methods to Context to provide easy use of the
> Contenxt's xentoollog_logger. These are not exported, but the LogLevel
> type is so that a later commit can allow the Context's log level to be
> configurable.
> 
> Becuase cgo does not support calling C functions with variable
> arguments, e.g. xtl_log, add an xtl_log_wrap function to the cgo preamble
> that accepts an already formatted string, and handle the formatting in
> Go.
> 
> Signed-off-by: Nick Rosbrook <rosbrookn@xxxxxxxxxxxx>

Looks good.  One comment:

> ---
> tools/golang/xenlight/xenlight.go | 45 +++++++++++++++++++++++++++++++
> 1 file changed, 45 insertions(+)
> 
> diff --git a/tools/golang/xenlight/xenlight.go 
> b/tools/golang/xenlight/xenlight.go
> index fc3eb0bf3f..f68d7b6e97 100644
> --- a/tools/golang/xenlight/xenlight.go
> +++ b/tools/golang/xenlight/xenlight.go
> @@ -32,6 +32,15 @@ static const libxl_childproc_hooks childproc_hooks = { 
> .chldowner = libxl_sigchl
> void xenlight_set_chldproc(libxl_ctx *ctx) {
>       libxl_childproc_setmode(ctx, &childproc_hooks, NULL);
> }
> +
> +void xtl_log_wrap(struct xentoollog_logger *logger,
> +               xentoollog_level level,
> +               int errnoval,
> +               const char *context,
> +               const char *msg)
> +{
> +    xtl_log(logger, level, errnoval, context, "%s", msg);
> +}
> */
> import "C"
> 
> @@ -192,6 +201,42 @@ func (ctx *Context) Close() error {
>       return nil
> }
> 
> +// LogLevel represents an xentoollog_level, and can be used to configre the 
> log
> +// level of a Context's logger.
> +type LogLevel int
> +
> +const (
> +     //LogLevelNone     LogLevel = C.XTL_NONE

Why are we not defining this one?  Don’t we want to be able to disable logging 
entirely?

> +     LogLevelDebug    LogLevel = C.XTL_DEBUG
> +     LogLevelVerbose  LogLevel = C.XTL_VERBOSE
> +     LogLevelDetail   LogLevel = C.XTL_DETAIL
> +     LogLevelProgress LogLevel = C.XTL_PROGRESS
> +     LogLevelInfo     LogLevel = C.XTL_INFO
> +     LogLevelNotice   LogLevel = C.XTL_NOTICE
> +     LogLevelWarn     LogLevel = C.XTL_WARN
> +     LogLevelError    LogLevel = C.XTL_ERROR
> +     LogLevelCritical LogLevel = C.XTL_CRITICAL
> +     //LogLevelNumLevels LogLevel = C.XTL_NUM_LEVELS
> +)
> +
> +func (ctx *Context) log(lvl LogLevel, errnoval int, format string, a 
> ...interface{}) {
> +     msg := C.CString(fmt.Sprintf(format, a...))
> +     defer C.free(unsafe.Pointer(msg))
> +     context := C.CString("xenlight")
> +     defer C.free(unsafe.Pointer(context))

Hmm, allocating and freeing a fixed string every time seems pretty wasteful.  
Would it make more sense to either use a static C string in the CGo code at the 
top instead?  Or if not, to make context a global variable we allocate once at 
the package level and re-use?

Also, is ‘xenlight’ informative enough?  I haven’t looked at the other 
“context” strings; would “go-xenlight” or something be better?

> +
> +     C.xtl_log_wrap((*C.xentoollog_logger)(unsafe.Pointer(ctx.logger)),
> +             C.xentoollog_level(lvl), C.int(errnoval), context, msg)
> +}

I think we want to make it possible long-term to configure your own logger or 
have no logger at all; so maybe we should add a `if (ctx.logger == nil) 
return;` at then top?

Other than that looks good, thanks!

 -George