Option to specify context when creating file and location objects

[NathanBaulch]

Something I find lacking in this library is the ability to trace outbound requests. It's possible to configure instrumented clients for most backends but common tracing tools like OpenTelemetry use context to pass state around.

Rather than breaking every API by adding context.Context arguments (as previously discussed), I propose the gs.WithContext pattern be applied more widely to NewLocation and NewFile. This would allow a web application to have a single shared FileSystem (or Location) and inject the current request context when working with specific files.

Here's a minimal example:

ctx := context.Background()
cfg, _ := config.LoadDefaultConfig(ctx)
cli := _s3.NewFromConfig(cfg, func(opts *_s3.Options) {
	// instrument S3 client
	opts.HTTPClient = &http.Client{Transport: otelhttp.NewTransport(http.DefaultTransport)}
})
// pass default global context to fs
fs := s3.NewFileSystem(s3.WithClient(cli), s3.WithContext(ctx))
backend.Register(fs.Name(), fs)

http.HandleFunc("/version", func(w http.ResponseWriter, r *http.Request) {
	// pass request context to file
	f, _ := fs.NewFile("", "version.txt", newfile.WithContext(r.Context()))
	c, _ := io.ReadAll(f)
	_, _ = fmt.Fprintf(w, "version: "+string(c))
})

_ = http.ListenAndServe(":8080", nil)

The other nice thing about this approach is it doesn't pollute the API for backends that don't support context, like mem, os and sftp.

This is a non-breaking additive change so in theory it could be included in v7, but I understand if it needs to wait for v8.

[funkyshu]

I'm still thinking this through but I think the s3 example suggestions make sense.