API to store Creds in net/http UNIX socket request context

[mdlayher]

After a couple of iterations, I came up with mdlayher/zedhook@333d2b6 for my HTTP server which accepts incoming requests via UNIX socket.

Would it make sense to incorporate a version of this in this package? Roughly:

package peercred

// A contextKey is an opaque structure used as a key for context.Context values.
type contextKey struct{ name string }

// ContextKey stores *Creds in a context.Context Value.
var ContextKey = &contextKey{"peercred"}

// ConnContext provides a hook to store *Creds for a UNIX socket client in the
// context for a *http.Request. Use this function with *http.Server's ConnContext field.
//
// TODO: better name?
func ConnContext(ctx context.Context, c net.Conn) context.Context {
    // Best effort; we don't know if net.Conn is *net.UnixConn.
    if creds, err := peercred.Get(c); err == nil {
        ctx = context.WithValue(ctx, ContextKey, creds)
    }
    return ctx
}

Usage:

srv := &http.Server{ConnContext: peercred.ConnContext}

func (h *handler) handle(w http.ResponseWriter, r *http.Request) {
    creds, ok := r.Context().Value(peercred.ContextKey).(*peercred.Cred)
    // If ok, use creds.
}

/cc @bradfitz