servitor/client/client.go

package client

import (
	"encoding/json"
	"errors"
	"fmt"
	"golang.org/x/sync/singleflight"
	"servitor/jtp"
	"servitor/object"
	"net/url"
	"os"
	"strings"
)

const MAX_REDIRECTS = 20

func FetchUnknown(input any, source *url.URL) (object.Object, *url.URL, error) {
	var obj object.Object
	switch narrowed := input.(type) {
	case string:
		ref, err := url.Parse(narrowed)
		if err != nil {
			return nil, nil, err
		}
		if source != nil {
			obj, source, err = FetchURL(source.ResolveReference(ref))
		} else {
			obj, source, err = FetchURL(ref)
		}
		if err != nil {
			return nil, nil, err
		}
	case map[string]any:
		obj = object.Object(narrowed)
	default:
		return nil, nil, fmt.Errorf("can't turn non-string, non-object %T into Item", input)
	}

	id, err := obj.GetURL("id")
	if errors.Is(err, object.ErrKeyNotPresent) {
		id = nil
	} else if err != nil {
		return nil, nil, err
	}
	/* Refetch if necessary */
	if id != nil && (source == nil || source.Host != id.Host || len(obj) <= 2) {
		obj, source, err = FetchURL(id)
		if err != nil {
			return nil, nil, err
		}
		/* Verify that now the id matches the source it came from */
		id, err = obj.GetURL("id")
		if errors.Is(err, object.ErrKeyNotPresent) {
			id = nil
		} else if err != nil {
			return nil, nil, err
		}
		if id != nil && source.Host != id.Host {
			return nil, nil, errors.New("received response with forged identifier")
		}
	}

	return obj, id, nil
}

var group singleflight.Group

type bundle struct {
	item   map[string]any
	source *url.URL
	err    error
}

/* A map of mutexes is used to ensure no two requests are made simultaneously.
   Instead, the subsequent ones will wait for the first one to finish (and will
   then naturally find its result in the cache) */

func FetchURL(uri *url.URL) (object.Object, *url.URL, error) {
	uriString := uri.String()
	b, _, _ := group.Do(uriString, func() (any, error) {
		json, source, err :=
			jtp.Get(
				uri,
				`application/activity+json,`+
					`application/ld+json; profile="https://www.w3.org/ns/activitystreams"`,
				[]string{
					"application/activity+json",
					"application/ld+json",
					"application/json",
				},
				MAX_REDIRECTS,
			)
		return bundle{
			item:   json,
			source: source,
			err:    err,
		}, nil
	})
	/* By this point the result has been cached in the LRU cache,
	   so it can be dropped from the singleflight cache */
	group.Forget(uriString)
	return b.(bundle).item, b.(bundle).source, b.(bundle).err
}

/*
converts a webfinger identifier to a url
see: https://datatracker.ietf.org/doc/html/rfc7033
*/
func ResolveWebfinger(username string) (string, error) {
	split := strings.SplitN(username, "@", 2)
	var account, domain string
	if len(split) != 2 {
		return "", errors.New("webfinger address must have a separating @ symbol")
	}
	account = split[0]
	domain = split[1]

	link := &url.URL{
		Scheme: "https",
		Host:   domain,
		Path:   "/.well-known/webfinger",
		RawQuery: (url.Values{
			"resource": []string{"acct:" + account + "@" + domain},
		}).Encode(),
	}

	json, _, err := jtp.Get(link, "application/jrd+json", []string{
		"application/jrd+json",
		"application/json",
	}, MAX_REDIRECTS)
	if err != nil {
		return "", err
	}
	response := object.Object(json)

	jrdLinks, err := response.GetList("links")
	if err != nil {
		return "", err
	}

	found := false
	var underlyingLink string

	for _, el := range jrdLinks {
		asMap, ok := el.(map[string]any)
		o := object.Object(asMap)
		if ok {
			rel, err := o.GetString("rel")
			if err != nil {
				return "", err
			}
			if rel != "self" {
				continue
			}
			mediaType, err := o.GetMediaType("type")
			if errors.Is(err, object.ErrKeyNotPresent) {
				continue
			} else if err != nil {
				return "", err
			}
			if !mediaType.Matches([]string{
				"application/activity+json",
				"application/ld+json",
			}) {
				continue
			}
			href, err := o.GetString("href")
			if err != nil {
				return "", err
			}
			found = true
			underlyingLink = href
			break
		} else {
			return "", fmt.Errorf("unrecognized type %T found in webfinger response", el)
		}
	}

	if !found {
		return "", errors.New("actor not found in webfinger listing")
	}

	return underlyingLink, nil
}

func FetchFromFile(name string) (object.Object, error) {
	file, err := os.Open(name)
	if err != nil {
		return nil, err
	}
	var obj object.Object
	json.NewDecoder(file).Decode(&obj)
	return obj, nil
}