gotosocial/internal/api/negotiate.go

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/*
GoToSocial
Copyright (C) 2021 GoToSocial Authors admin@gotosocial.org
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package api
import (
"errors"
"fmt"
"github.com/gin-gonic/gin"
)
// Offer represents an offered mime-type.
type Offer string
const (
AppJSON Offer = `application/json` // AppJSON is the mime type for 'application/json'.
AppActivityJSON Offer = `application/activity+json` // AppActivityJSON is the mime type for 'application/activity+json'.
AppActivityLDJSON Offer = `application/ld+json; profile="https://www.w3.org/ns/activitystreams"` // AppActivityLDJSON is the mime type for 'application/ld+json; profile="https://www.w3.org/ns/activitystreams"'
TextHTML Offer = `text/html` // TextHTML is the mime type for 'text/html'.
)
// ActivityPubAcceptHeaders represents the Accept headers mentioned here:
// https://www.w3.org/TR/activitypub/#retrieving-objects
var ActivityPubAcceptHeaders = []Offer{
AppActivityJSON,
AppActivityLDJSON,
}
// JSONAcceptHeaders is a slice of offers that just contains application/json types.
var JSONAcceptHeaders = []Offer{
AppJSON,
}
// HTMLAcceptHeaders is a slice of offers that just contains text/html types.
var HTMLAcceptHeaders = []Offer{
TextHTML,
}
// NegotiateAccept takes the *gin.Context from an incoming request, and a
// slice of Offers, and performs content negotiation for the given request
// with the given content-type offers. It will return a string representation
// of the first suitable content-type, or an error if something goes wrong or
// a suitable content-type cannot be matched.
//
// For example, if the request in the *gin.Context has Accept headers of value
// [application/json, text/html], and the provided offers are of value
// [application/json, application/xml], then the returned string will be
// 'application/json', which indicates the content-type that should be returned.
//
// If the length of offers is 0, then an error will be returned, so this function
// should only be called in places where format negotiation is actually needed.
//
// If there are no Accept headers in the request, then the first offer will be returned,
// under the assumption that it's better to serve *something* than error out completely.
//
// Callers can use the offer slices exported in this package as shortcuts for
// often-used Accept types.
//
// See https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#server-driven_content_negotiation
func NegotiateAccept(c *gin.Context, offers ...Offer) (string, error) {
if len(offers) == 0 {
return "", errors.New("no format offered")
}
strings := []string{}
for _, o := range offers {
strings = append(strings, string(o))
}
accepts := c.Request.Header.Values("Accept")
if len(accepts) == 0 {
// there's no accept header set, just return the first offer
return strings[0], nil
}
format := c.NegotiateFormat(strings...)
if format == "" {
return "", fmt.Errorf("no format can be offered for requested Accept header(s) %s; this endpoint offers %s", accepts, offers)
}
return format, nil
}