1 files changed, 104 insertions, 0 deletions
diff --git a/internal/api/util/negotiate.go b/internal/api/util/negotiate.go
new file mode 100644
index 000000000..2854ea09c
--- /dev/null
+++ b/internal/api/util/negotiate.go
@@ -0,0 +1,104 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 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 util
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/gin-gonic/gin"
+)
+
+// ActivityPubAcceptHeaders represents the Accept headers mentioned here:
+var ActivityPubAcceptHeaders = []MIME{
+	AppActivityJSON,
+	AppActivityLDJSON,
+}
+
+// JSONAcceptHeaders is a slice of offers that just contains application/json types.
+var JSONAcceptHeaders = []MIME{
+	AppJSON,
+}
+
+// HTMLOrJSONAcceptHeaders is a slice of offers that prefers TextHTML and will
+// fall back to JSON if necessary. This is useful for error handling, since it can
+// be used to serve a nice HTML page if the caller accepts that, or just JSON if not.
+var HTMLOrJSONAcceptHeaders = []MIME{
+	TextHTML,
+	AppJSON,
+}
+
+// HTMLAcceptHeaders is a slice of offers that just contains text/html types.
+var HTMLAcceptHeaders = []MIME{
+	TextHTML,
+}
+
+// HTMLOrActivityPubHeaders matches text/html first, then activitypub types.
+// This is useful for user URLs that a user might go to in their browser.
+// https://www.w3.org/TR/activitypub/#retrieving-objects
+var HTMLOrActivityPubHeaders = []MIME{
+	TextHTML,
+	AppActivityJSON,
+	AppActivityLDJSON,
+}
+
+// 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 ...MIME) (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
+}