[bugfix] Fix NegotiateFormat with multiple accept headers (#1797)

* [bugfix] Fix NegotiateAccept with multi accept

There's a bug in Gin's NegotiateFormat that doesn't handle the presence
of multilpe accept headers. This lifts the code from the PR @tsmethurst
sent a year ago to Gin into our codebase to fix the issue.

* [bugfix] Concat accept header in webfinger

Some implementations bug out when there's multiple accept headers,
including Gin (see 7050112af1). But things
seem to work reliably with a single accept header with multiple parts.

Fixes: #1793

internal/api/util/negotiate.go

@@ -20,6 +20,7 @@ package util
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/gin-gonic/gin"
 )
@@ -108,10 +109,63 @@ func NegotiateAccept(c *gin.Context, offers ...MIME) (string, error) {
 		return strings[0], nil
 	}
 
-	format := c.NegotiateFormat(strings...)
+	format := NegotiateFormat(c, 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
 }
+
+// This is the exact same thing as gin.Context.NegotiateFormat except it contains
+// tsmethurst's fix to make it work properly with multiple accept headers.
+//
+// https://github.com/gin-gonic/gin/pull/3156
+func NegotiateFormat(c *gin.Context, offered ...string) string {
+	if len(offered) == 0 {
+		panic("you must provide at least one offer")
+	}
+
+	if c.Accepted == nil {
+		for _, a := range c.Request.Header.Values("Accept") {
+			c.Accepted = append(c.Accepted, parseAccept(a)...)
+		}
+	}
+	if len(c.Accepted) == 0 {
+		return offered[0]
+	}
+	for _, accepted := range c.Accepted {
+		for _, offer := range offered {
+			// According to RFC 2616 and RFC 2396, non-ASCII characters are not allowed in headers,
+			// therefore we can just iterate over the string without casting it into []rune
+			i := 0
+			for ; i < len(accepted); i++ {
+				if accepted[i] == '*' || offer[i] == '*' {
+					return offer
+				}
+				if accepted[i] != offer[i] {
+					break
+				}
+			}
+			if i == len(accepted) {
+				return offer
+			}
+		}
+	}
+	return ""
+}
+
+// https://github.com/gin-gonic/gin/blob/4787b8203b79012877ac98d7806422da3a678ba2/utils.go#L103
+func parseAccept(acceptHeader string) []string {
+	parts := strings.Split(acceptHeader, ",")
+	out := make([]string, 0, len(parts))
+	for _, part := range parts {
+		if i := strings.IndexByte(part, ';'); i > 0 {
+			part = part[:i]
+		}
+		if part = strings.TrimSpace(part); part != "" {
+			out = append(out, part)
+		}
+	}
+	return out
+}