Add auth package

- Extract the access token from the HTTP request

src/github.com/matrix-org/dendrite/clientapi/auth/auth.go

@@ -0,0 +1,43 @@
+package auth
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+// VerifyAccessToken verifies that an access token was supplied in the given HTTP request
+// and returns the user ID it corresponds to. Returns an error if there is no access token
+// or the token is invalid.
+func VerifyAccessToken(req *http.Request) (userID string, err error) {
+	_, tokenErr := extractAccessToken(req)
+	if tokenErr != nil {
+		// err = MatrixError(MatrixError.M_MISSING_TOKEN, tokenErr.Error())
+		return
+	}
+	// TODO: Do something with the token
+	return
+}
+
+// extractAccessToken from a request, or return an error detailing what went wrong.
+func extractAccessToken(req *http.Request) (string, error) {
+	authBearer := req.Header.Get("Authorization")
+	queryToken := req.URL.Query().Get("access_token")
+	if authBearer != "" && queryToken != "" {
+		return "", fmt.Errorf("mixing Authorization headers and access_token query parameters")
+	}
+
+	if queryToken != "" {
+		return queryToken, nil
+	}
+
+	if authBearer != "" {
+		parts := strings.SplitN(authBearer, " ", 2)
+		if len(parts) != 2 || parts[0] != "Bearer" {
+			return "", fmt.Errorf("invalid Authorization header")
+		}
+		return parts[1], nil
+	}
+
+	return "", fmt.Errorf("missing access token")
+}