Handle guest access [1/2?] (#2872)

Needs https://github.com/matrix-org/sytest/pull/1315, as otherwise the
membership events aren't persisted yet when hitting `/state` after
kicking guest users.

Makes the following tests pass:
```
Guest users denied access over federation if guest access prohibited
Guest users are kicked from guest_access rooms on revocation of guest_access
Guest users are kicked from guest_access rooms on revocation of guest_access over federation
```

Todo (in a follow up PR):
- Restrict access to CS API Endpoints as per
https://spec.matrix.org/v1.4/client-server-api/#client-behaviour-14

Co-authored-by: kegsay <kegan@matrix.org>

setup/config/config_global.go

@@ -174,7 +174,7 @@ func (c *Global) SigningIdentityFor(serverName gomatrixserverlib.ServerName) (*g
 			return id, nil
 		}
 	}
-	return nil, fmt.Errorf("no signing identity %q", serverName)
+	return nil, fmt.Errorf("no signing identity for %q", serverName)
 }
 
 func (c *Global) SigningIdentities() []*gomatrixserverlib.SigningIdentity {

setup/config/config_test.go

@@ -16,8 +16,10 @@ package config
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
 
+	"github.com/matrix-org/gomatrixserverlib"
 	"gopkg.in/yaml.v2"
 )
 
@@ -290,3 +292,55 @@ func TestUnmarshalDataUnit(t *testing.T) {
 		}
 	}
 }
+
+func Test_SigningIdentityFor(t *testing.T) {
+	tests := []struct {
+		name         string
+		virtualHosts []*VirtualHost
+		serverName   gomatrixserverlib.ServerName
+		want         *gomatrixserverlib.SigningIdentity
+		wantErr      bool
+	}{
+		{
+			name:    "no virtual hosts defined",
+			wantErr: true,
+		},
+		{
+			name:       "no identity found",
+			serverName: gomatrixserverlib.ServerName("doesnotexist"),
+			wantErr:    true,
+		},
+		{
+			name:       "found identity",
+			serverName: gomatrixserverlib.ServerName("main"),
+			want:       &gomatrixserverlib.SigningIdentity{ServerName: "main"},
+		},
+		{
+			name:       "identity found on virtual hosts",
+			serverName: gomatrixserverlib.ServerName("vh2"),
+			virtualHosts: []*VirtualHost{
+				{SigningIdentity: gomatrixserverlib.SigningIdentity{ServerName: "vh1"}},
+				{SigningIdentity: gomatrixserverlib.SigningIdentity{ServerName: "vh2"}},
+			},
+			want: &gomatrixserverlib.SigningIdentity{ServerName: "vh2"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := &Global{
+				VirtualHosts: tt.virtualHosts,
+				SigningIdentity: gomatrixserverlib.SigningIdentity{
+					ServerName: "main",
+				},
+			}
+			got, err := c.SigningIdentityFor(tt.serverName)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("SigningIdentityFor() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("SigningIdentityFor() got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}