ssh: add ServerConfig.NoClientAuthCallback

It was possible to accept auth type "none" before, but not dynamically
at runtime as a function of the ConnMetadata like the other auth types'
callback hooks.

Fixes golang/go#51994

Change-Id: I83ea80901d4977d8f78523e3d1e16e0a7df5b172
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/395314
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Julie Qiu <julieqiu@google.com>

ssh/session_test.go

@@ -779,3 +779,54 @@ func TestHostKeyAlgorithms(t *testing.T) {
 		t.Fatal("succeeded connecting with unknown hostkey algorithm")
 	}
 }
+
+func TestServerClientAuthCallback(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	userCh := make(chan string, 1)
+
+	serverConf := &ServerConfig{
+		NoClientAuth: true,
+		NoClientAuthCallback: func(conn ConnMetadata) (*Permissions, error) {
+			userCh <- conn.User()
+			return nil, nil
+		},
+	}
+	const someUsername = "some-username"
+
+	serverConf.AddHostKey(testSigners["ecdsa"])
+	clientConf := &ClientConfig{
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		User:            someUsername,
+	}
+
+	go func() {
+		_, chans, reqs, err := NewServerConn(c1, serverConf)
+		if err != nil {
+			t.Errorf("server handshake: %v", err)
+			userCh <- "error"
+			return
+		}
+		go DiscardRequests(reqs)
+		for ch := range chans {
+			ch.Reject(Prohibited, "")
+		}
+	}()
+
+	conn, _, _, err := NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client handshake: %v", err)
+		return
+	}
+	conn.Close()
+
+	got := <-userCh
+	if got != someUsername {
+		t.Errorf("username = %q; want %q", got, someUsername)
+	}
+}