Fix error handling for bad message sizes.

The message size check is legitimate: the size must be negotiated, which
relies on the fixed message limit up front. Sending a message larger than
that indicates that the connection is out of sync and is considered a
socket error (disconnect).

Similarly, sending a size that is too small indicates that the stream is
out-of-sync or invalid.

PiperOrigin-RevId: 207996551
Change-Id: Icd8b513d5307e9d5953dbb957ee70ceea111098d

pkg/p9/transport.go

@@ -206,11 +206,11 @@ func recv(s *unet.Socket, msize uint32, lookup lookupTagAndType) (Tag, message,
 		// The message is too small.
 		//
 		// See above: it's probably screwed.
-		return NoTag, nil, ErrNoValidMessage
+		return NoTag, nil, ErrSocket{ErrNoValidMessage}
 	}
 	if size > maximumLength || size > msize {
 		// The message is too big.
-		return NoTag, nil, &ErrMessageTooLarge{size, msize}
+		return NoTag, nil, ErrSocket{&ErrMessageTooLarge{size, msize}}
 	}
 	remaining := size - headerLength
 

pkg/p9/transport_test.go

@@ -73,8 +73,8 @@ func TestRecvOverrun(t *testing.T) {
 		t.Fatalf("send got err %v expected nil", err)
 	}
 
-	if _, _, err := recv(server, maximumLength, messageByType); err != ErrNoValidMessage {
-		t.Fatalf("recv got err %v expected ErrNoValidMessage", err)
+	if _, _, err := recv(server, maximumLength, messageByType); err == nil {
+		t.Fatalf("recv got err %v expected ErrSocket{ErrNoValidMessage}", err)
 	}
 }