rpc: avoid a panic in delayingConn

If the remote server shuts down while the connection is established,
the delaying conn may not complete its handshake. Avoid a panic in
that case.

Release note: None

pkg/rpc/context.go

@@ -904,6 +904,8 @@ func (d delayingConn) Write(b []byte) (n int, err error) {
 	return n, err
 }
 
+var errMagicNotFound = errors.New("didn't get expected magic bytes header")
+
 func (d *delayingConn) Read(b []byte) (n int, err error) {
 	if d.readBuf.Len() == 0 {
 		var hdr delayingHeader
@@ -912,17 +914,17 @@ func (d *delayingConn) Read(b []byte) (n int, err error) {
 		}
 		// If we somehow don't get our expected magic, throw an error.
 		if hdr.Magic != magic {
-			panic(errors.New("didn't get expected magic bytes header"))
-		} else {
-			// Once we receive our first packet, we set our delay to the expected
-			// delay that was sent on the write side.
-			d.latency = time.Duration(hdr.DelayMS) * time.Millisecond
-			defer func() {
-				time.Sleep(timeutil.Until(timeutil.Unix(0, hdr.ReadTime)))
-			}()
-			if _, err := io.CopyN(d.readBuf, d.Conn, int64(hdr.Sz)); err != nil {
-				return 0, err
-			}
+			return 0, errors.WithStack(errMagicNotFound)
+		}
+
+		// Once we receive our first packet, we set our delay to the expected
+		// delay that was sent on the write side.
+		d.latency = time.Duration(hdr.DelayMS) * time.Millisecond
+		defer func() {
+			time.Sleep(timeutil.Until(timeutil.Unix(0, hdr.ReadTime)))
+		}()
+		if _, err := io.CopyN(d.readBuf, d.Conn, int64(hdr.Sz)); err != nil {
+			return 0, err
 		}
 	}
 	return d.readBuf.Read(b)