use SO_RCVBUFFORCE to force receive buffer increase on Linux (#3804)

* Add ability to force change the receive buffer size using SO_RCVBUFFORCE in Linux

* Fix imports

* Update test

* Add sys_conn_helper_not_linux

* Rename file

* ignore the error on SetReadBuffer

* also run unit tests as root

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

sys_conn_helper_linux_test.go

@@ -0,0 +1,37 @@
+// We need root permissions to use RCVBUFFORCE.
+// This test is therefore only compiled when the root build flag is set.
+// It can only succeed if the tests are then also run with root permissions.
+//go:build linux && root
+
+package quic
+
+import (
+	"net"
+	"os"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Can change the receive buffer size", func() {
+	It("Force a change (if we have CAP_NET_ADMIN)", func() {
+		if os.Getuid() != 0 {
+			Fail("Must be root to force change the receive buffer size")
+		}
+
+		c, err := net.ListenPacket("udp", "127.0.0.1:0")
+		Expect(err).ToNot(HaveOccurred())
+		forceSetReceiveBuffer(c, 256<<10)
+
+		size, err := inspectReadBuffer(c)
+		Expect(err).ToNot(HaveOccurred())
+		//  The kernel doubles this value (to allow space for bookkeeping overhead)
+		Expect(size).To(Equal(512 << 10))
+
+		forceSetReceiveBuffer(c, 512<<10)
+		size, err = inspectReadBuffer(c)
+		Expect(err).ToNot(HaveOccurred())
+		//  The kernel doubles this value (to allow space for bookkeeping overhead)
+		Expect(size).To(Equal(1024 << 10))
+	})
+})