Add function to get error from a tcpip.Endpoint

In an upcoming CL, socket option types are made to implement a marker
interface with pointer receivers. Since this results in calling methods
of an interface with a pointer, we incur an allocation when attempting
to get an Endpoint's last error with the current implementation.

When calling the method of an interface, the compiler is unable to
determine what the interface implementation does with the pointer
(since calling a method on an interface uses virtual dispatch at runtime
so the compiler does not know what the interface method will do) so it
allocates on the heap to be safe incase an implementation continues to
hold the pointer after the functioon returns (the reference escapes the
scope of the object).

In the example below, the compiler does not know what b.foo does with
the reference to a it allocates a on the heap as the reference to a may
escape the scope of a.
```
var a int
var b someInterface
b.foo(&a)
```

This change removes the opportunity for that allocation.

RELNOTES: n/a
PiperOrigin-RevId: 328796559

1 files changed, 1 insertions, 1 deletions

diff --git a/pkg/tcpip/sample/tun_tcp_connect/main.go b/pkg/tcpip/sample/tun_tcp_connect/main.go
index 0ab089208..91fc26722 100644
--- a/pkg/tcpip/sample/tun_tcp_connect/main.go
+++ b/pkg/tcpip/sample/tun_tcp_connect/main.go
@@ -182,7 +182,7 @@ func main() {
 	if terr == tcpip.ErrConnectStarted {
 		fmt.Println("Connect is pending...")
 		<-notifyCh
-		terr = ep.GetSockOpt(tcpip.ErrorOption{})
+		terr = ep.LastError()
 	}
 	wq.EventUnregister(&waitEntry)