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

5 files changed, 12 insertions, 18 deletions

diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 290172ac9..72df5c2a1 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -491,7 +491,7 @@ func (h *handshake) resolveRoute() *tcpip.Error {
 				h.ep.mu.Lock()
 			}
 			if n&notifyError != 0 {
-				return h.ep.takeLastError()
+				return h.ep.LastError()
 			}
 		}
 
@@ -620,7 +620,7 @@ func (h *handshake) execute() *tcpip.Error {
 				h.ep.mu.Lock()
 			}
 			if n&notifyError != 0 {
-				return h.ep.takeLastError()
+				return h.ep.LastError()
 			}
 
 		case wakerForNewSegment:
diff --git a/pkg/tcpip/transport/tcp/dual_stack_test.go b/pkg/tcpip/transport/tcp/dual_stack_test.go
index 804e95aea..6074cc24e 100644
--- a/pkg/tcpip/transport/tcp/dual_stack_test.go
+++ b/pkg/tcpip/transport/tcp/dual_stack_test.go
@@ -86,8 +86,7 @@ func testV4Connect(t *testing.T, c *context.Context, checkers ...checker.Network
 	// Wait for connection to be established.
 	select {
 	case <-ch:
-		err = c.EP.GetSockOpt(tcpip.ErrorOption{})
-		if err != nil {
+		if err := c.EP.LastError(); err != nil {
 			t.Fatalf("Unexpected error when connecting: %v", err)
 		}
 	case <-time.After(1 * time.Second):
@@ -194,8 +193,7 @@ func testV6Connect(t *testing.T, c *context.Context, checkers ...checker.Network
 	// Wait for connection to be established.
 	select {
 	case <-ch:
-		err = c.EP.GetSockOpt(tcpip.ErrorOption{})
-		if err != nil {
+		if err := c.EP.LastError(); err != nil {
 			t.Fatalf("Unexpected error when connecting: %v", err)
 		}
 	case <-time.After(1 * time.Second):
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index ff9b8804d..8a5e993b5 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -1234,7 +1234,7 @@ func (e *endpoint) SetOwner(owner tcpip.PacketOwner) {
 	e.owner = owner
 }
 
-func (e *endpoint) takeLastError() *tcpip.Error {
+func (e *endpoint) LastError() *tcpip.Error {
 	e.lastErrorMu.Lock()
 	defer e.lastErrorMu.Unlock()
 	err := e.lastError
@@ -1995,9 +1995,6 @@ func (e *endpoint) GetSockOptInt(opt tcpip.SockOptInt) (int, *tcpip.Error) {
 // GetSockOpt implements tcpip.Endpoint.GetSockOpt.
 func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 	switch o := opt.(type) {
-	case tcpip.ErrorOption:
-		return e.takeLastError()
-
 	case *tcpip.BindToDeviceOption:
 		e.LockUser()
 		*o = tcpip.BindToDeviceOption(e.bindToDevice)
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 9650bb06c..3d3034d50 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -74,8 +74,8 @@ func TestGiveUpConnect(t *testing.T) {
 
 	// Wait for ep to become writable.
 	<-notifyCh
-	if err := ep.GetSockOpt(tcpip.ErrorOption{}); err != tcpip.ErrAborted {
-		t.Fatalf("got ep.GetSockOpt(tcpip.ErrorOption{}) = %s, want = %s", err, tcpip.ErrAborted)
+	if err := ep.LastError(); err != tcpip.ErrAborted {
+		t.Fatalf("got ep.LastError() = %s, want = %s", err, tcpip.ErrAborted)
 	}
 
 	// Call Connect again to retreive the handshake failure status
@@ -3023,8 +3023,8 @@ func TestSynOptionsOnActiveConnect(t *testing.T) {
 	// Wait for connection to be established.
 	select {
 	case <-ch:
-		if err := c.EP.GetSockOpt(tcpip.ErrorOption{}); err != nil {
-			t.Fatalf("GetSockOpt failed: %s", err)
+		if err := c.EP.LastError(); err != nil {
+			t.Fatalf("Connect failed: %s", err)
 		}
 	case <-time.After(1 * time.Second):
 		t.Fatalf("Timed out waiting for connection")
@@ -4411,7 +4411,7 @@ func TestSelfConnect(t *testing.T) {
 	}
 
 	<-notifyCh
-	if err := ep.GetSockOpt(tcpip.ErrorOption{}); err != nil {
+	if err := ep.LastError(); err != nil {
 		t.Fatalf("Connect failed: %s", err)
 	}
 
diff --git a/pkg/tcpip/transport/tcp/testing/context/context.go b/pkg/tcpip/transport/tcp/testing/context/context.go
index b6031354e..1f5340cd0 100644
--- a/pkg/tcpip/transport/tcp/testing/context/context.go
+++ b/pkg/tcpip/transport/tcp/testing/context/context.go
@@ -638,7 +638,7 @@ func (c *Context) Connect(iss seqnum.Value, rcvWnd seqnum.Size, options []byte)
 	// Wait for connection to be established.
 	select {
 	case <-notifyCh:
-		if err := c.EP.GetSockOpt(tcpip.ErrorOption{}); err != nil {
+		if err := c.EP.LastError(); err != nil {
 			c.t.Fatalf("Unexpected error when connecting: %v", err)
 		}
 	case <-time.After(1 * time.Second):
@@ -882,8 +882,7 @@ func (c *Context) CreateConnectedWithOptions(wantOptions header.TCPSynOptions) *
 	// Wait for connection to be established.
 	select {
 	case <-notifyCh:
-		err = c.EP.GetSockOpt(tcpip.ErrorOption{})
-		if err != nil {
+		if err := c.EP.LastError(); err != nil {
 			c.t.Fatalf("Unexpected error when connecting: %v", err)
 		}
 	case <-time.After(1 * time.Second):