Fix Layer merge and add unit tests

mergo was improperly merging nil and empty strings

PiperOrigin-RevId: 308170862

3 files changed, 141 insertions, 18 deletions

diff --git a/test/packetimpact/testbench/BUILD b/test/packetimpact/testbench/BUILD
index b6a254882..3ceceb9d7 100644
--- a/test/packetimpact/testbench/BUILD
+++ b/test/packetimpact/testbench/BUILD
@@ -23,7 +23,6 @@ go_library(
         "//test/packetimpact/proto:posix_server_go_proto",
         "@com_github_google_go-cmp//cmp:go_default_library",
         "@com_github_google_go-cmp//cmp/cmpopts:go_default_library",
-        "@com_github_imdario_mergo//:go_default_library",
         "@com_github_mohae_deepcopy//:go_default_library",
         "@org_golang_google_grpc//:go_default_library",
         "@org_golang_google_grpc//keepalive:go_default_library",
@@ -37,5 +36,8 @@ go_test(
     size = "small",
     srcs = ["layers_test.go"],
     library = ":testbench",
-    deps = ["//pkg/tcpip"],
+    deps = [
+        "//pkg/tcpip",
+        "@com_github_mohae_deepcopy//:go_default_library",
+    ],
 )
diff --git a/test/packetimpact/testbench/layers.go b/test/packetimpact/testbench/layers.go
index 5ce324f0d..01e99567d 100644
--- a/test/packetimpact/testbench/layers.go
+++ b/test/packetimpact/testbench/layers.go
@@ -22,7 +22,6 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
-	"github.com/imdario/mergo"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
@@ -111,13 +110,31 @@ func equalLayer(x, y Layer) bool {
 	return cmp.Equal(x, y, opt, cmpopts.IgnoreTypes(LayerBase{}))
 }
 
-// mergeLayer merges other in layer. Any non-nil value in other overrides the
-// corresponding value in layer. If other is nil, no action is performed.
-func mergeLayer(layer, other Layer) error {
-	if other == nil {
+// mergeLayer merges y into x. Any fields for which y has a non-nil value, that
+// value overwrite the corresponding fields in x.
+func mergeLayer(x, y Layer) error {
+	if y == nil {
 		return nil
 	}
-	return mergo.Merge(layer, other, mergo.WithOverride)
+	if reflect.TypeOf(x) != reflect.TypeOf(y) {
+		return fmt.Errorf("can't merge %T into %T", y, x)
+	}
+	vx := reflect.ValueOf(x).Elem()
+	vy := reflect.ValueOf(y).Elem()
+	t := vy.Type()
+	for i := 0; i < vy.NumField(); i++ {
+		t := t.Field(i)
+		if t.Anonymous {
+			// Ignore the LayerBase in the Layer struct.
+			continue
+		}
+		v := vy.Field(i)
+		if v.IsNil() {
+			continue
+		}
+		vx.Field(i).Set(v)
+	}
+	return nil
 }
 
 func stringLayer(l Layer) string {
@@ -243,8 +260,7 @@ func (l *Ether) length() int {
 	return header.EthernetMinimumSize
 }
 
-// merge overrides the values in l with the values from other but only in fields
-// where the value is not nil.
+// merge implements Layer.merge.
 func (l *Ether) merge(other Layer) error {
 	return mergeLayer(l, other)
 }
@@ -399,8 +415,7 @@ func (l *IPv4) length() int {
 	return int(*l.IHL)
 }
 
-// merge overrides the values in l with the values from other but only in fields
-// where the value is not nil.
+// merge implements Layer.merge.
 func (l *IPv4) merge(other Layer) error {
 	return mergeLayer(l, other)
 }
@@ -544,8 +559,7 @@ func (l *TCP) length() int {
 	return int(*l.DataOffset)
 }
 
-// merge overrides the values in l with the values from other but only in fields
-// where the value is not nil.
+// merge implements Layer.merge.
 func (l *TCP) merge(other Layer) error {
 	return mergeLayer(l, other)
 }
@@ -622,8 +636,7 @@ func (l *UDP) length() int {
 	return int(*l.Length)
 }
 
-// merge overrides the values in l with the values from other but only in fields
-// where the value is not nil.
+// merge implements Layer.merge.
 func (l *UDP) merge(other Layer) error {
 	return mergeLayer(l, other)
 }
@@ -659,8 +672,7 @@ func (l *Payload) length() int {
 	return len(l.Bytes)
 }
 
-// merge overrides the values in l with the values from other but only in fields
-// where the value is not nil.
+// merge implements Layer.merge.
 func (l *Payload) merge(other Layer) error {
 	return mergeLayer(l, other)
 }
diff --git a/test/packetimpact/testbench/layers_test.go b/test/packetimpact/testbench/layers_test.go
index c99cf6312..f07ec5eb2 100644
--- a/test/packetimpact/testbench/layers_test.go
+++ b/test/packetimpact/testbench/layers_test.go
@@ -17,6 +17,7 @@ package testbench
 import (
 	"testing"
 
+	"github.com/mohae/deepcopy"
 	"gvisor.dev/gvisor/pkg/tcpip"
 )
 
@@ -52,6 +53,114 @@ func TestLayerMatch(t *testing.T) {
 	}
 }
 
+func TestLayerMergeMismatch(t *testing.T) {
+	tcp := &TCP{}
+	otherTCP := &TCP{}
+	ipv4 := &IPv4{}
+	ether := &Ether{}
+	for _, tt := range []struct {
+		a, b    Layer
+		success bool
+	}{
+		{tcp, tcp, true},
+		{tcp, otherTCP, true},
+		{tcp, ipv4, false},
+		{tcp, ether, false},
+		{tcp, nil, true},
+
+		{otherTCP, otherTCP, true},
+		{otherTCP, ipv4, false},
+		{otherTCP, ether, false},
+		{otherTCP, nil, true},
+
+		{ipv4, ipv4, true},
+		{ipv4, ether, false},
+		{ipv4, nil, true},
+
+		{ether, ether, true},
+		{ether, nil, true},
+	} {
+		if err := tt.a.merge(tt.b); (err == nil) != tt.success {
+			t.Errorf("%s.merge(%s) got %s, wanted the opposite", tt.a, tt.b, err)
+		}
+		if tt.b != nil {
+			if err := tt.b.merge(tt.a); (err == nil) != tt.success {
+				t.Errorf("%s.merge(%s) got %s, wanted the opposite", tt.b, tt.a, err)
+			}
+		}
+	}
+}
+
+func TestLayerMerge(t *testing.T) {
+	zero := Uint32(0)
+	one := Uint32(1)
+	two := Uint32(2)
+	empty := []byte{}
+	foo := []byte("foo")
+	bar := []byte("bar")
+	for _, tt := range []struct {
+		a, b Layer
+		want Layer
+	}{
+		{&TCP{AckNum: nil}, &TCP{AckNum: nil}, &TCP{AckNum: nil}},
+		{&TCP{AckNum: nil}, &TCP{AckNum: zero}, &TCP{AckNum: zero}},
+		{&TCP{AckNum: nil}, &TCP{AckNum: one}, &TCP{AckNum: one}},
+		{&TCP{AckNum: nil}, &TCP{AckNum: two}, &TCP{AckNum: two}},
+		{&TCP{AckNum: nil}, nil, &TCP{AckNum: nil}},
+
+		{&TCP{AckNum: zero}, &TCP{AckNum: nil}, &TCP{AckNum: zero}},
+		{&TCP{AckNum: zero}, &TCP{AckNum: zero}, &TCP{AckNum: zero}},
+		{&TCP{AckNum: zero}, &TCP{AckNum: one}, &TCP{AckNum: one}},
+		{&TCP{AckNum: zero}, &TCP{AckNum: two}, &TCP{AckNum: two}},
+		{&TCP{AckNum: zero}, nil, &TCP{AckNum: zero}},
+
+		{&TCP{AckNum: one}, &TCP{AckNum: nil}, &TCP{AckNum: one}},
+		{&TCP{AckNum: one}, &TCP{AckNum: zero}, &TCP{AckNum: zero}},
+		{&TCP{AckNum: one}, &TCP{AckNum: one}, &TCP{AckNum: one}},
+		{&TCP{AckNum: one}, &TCP{AckNum: two}, &TCP{AckNum: two}},
+		{&TCP{AckNum: one}, nil, &TCP{AckNum: one}},
+
+		{&TCP{AckNum: two}, &TCP{AckNum: nil}, &TCP{AckNum: two}},
+		{&TCP{AckNum: two}, &TCP{AckNum: zero}, &TCP{AckNum: zero}},
+		{&TCP{AckNum: two}, &TCP{AckNum: one}, &TCP{AckNum: one}},
+		{&TCP{AckNum: two}, &TCP{AckNum: two}, &TCP{AckNum: two}},
+		{&TCP{AckNum: two}, nil, &TCP{AckNum: two}},
+
+		{&Payload{Bytes: nil}, &Payload{Bytes: nil}, &Payload{Bytes: nil}},
+		{&Payload{Bytes: nil}, &Payload{Bytes: empty}, &Payload{Bytes: empty}},
+		{&Payload{Bytes: nil}, &Payload{Bytes: foo}, &Payload{Bytes: foo}},
+		{&Payload{Bytes: nil}, &Payload{Bytes: bar}, &Payload{Bytes: bar}},
+		{&Payload{Bytes: nil}, nil, &Payload{Bytes: nil}},
+
+		{&Payload{Bytes: empty}, &Payload{Bytes: nil}, &Payload{Bytes: empty}},
+		{&Payload{Bytes: empty}, &Payload{Bytes: empty}, &Payload{Bytes: empty}},
+		{&Payload{Bytes: empty}, &Payload{Bytes: foo}, &Payload{Bytes: foo}},
+		{&Payload{Bytes: empty}, &Payload{Bytes: bar}, &Payload{Bytes: bar}},
+		{&Payload{Bytes: empty}, nil, &Payload{Bytes: empty}},
+
+		{&Payload{Bytes: foo}, &Payload{Bytes: nil}, &Payload{Bytes: foo}},
+		{&Payload{Bytes: foo}, &Payload{Bytes: empty}, &Payload{Bytes: empty}},
+		{&Payload{Bytes: foo}, &Payload{Bytes: foo}, &Payload{Bytes: foo}},
+		{&Payload{Bytes: foo}, &Payload{Bytes: bar}, &Payload{Bytes: bar}},
+		{&Payload{Bytes: foo}, nil, &Payload{Bytes: foo}},
+
+		{&Payload{Bytes: bar}, &Payload{Bytes: nil}, &Payload{Bytes: bar}},
+		{&Payload{Bytes: bar}, &Payload{Bytes: empty}, &Payload{Bytes: empty}},
+		{&Payload{Bytes: bar}, &Payload{Bytes: foo}, &Payload{Bytes: foo}},
+		{&Payload{Bytes: bar}, &Payload{Bytes: bar}, &Payload{Bytes: bar}},
+		{&Payload{Bytes: bar}, nil, &Payload{Bytes: bar}},
+	} {
+		a := deepcopy.Copy(tt.a).(Layer)
+		if err := a.merge(tt.b); err != nil {
+			t.Errorf("%s.merge(%s) = %s, wanted nil", tt.a, tt.b, err)
+			continue
+		}
+		if a.String() != tt.want.String() {
+			t.Errorf("%s.merge(%s) merge result got %s, want %s", tt.a, tt.b, a, tt.want)
+		}
+	}
+}
+
 func TestLayerStringFormat(t *testing.T) {
 	for _, tt := range []struct {
 		name string