netstack: Refactor tcpip.Endpoint.Read

Read now takes a destination io.Writer, count, options. Keeping the method name
Read, in contrast to the Write method.

This enables:
* direct transfer of views under VV
* zero copy

It also eliminates the need for sentry to keep a slice of view because
userspace had requested a read that is smaller than the view returned, removing
the complexity there.

Read/Peek/ReadPacket are now consolidated together and some duplicate code is
removed.

PiperOrigin-RevId: 350636322

2 files changed, 92 insertions, 13 deletions

diff --git a/pkg/tcpip/buffer/view.go b/pkg/tcpip/buffer/view.go
index 8db70a700..5dd1b1b6b 100644
--- a/pkg/tcpip/buffer/view.go
+++ b/pkg/tcpip/buffer/view.go
@@ -105,18 +105,18 @@ func (vv *VectorisedView) TrimFront(count int) {
 }
 
 // Read implements io.Reader.
-func (vv *VectorisedView) Read(v View) (copied int, err error) {
-	count := len(v)
+func (vv *VectorisedView) Read(b []byte) (copied int, err error) {
+	count := len(b)
 	for count > 0 && len(vv.views) > 0 {
 		if count < len(vv.views[0]) {
 			vv.size -= count
-			copy(v[copied:], vv.views[0][:count])
+			copy(b[copied:], vv.views[0][:count])
 			vv.views[0].TrimFront(count)
 			copied += count
 			return copied, nil
 		}
 		count -= len(vv.views[0])
-		copy(v[copied:], vv.views[0])
+		copy(b[copied:], vv.views[0])
 		copied += len(vv.views[0])
 		vv.removeFirst()
 	}
@@ -145,6 +145,35 @@ func (vv *VectorisedView) ReadToVV(dstVV *VectorisedView, count int) (copied int
 	return copied
 }
 
+// ReadTo reads up to count bytes from vv to dst. It also removes them from vv
+// unless peek is true.
+func (vv *VectorisedView) ReadTo(dst io.Writer, count int, peek bool) (int, error) {
+	var err error
+	done := 0
+	for _, v := range vv.Views() {
+		remaining := count - done
+		if remaining <= 0 {
+			break
+		}
+		if len(v) > remaining {
+			v = v[:remaining]
+		}
+
+		var n int
+		n, err = dst.Write(v)
+		if n > 0 {
+			done += n
+		}
+		if err != nil {
+			break
+		}
+	}
+	if !peek {
+		vv.TrimFront(done)
+	}
+	return done, err
+}
+
 // CapLength irreversibly reduces the length of the vectorised view.
 func (vv *VectorisedView) CapLength(length int) {
 	if length < 0 {
diff --git a/pkg/tcpip/buffer/view_test.go b/pkg/tcpip/buffer/view_test.go
index 726e54de9..e0ef8a94d 100644
--- a/pkg/tcpip/buffer/view_test.go
+++ b/pkg/tcpip/buffer/view_test.go
@@ -235,14 +235,16 @@ func TestToClone(t *testing.T) {
 	}
 }
 
-func TestVVReadToVV(t *testing.T) {
-	testCases := []struct {
-		comment     string
-		vv          VectorisedView
-		bytesToRead int
-		wantBytes   string
-		leftVV      VectorisedView
-	}{
+type readToTestCases struct {
+	comment     string
+	vv          VectorisedView
+	bytesToRead int
+	wantBytes   string
+	leftVV      VectorisedView
+}
+
+func createReadToTestCases() []readToTestCases {
+	return []readToTestCases{
 		{
 			comment:     "large VV, short read",
 			vv:          vv(30, "012345678901234567890123456789"),
@@ -279,8 +281,10 @@ func TestVVReadToVV(t *testing.T) {
 			leftVV:      vv(0, ""),
 		},
 	}
+}
 
-	for _, tc := range testCases {
+func TestVVReadToVV(t *testing.T) {
+	for _, tc := range createReadToTestCases() {
 		t.Run(tc.comment, func(t *testing.T) {
 			var readTo VectorisedView
 			inSize := tc.vv.Size()
@@ -301,6 +305,52 @@ func TestVVReadToVV(t *testing.T) {
 	}
 }
 
+func TestVVReadTo(t *testing.T) {
+	for _, tc := range createReadToTestCases() {
+		t.Run(tc.comment, func(t *testing.T) {
+			var dst bytes.Buffer
+			origSize := tc.vv.Size()
+			copied, err := tc.vv.ReadTo(&dst, tc.bytesToRead, false /* peek */)
+			if got, want := copied, len(tc.wantBytes); err != nil || got != want {
+				t.Errorf("got ReadTo(&dst, %d, false) = %d, %v; want %d, nil", tc.bytesToRead, got, err, want)
+			}
+			if got, want := string(dst.Bytes()), tc.wantBytes; got != want {
+				t.Errorf("got dst = %q, want %q", got, want)
+			}
+			if got, want := tc.vv.Size(), origSize-copied; got != want {
+				t.Errorf("got after-read tc.vv.Size() = %d, want %d", got, want)
+			}
+			if got, want := string(tc.vv.ToView()), string(tc.leftVV.ToView()); got != want {
+				t.Errorf("got after-read data in tc.vv = %q, want %q", got, want)
+			}
+		})
+	}
+}
+
+func TestVVReadToPeek(t *testing.T) {
+	for _, tc := range createReadToTestCases() {
+		t.Run(tc.comment, func(t *testing.T) {
+			var dst bytes.Buffer
+			origSize := tc.vv.Size()
+			origData := string(tc.vv.ToView())
+			copied, err := tc.vv.ReadTo(&dst, tc.bytesToRead, true /* peek */)
+			if got, want := copied, len(tc.wantBytes); err != nil || got != want {
+				t.Errorf("got ReadTo(&dst, %d, false) = %d, %v; want %d, nil", tc.bytesToRead, got, err, want)
+			}
+			if got, want := string(dst.Bytes()), tc.wantBytes; got != want {
+				t.Errorf("got dst = %q, want %q", got, want)
+			}
+			// Expect tc.vv is unchanged.
+			if got, want := tc.vv.Size(), origSize; got != want {
+				t.Errorf("got after-read tc.vv.Size() = %d, want %d", got, want)
+			}
+			if got, want := string(tc.vv.ToView()), origData; got != want {
+				t.Errorf("got after-read data in tc.vv = %q, want %q", got, want)
+			}
+		})
+	}
+}
+
 func TestVVRead(t *testing.T) {
 	testCases := []struct {
 		comment     string