3 files changed, 66 insertions, 7 deletions
diff --git a/pkg/tcpip/buffer/BUILD b/pkg/tcpip/buffer/BUILD
index 563bc78ea..5e135c50d 100644
--- a/pkg/tcpip/buffer/BUILD
+++ b/pkg/tcpip/buffer/BUILD
@@ -14,6 +14,9 @@ go_library(
 go_test(
     name = "buffer_test",
     size = "small",
-    srcs = ["view_test.go"],
+    srcs = [
+        "prependable_test.go",
+        "view_test.go",
+    ],
     library = ":buffer",
 )
diff --git a/pkg/tcpip/buffer/prependable.go b/pkg/tcpip/buffer/prependable.go
index ba21f4eca..57d1922ab 100644
--- a/pkg/tcpip/buffer/prependable.go
+++ b/pkg/tcpip/buffer/prependable.go
@@ -32,13 +32,19 @@ func NewPrependable(size int) Prependable {
 	return Prependable{buf: NewView(size), usedIdx: size}
 }
 
-// NewPrependableFromView creates an entirely-used Prependable from a View.
+// NewPrependableFromView creates a Prependable from a View and allocates
+// additional space if needed.
 //
-// NewPrependableFromView takes ownership of v. Note that since the entire
-// prependable is used, further attempts to call Prepend will note that size >
-// p.usedIdx and return nil.
-func NewPrependableFromView(v View) Prependable {
-	return Prependable{buf: v, usedIdx: 0}
+// NewPrependableFromView takes ownership of v. Note that if the entire
+// prependable is used, further attempts to call Prepend will note that
+// size > p.usedIdx and return nil.
+func NewPrependableFromView(v View, extraCap int) Prependable {
+	if extraCap == 0 {
+		return Prependable{buf: v, usedIdx: 0}
+	}
+	buf := make([]byte, extraCap, extraCap+len(v))
+	buf = append(buf, v...)
+	return Prependable{buf: buf, usedIdx: extraCap}
 }
 
 // NewEmptyPrependableFromView creates a new prependable buffer from a View.
diff --git a/pkg/tcpip/buffer/prependable_test.go b/pkg/tcpip/buffer/prependable_test.go
new file mode 100644
index 000000000..435a94a61
--- /dev/null
+++ b/pkg/tcpip/buffer/prependable_test.go
@@ -0,0 +1,50 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package buffer
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNewPrependableFromView(t *testing.T) {
+	tests := []struct {
+		comment   string
+		view      View
+		extraSize int
+		want      Prependable
+	}{
+		{
+			comment:   "Reserve extra space",
+			view:      View("abc"),
+			extraSize: 2,
+			want:      Prependable{buf: View("\x00\x00abc"), usedIdx: 2},
+		},
+		{
+			comment:   "Don't reserve extra space",
+			view:      View("abc"),
+			extraSize: 0,
+			want:      Prependable{buf: View("abc"), usedIdx: 0},
+		},
+	}
+
+	for _, testCase := range tests {
+		t.Run(testCase.comment, func(t *testing.T) {
+			prep := NewPrependableFromView(testCase.view, testCase.extraSize)
+			if !reflect.DeepEqual(prep, testCase.want) {
+				t.Errorf("NewPrependableFromView(%#v, %d) = %#v; want %#v", testCase.view, testCase.extraSize, prep, testCase.want)
+			}
+		})
+	}
+}