3 files changed, 175 insertions, 0 deletions
diff --git a/pkg/linewriter/BUILD b/pkg/linewriter/BUILD
new file mode 100644
index 000000000..4a96c6f1d
--- /dev/null
+++ b/pkg/linewriter/BUILD
@@ -0,0 +1,16 @@
+package(licenses = ["notice"])  # Apache 2.0
+
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "linewriter",
+    srcs = ["linewriter.go"],
+    importpath = "gvisor.googlesource.com/gvisor/pkg/linewriter",
+    visibility = ["//visibility:public"],
+)
+
+go_test(
+    name = "linewriter_test",
+    srcs = ["linewriter_test.go"],
+    embed = [":linewriter"],
+)
diff --git a/pkg/linewriter/linewriter.go b/pkg/linewriter/linewriter.go
new file mode 100644
index 000000000..98f974410
--- /dev/null
+++ b/pkg/linewriter/linewriter.go
@@ -0,0 +1,78 @@
+// Copyright 2018 Google Inc.
+//
+// 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 linewriter provides an io.Writer which calls an emitter on each line.
+package linewriter
+
+import (
+	"bytes"
+	"sync"
+)
+
+// Writer is an io.Writer which buffers input, flushing
+// individual lines through an emitter function.
+type Writer struct {
+	// the mutex locks buf.
+	sync.Mutex
+
+	// buf holds the data we haven't emitted yet.
+	buf bytes.Buffer
+
+	// emit is used to flush individual lines.
+	emit func(p []byte)
+}
+
+// NewWriter creates a Writer which emits using emitter.
+// The emitter must not retain p. It may change after emitter returns.
+func NewWriter(emitter func(p []byte)) *Writer {
+	return &Writer{emit: emitter}
+}
+
+// Write implements io.Writer.Write.
+// It calls emit on each line of input, not including the newline.
+// Write may be called concurrently.
+func (w *Writer) Write(p []byte) (int, error) {
+	w.Lock()
+	defer w.Unlock()
+
+	total := 0
+	for len(p) > 0 {
+		emit := true
+		i := bytes.IndexByte(p, '\n')
+		if i < 0 {
+			// No newline, we will buffer everything.
+			i = len(p)
+			emit = false
+		}
+
+		n, err := w.buf.Write(p[:i])
+		if err != nil {
+			return total, err
+		}
+		total += n
+
+		p = p[i:]
+
+		if emit {
+			// Skip the newline, but still count it.
+			p = p[1:]
+			total++
+
+			w.emit(w.buf.Bytes())
+			w.buf.Reset()
+		}
+	}
+
+	return total, nil
+}
diff --git a/pkg/linewriter/linewriter_test.go b/pkg/linewriter/linewriter_test.go
new file mode 100644
index 000000000..ce97cca05
--- /dev/null
+++ b/pkg/linewriter/linewriter_test.go
@@ -0,0 +1,81 @@
+// Copyright 2018 Google Inc.
+//
+// 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 linewriter
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestWriter(t *testing.T) {
+	testCases := []struct {
+		input []string
+		want  []string
+	}{
+		{
+			input: []string{"1\n", "2\n"},
+			want:  []string{"1", "2"},
+		},
+		{
+			input: []string{"1\n", "\n", "2\n"},
+			want:  []string{"1", "", "2"},
+		},
+		{
+			input: []string{"1\n2\n", "3\n"},
+			want:  []string{"1", "2", "3"},
+		},
+		{
+			input: []string{"1", "2\n"},
+			want:  []string{"12"},
+		},
+		{
+			// Data with no newline yet is omitted.
+			input: []string{"1\n", "2\n", "3"},
+			want:  []string{"1", "2"},
+		},
+	}
+
+	for _, c := range testCases {
+		var lines [][]byte
+
+		w := NewWriter(func(p []byte) {
+			// We must not retain p, so we must make a copy.
+			b := make([]byte, len(p))
+			copy(b, p)
+
+			lines = append(lines, b)
+		})
+
+		for _, in := range c.input {
+			n, err := w.Write([]byte(in))
+			if err != nil {
+				t.Errorf("Write(%q) err got %v want nil (case %+v)", in, err, c)
+			}
+			if n != len(in) {
+				t.Errorf("Write(%q) b got %d want %d (case %+v)", in, n, len(in), c)
+			}
+		}
+
+		if len(lines) != len(c.want) {
+			t.Errorf("len(lines) got %d want %d (case %+v)", len(lines), len(c.want), c)
+		}
+
+		for i := range lines {
+			if !bytes.Equal(lines[i], []byte(c.want[i])) {
+				t.Errorf("item %d got %q want %q (case %+v)", i, lines[i], c.want[i], c)
+			}
+		}
+	}
+}