io: MultiReader and MultiWriter

Little helpers I've found useful.

R=adg, rsc, r, gri
CC=golang-dev
https://golang.org/cl/1764043

src/pkg/io/Makefile

@@ -7,6 +7,8 @@ include ../../Make.$(GOARCH)
 TARG=io
 GOFILES=\
 	io.go\
+	multi_reader.go\
+	multi_writer.go\
 	pipe.go\
 
 include ../../Make.pkg

src/pkg/io/multi_reader.go

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io
+
+import "os"
+
+type multiReader struct {
+	readers []Reader
+}
+
+func (mr *multiReader) Read(p []byte) (n int, err os.Error) {
+	for len(mr.readers) > 0 {
+		n, err = mr.readers[0].Read(p)
+		if n > 0 || err != os.EOF {
+			if err == os.EOF {
+				// This shouldn't happen.
+				// Well-behaved Readers should never
+				// return non-zero bytes read with an
+				// EOF.  But if so, we clean it.
+				err = nil
+			}
+			return
+		}
+		mr.readers = mr.readers[1:]
+	}
+	return 0, os.EOF
+}
+
+// MultiReader returns a Reader that's the logical concatenation of
+// the provided input readers.  They're read sequentially.  Once all
+// inputs are drained, Read will return os.EOF.
+func MultiReader(readers ...Reader) Reader {
+	return &multiReader{readers}
+}

src/pkg/io/multi_reader_test.go

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io_test
+
+import (
+	. "io"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestMultiReader(t *testing.T) {
+	var mr Reader
+	var buf []byte
+	nread := 0
+	withFooBar := func(tests func()) {
+		r1 := strings.NewReader("foo ")
+		r2 := strings.NewReader("bar")
+		mr = MultiReader(r1, r2)
+		buf = make([]byte, 20)
+		tests()
+	}
+	expectRead := func(size int, expected string, eerr os.Error) {
+		nread++
+		n, gerr := mr.Read(buf[0:size])
+		if n != len(expected) {
+			t.Errorf("#%d, expected %d bytes; got %d",
+				nread, len(expected), n)
+		}
+		got := string(buf[0:n])
+		if got != expected {
+			t.Errorf("#%d, expected %q; got %q",
+				nread, expected, got)
+		}
+		if gerr != eerr {
+			t.Errorf("#%d, expected error %v; got %v",
+				nread, eerr, gerr)
+		}
+		buf = buf[n:]
+	}
+	withFooBar(func() {
+		expectRead(2, "fo", nil)
+		expectRead(5, "o ", nil)
+		expectRead(5, "bar", nil)
+		expectRead(5, "", os.EOF)
+	})
+	withFooBar(func() {
+		expectRead(4, "foo ", nil)
+		expectRead(1, "b", nil)
+		expectRead(3, "ar", nil)
+		expectRead(1, "", os.EOF)
+	})
+	withFooBar(func() {
+		expectRead(5, "foo ", nil)
+	})
+}

src/pkg/io/multi_writer.go

+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io
+
+import "os"
+
+type multiWriter struct {
+	writers []Writer
+}
+
+func (t *multiWriter) Write(p []byte) (n int, err os.Error) {
+	for _, w := range t.writers {
+		n, err = w.Write(p)
+		if err != nil {
+			return
+		}
+		if n != len(p) {
+			err = ErrShortWrite
+			return
+		}
+	}
+	return len(p), nil
+}
+
+// MultiWriter creates a writer that duplicates its writes to all the
+// provided writers, similar to the Unix tee(1) command.
+func MultiWriter(writers ...Writer) Writer {
+	return &multiWriter{writers}
+}

src/pkg/io/multi_writer_test.go

+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package io_test
+
+import (
+	. "io"
+	"bytes"
+	"crypto/sha1"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestMultiWriter(t *testing.T) {
+	sha1 := sha1.New()
+	sink := new(bytes.Buffer)
+	mw := MultiWriter(sha1, sink)
+
+	sourceString := "My input text."
+	source := strings.NewReader(sourceString)
+	written, err := Copy(mw, source)
+
+	if written != int64(len(sourceString)) {
+		t.Errorf("short write of %d, not %d", written, len(sourceString))
+	}
+
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	sha1hex := fmt.Sprintf("%x", sha1.Sum())
+	if sha1hex != "01cb303fa8c30a64123067c5aa6284ba7ec2d31b" {
+		t.Error("incorrect sha1 value")
+	}
+
+	if sink.String() != sourceString {
+		t.Error("expected %q; got %q", sourceString, sink.String())
+	}
+}