diff --git a/src/lib/Makefile b/src/lib/Makefile
index 9aaa71e8225..8c605cb8803 100644
--- a/src/lib/Makefile
+++ b/src/lib/Makefile
@@ -45,6 +45,7 @@ DIRS=\
 	tabwriter\
 	template\
 	testing\
+	testing/iotest\
 	time\
 	unicode\
 	utf8\
@@ -115,6 +116,7 @@ nuke: nuke.dirs
 
 bignum.install: fmt.install
 bufio.install: io.install os.install
+crypto/aes.install: os.install
 exec.install: os.install strings.install
 exvar.install: fmt.install http.install log.install strconv.install sync.install
 flag.install: fmt.install os.install strconv.install
@@ -123,7 +125,7 @@ path.install: io.install
 once.install: sync.install
 strings.install: utf8.install
 testing.install: flag.install fmt.install runtime.install
-
+testing/iotest.install: io.install os.install
 fmt.install: io.install reflect.install strconv.install
 GODEPS=fmt.install io.install once.install regexp.install sort.install \
 	strconv.install strings.install unicode.install utf8.install container/vector.install
diff --git a/src/lib/bufio/bufio.go b/src/lib/bufio/bufio.go
index 3b4aeb82022..7bfbb089f72 100644
--- a/src/lib/bufio/bufio.go
+++ b/src/lib/bufio/bufio.go
@@ -421,21 +421,17 @@ func (b *Writer) Flush() os.Error {
 	if b.err != nil {
 		return b.err
 	}
-	n := 0;
-	for n < b.n {
-		m, e := b.wr.Write(b.buf[n:b.n]);
-		n += m;
-		if m == 0 && e == nil {
-			e = io.ErrShortWrite
-		}
-		if e != nil {
-			if n < b.n {
-				copySlice(b.buf[0:b.n-n], b.buf[n:b.n])
-			}
-			b.n -= n;
-			b.err = e;
-			return e
+	n, e := b.wr.Write(b.buf[0:b.n]);
+	if n < b.n && e == nil {
+		e = io.ErrShortWrite;
+	}
+	if e != nil {
+		if n > 0 && n < b.n {
+			copySlice(b.buf[0:b.n-n], b.buf[n:b.n])
 		}
+		b.n -= n;
+		b.err = e;
+		return e
 	}
 	b.n = 0;
 	return nil
@@ -505,14 +501,14 @@ func (b *Writer) WriteByte(c byte) os.Error {
 
 // buffered input and output
 
-// ReadWriter stores (a pointer to) a Reader and a Writer.
+// ReadWriter stores pointers to a Reader and a Writer.
 // It implements io.ReadWriter.
 type ReadWriter struct {
 	*Reader;
 	*Writer;
 }
 
-// NewReadWriter allocates a new ReadWriter holding r and w.
+// NewReadWriter allocates a new ReadWriter that dispatches to r and w.
 func NewReadWriter(r *Reader, w *Writer) *ReadWriter {
 	return &ReadWriter{r, w}
 }
diff --git a/src/lib/bufio/bufio_test.go b/src/lib/bufio/bufio_test.go
index 240a6d03734..6e5135df782 100644
--- a/src/lib/bufio/bufio_test.go
+++ b/src/lib/bufio/bufio_test.go
@@ -10,61 +10,9 @@ import (
 	"io";
 	"os";
 	"testing";
+	"testing/iotest";
 )
 
-// Should be in language!
-func copy(p []byte, q []byte) {
-	for i := 0; i < len(p); i++ {
-		p[i] = q[i]
-	}
-}
-
-// Reads from p.
-type byteReader struct {
-	p []byte
-}
-
-func newByteReader(p []byte) io.Reader {
-	b := new(byteReader);
-	b.p = p;
-	return b
-}
-
-func (b *byteReader) Read(p []byte) (int, os.Error) {
-	n := len(p);
-	if n > len(b.p) {
-		n = len(b.p)
-	}
-	copy(p[0:n], b.p[0:n]);
-	b.p = b.p[n:len(b.p)];
-	return n, nil
-}
-
-
-// Reads from p but only returns half of what you asked for.
-type halfByteReader struct {
-	p []byte
-}
-
-func newHalfByteReader(p []byte) io.Reader {
-	b := new(halfByteReader);
-	b.p = p;
-	return b
-}
-
-func (b *halfByteReader) Read(p []byte) (int, os.Error) {
-	n := len(p)/2;
-	if n == 0 && len(p) > 0 {
-		n = 1
-	}
-	if n > len(b.p) {
-		n = len(b.p)
-	}
-	copy(p[0:n], b.p[0:n]);
-	b.p = b.p[n:len(b.p)];
-	return n, nil
-}
-
 // Reads from a reader and rot13s the result.
 type rot13Reader struct {
 	r io.Reader
@@ -82,24 +30,56 @@ func (r13 *rot13Reader) Read(p []byte) (int, os.Error) {
 		return n, e
 	}
 	for i := 0; i < n; i++ {
-		if 'a' <= p[i] && p[i] <= 'z' || 'A' <= p[i] && p[i] <= 'Z' {
-			if 'a' <= p[i] && p[i] <= 'm' || 'A' <= p[i] && p[i] <= 'M' {
-				p[i] += 13;
-			} else {
-				p[i] -= 13;
-			}
+		c := p[i] | 0x20;	// lowercase byte
+		if 'a' <= c && c <= 'm' {
+			p[i] += 13;
+		} else if 'n' <= c && c <= 'z' {
+			p[i] -= 13;
 		}
 	}
 	return n, nil
 }
 
+// Call ReadByte to accumulate the text of a file
+func readBytes(buf *Reader) string {
+	var b [1000]byte;
+	nb := 0;
+	for {
+		c, e := buf.ReadByte();
+		if e == io.ErrEOF {
+			break
+		}
+		if e != nil {
+			panic("Data: "+e.String())
+		}
+		b[nb] = c;
+		nb++;
+	}
+	return string(b[0:nb])
+}
+
+func TestReaderSimple(t *testing.T) {
+	data := io.StringBytes("hello world");
+	b := NewReader(io.NewByteReader(data));
+	if s := readBytes(b); s != "hello world" {
+		t.Errorf("simple hello world test failed: got %q", s);
+	}
+
+	b = NewReader(newRot13Reader(io.NewByteReader(data)));
+	if s := readBytes(b); s != "uryyb jbeyq" {
+		t.Error("rot13 hello world test failed: got %q", s);
+	}
+}
+
+
 type readMaker struct {
 	name string;
-	fn func([]byte) io.Reader;
+	fn func(io.Reader) io.Reader;
 }
 var readMakers = []readMaker {
-	readMaker{ "full", func(p []byte) io.Reader { return newByteReader(p) } },
-	readMaker{ "half", func(p []byte) io.Reader { return newHalfByteReader(p) } },
+	readMaker{ "full", func(r io.Reader) io.Reader { return r } },
+	readMaker{ "byte", iotest.OneByteReader },
+	readMaker{ "half", iotest.HalfReader },
 }
 
 // Call ReadLineString (which ends up calling everything else)
@@ -119,25 +99,6 @@ func readLines(b *Reader) string {
 	return s
 }
 
-// Call ReadByte to accumulate the text of a file
-func readBytes(buf *Reader) string {
-	var b [1000]byte;
-	nb := 0;
-	for {
-		c, e := buf.ReadByte();
-		if e == io.ErrEOF {
-			break
-		}
-		if e != nil {
-			panic("GetBytes: "+e.String())
-		}
-		b[nb] = c;
-		nb++;
-	}
-	// BUG return string(b[0:nb]) ?
-	return string(b[0:nb])
-}
-
 // Call Read to accumulate the text of a file
 func reads(buf *Reader, m int) string {
 	var b [1000]byte;
@@ -172,18 +133,6 @@ var bufsizes = []int {
 	23, 32, 46, 64, 93, 128, 1024, 4096
 }
 
-func TestReaderSimple(t *testing.T) {
-	b := NewReader(newByteReader(io.StringBytes("hello world")));
-	if s := readBytes(b); s != "hello world" {
-		t.Errorf("simple hello world test failed: got %q", s);
-	}
-
-	b = NewReader(newRot13Reader(newByteReader(io.StringBytes("hello world"))));
-	if s := readBytes(b); s != "uryyb jbeyq" {
-		t.Error("rot13 hello world test failed: got %q", s);
-	}
-}
-
 func TestReader(t *testing.T) {
 	var texts [31]string;
 	str := "";
@@ -204,7 +153,7 @@ func TestReader(t *testing.T) {
 					readmaker := readMakers[i];
 					bufreader := bufreaders[j];
 					bufsize := bufsizes[k];
-					read := readmaker.fn(textbytes);
+					read := readmaker.fn(io.NewByteReader(textbytes));
 					buf, e := NewReaderSize(read, bufsize);
 					s := bufreader.fn(buf);
 					if s != text {
@@ -217,122 +166,92 @@ func TestReader(t *testing.T) {
 	}
 }
 
-type writeBuffer interface {
-	Write(p []byte) (int, os.Error);
-	GetBytes() []byte
-}
-
-// Accumulates bytes into a byte array.
-type byteWriter struct {
-	p []byte;
-	n int
-}
-
-func newByteWriter() writeBuffer {
-	return new(byteWriter)
-}
-
-func (w *byteWriter) Write(p []byte) (int, os.Error) {
-	if w.p == nil {
-		w.p = make([]byte, len(p)+100)
-	} else if w.n + len(p) >= len(w.p) {
-		newp := make([]byte, len(w.p)*2 + len(p));
-		copy(newp[0:w.n], w.p[0:w.n]);
-		w.p = newp
-	}
-	copy(w.p[w.n:w.n+len(p)], p);
-	w.n += len(p);
-	return len(p), nil
-}
-
-func (w *byteWriter) GetBytes() []byte {
-	return w.p[0:w.n]
-}
-
-// Accumulates bytes written into a byte array
-// but Write only takes half of what you give it.
-// TODO: Could toss this -- Write() is not supposed to do that.
-type halfByteWriter struct {
-	bw writeBuffer
-}
-
-func newHalfByteWriter() writeBuffer {
-	w := new(halfByteWriter);
-	w.bw = newByteWriter();
-	return w
-}
-
-func (w *halfByteWriter) Write(p []byte) (int, os.Error) {
-	n := (len(p)+1) / 2;
-	// BUG return w.bw.Write(p[0:n])
-	r, e := w.bw.Write(p[0:n]);
-	return r, e
-}
-
-func (w *halfByteWriter) GetBytes() []byte {
-	return w.bw.GetBytes()
-}
-
-type writeMaker struct {
-	name string;
-	fn func()writeBuffer;
-}
 func TestWriter(t *testing.T) {
 	var data [8192]byte;
 
-	var writers = []writeMaker {
-		writeMaker{ "full", newByteWriter },
-		writeMaker{ "half", newHalfByteWriter },
-	};
-
 	for i := 0; i < len(data); i++ {
 		data[i] = byte(' '+ i%('~'-' '));
 	}
+	w := new(io.ByteBuffer);
 	for i := 0; i < len(bufsizes); i++ {
 		for j := 0; j < len(bufsizes); j++ {
-			for k := 0; k < len(writers); k++ {
-				nwrite := bufsizes[i];
-				bs := bufsizes[j];
+			nwrite := bufsizes[i];
+			bs := bufsizes[j];
 
-				// Write nwrite bytes using buffer size bs.
-				// Check that the right amount makes it out
-				// and that the data is correct.
+			// Write nwrite bytes using buffer size bs.
+			// Check that the right amount makes it out
+			// and that the data is correct.
 
-				write := writers[k].fn();
-				buf, e := NewWriterSize(write, bs);
-				context := fmt.Sprintf("write=%s nwrite=%d bufsize=%d", writers[k].name, nwrite, bs);
-				if e != nil {
-					t.Errorf("%s: NewWriterSize %d: %v", context, bs, e);
-					continue;
-				}
-				n, e1 := buf.Write(data[0:nwrite]);
-				if e1 != nil || n != nwrite {
-					t.Errorf("%s: buf.Write %d = %d, %v", context, nwrite, n, e1);
-					continue;
-				}
-				if e = buf.Flush(); e != nil {
-					t.Errorf("%s: buf.Flush = %v", context, e);
-				}
+			w.Reset();
+			buf, e := NewWriterSize(w, bs);
+			context := fmt.Sprintf("nwrite=%d bufsize=%d", nwrite, bs);
+			if e != nil {
+				t.Errorf("%s: NewWriterSize %d: %v", context, bs, e);
+				continue;
+			}
+			n, e1 := buf.Write(data[0:nwrite]);
+			if e1 != nil || n != nwrite {
+				t.Errorf("%s: buf.Write %d = %d, %v", context, nwrite, n, e1);
+				continue;
+			}
+			if e = buf.Flush(); e != nil {
+				t.Errorf("%s: buf.Flush = %v", context, e);
+			}
 
-				written := write.GetBytes();
-				if len(written) != nwrite {
-					t.Errorf("%s: %d bytes written", context, len(written));
-				}
-				for l := 0; l < len(written); l++ {
-					if written[i] != data[i] {
-						t.Errorf("%s: wrong bytes written");
-						t.Errorf("want=%s", data[0:len(written)]);
-						t.Errorf("have=%s", written);
-					}
+			written := w.Data();
+			if len(written) != nwrite {
+				t.Errorf("%s: %d bytes written", context, len(written));
+			}
+			for l := 0; l < len(written); l++ {
+				if written[i] != data[i] {
+					t.Errorf("%s: wrong bytes written");
+					t.Errorf("want=%s", data[0:len(written)]);
+					t.Errorf("have=%s", written);
 				}
 			}
 		}
 	}
 }
 
+// Check that write errors are returned properly.
+
+type errorWriterTest struct {
+	n, m int;
+	err os.Error;
+	expect os.Error;
+}
+
+func (w errorWriterTest) Write(p []byte) (int, os.Error) {
+	return len(p)*w.n/w.m, w.err;
+}
+
+var errorWriterTests = []errorWriterTest {
+	errorWriterTest{ 0, 1, nil, io.ErrShortWrite },
+	errorWriterTest{ 1, 2, nil, io.ErrShortWrite },
+	errorWriterTest{ 1, 1, nil, nil },
+	errorWriterTest{ 0, 1, os.EPIPE, os.EPIPE },
+	errorWriterTest{ 1, 2, os.EPIPE, os.EPIPE },
+	errorWriterTest{ 1, 1, os.EPIPE, os.EPIPE },
+}
+
+func TestWriteErrors(t *testing.T) {
+	for i, w := range errorWriterTests {
+		buf := NewWriter(w);
+		n, e := buf.Write(io.StringBytes("hello world"));
+		if e != nil {
+			t.Errorf("Write hello to %v: %v", w, e);
+			continue;
+		}
+		e = buf.Flush();
+		if e != w.expect {
+			t.Errorf("Flush %v: got %v, wanted %v", w, e, w.expect);
+		}
+	}
+}
+
 func TestNewReaderSizeIdempotent(t *testing.T) {
 	const BufSize = 1000;
-	b, err := NewReaderSize(newByteReader(io.StringBytes("hello world")), BufSize);
+	b, err := NewReaderSize(io.NewByteReader(io.StringBytes("hello world")), BufSize);
 	if err != nil {
 		t.Error("NewReaderSize create fail", err);
 	}
@@ -356,7 +275,7 @@ func TestNewReaderSizeIdempotent(t *testing.T) {
 
 func TestNewWriterSizeIdempotent(t *testing.T) {
 	const BufSize = 1000;
-	b, err := NewWriterSize(newByteWriter(), BufSize);
+	b, err := NewWriterSize(new(io.ByteBuffer), BufSize);
 	if err != nil {
 		t.Error("NewWriterSize create fail", err);
 	}
diff --git a/src/lib/io/bytebuffer.go b/src/lib/io/bytebuffer.go
index c862818fd77..921ddb17aea 100644
--- a/src/lib/io/bytebuffer.go
+++ b/src/lib/io/bytebuffer.go
@@ -42,13 +42,17 @@ func (b *ByteBuffer) Len() int {
 // Truncate discards all but the first n unread bytes from the buffer.
 // It is an error to call b.Truncate(n) with n > b.Len().
 func (b *ByteBuffer) Truncate(n int) {
+	if n == 0 {
+		// Reuse buffer space.
+		b.off = 0;
+	}
 	b.buf = b.buf[0 : b.off + n];
 }
 
 // Reset resets the buffer so it has no content.
 // b.Reset() is the same as b.Truncate(0).
 func (b *ByteBuffer) Reset() {
-	b.buf = b.buf[0 : b.off];
+	b.Truncate(0);
 }
 
 // Write appends the contents of p to the buffer.  The return
diff --git a/src/lib/io/io.go b/src/lib/io/io.go
index 70c82d56293..91b6ffd8b9b 100644
--- a/src/lib/io/io.go
+++ b/src/lib/io/io.go
@@ -185,3 +185,29 @@ func Copy(src Reader, dst Writer) (written int64, err os.Error) {
 	}
 	return written, err
 }
+
+// A ByteReader satisfies Reads by consuming data from a slice of bytes.
+// Clients can call NewByteReader to create one or wrap pointers
+// to their own slices: r := ByteReader{&data}.
+type ByteReader struct {
+	Data *[]byte
+}
+
+func (r ByteReader) Read(p []byte) (int, os.Error) {
+	n := len(p);
+	b := r.Data;
+	if n > len(b) {
+		n = len(b);
+	}
+	for i := 0; i < n; i++ {
+		p[i] = b[i];
+	}
+	*b = b[n:len(b)];
+	return n, nil;
+}
+
+// NewByteReader returns a new ByteReader reading from data.
+func NewByteReader(data []byte) ByteReader {
+	return ByteReader{ &data };
+}
+
diff --git a/src/lib/testing/iotest/Makefile b/src/lib/testing/iotest/Makefile
new file mode 100644
index 00000000000..5bfa513c735
--- /dev/null
+++ b/src/lib/testing/iotest/Makefile
@@ -0,0 +1,68 @@
+# Copyright 2009 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.
+
+# DO NOT EDIT.  Automatically generated by gobuild.
+# gobuild -m >Makefile
+
+D=/testing/
+
+O_arm=5
+O_amd64=6
+O_386=8
+OS=568vq
+
+O=$(O_$(GOARCH))
+GC=$(O)g -I_obj
+CC=$(O)c -FVw
+AS=$(O)a
+AR=6ar
+
+default: packages
+
+clean:
+	rm -rf *.[$(OS)] *.a [$(OS)].out _obj
+
+test: packages
+	gotest
+
+coverage: packages
+	gotest
+	6cov -g `pwd` | grep -v '_test\.go:'
+
+%.$O: %.go
+	$(GC) $*.go
+
+%.$O: %.c
+	$(CC) $*.c
+
+%.$O: %.s
+	$(AS) $*.s
+
+O1=\
+	reader.$O\
+
+
+phases: a1
+_obj$D/iotest.a: phases
+
+a1: $(O1)
+	$(AR) grc _obj$D/iotest.a reader.$O
+	rm -f $(O1)
+
+
+newpkg: clean
+	mkdir -p _obj$D
+	$(AR) grc _obj$D/iotest.a
+
+$(O1): newpkg
+$(O2): a1
+
+nuke: clean
+	rm -f $(GOROOT)/pkg$D/iotest.a
+
+packages: _obj$D/iotest.a
+
+install: packages
+	test -d $(GOROOT)/pkg && mkdir -p $(GOROOT)/pkg$D
+	cp _obj$D/iotest.a $(GOROOT)/pkg$D/iotest.a
diff --git a/src/lib/testing/iotest/reader.go b/src/lib/testing/iotest/reader.go
new file mode 100644
index 00000000000..0bb8633388a
--- /dev/null
+++ b/src/lib/testing/iotest/reader.go
@@ -0,0 +1,44 @@
+// Copyright 2009 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.
+
+// The iotest package implements Readers and Writers
+// useful only for testing.
+package iotest
+
+import (
+	"io";
+	"os";
+)
+
+type oneByteReader struct {
+	r io.Reader;
+}
+
+func (r *oneByteReader) Read(p []byte) (int, os.Error) {
+	if len(p) == 0 {
+		return 0, nil;
+	}
+	return r.r.Read(p[0:1]);
+}
+
+// OneByteReader returns a Reader that implements
+// each non-empty Read by reading one byte from r.
+func OneByteReader(r io.Reader) io.Reader {
+	return &oneByteReader{r};
+}
+
+type halfReader struct {
+	r io.Reader;
+}
+
+func (r *halfReader) Read(p []byte) (int, os.Error) {
+	return r.r.Read(p[0:(len(p)+1)/2]);
+}
+
+// HalfReader returns a Reader that implements Read
+// by reading half as many requested bytes from r.
+func HalfReader(r io.Reader) io.Reader {
+	return &halfReader{r};
+}
+