Clean-up buffer implementation.

This also adds substantial test cases.

The Read/Write interfaces are dropped as they are not necessary.

PiperOrigin-RevId: 300461547

6 files changed, 715 insertions, 274 deletions

diff --git a/pkg/buffer/BUILD b/pkg/buffer/BUILD
index a77a3beea..dcd086298 100644
--- a/pkg/buffer/BUILD
+++ b/pkg/buffer/BUILD
@@ -10,8 +10,8 @@ go_template_instance(
     prefix = "buffer",
     template = "//pkg/ilist:generic_list",
     types = {
-        "Element": "*Buffer",
-        "Linker": "*Buffer",
+        "Element": "*buffer",
+        "Linker": "*buffer",
     },
 )
 
@@ -34,6 +34,10 @@ go_library(
 go_test(
     name = "buffer_test",
     size = "small",
-    srcs = ["view_test.go"],
+    srcs = [
+        "safemem_test.go",
+        "view_test.go",
+    ],
     library = ":buffer",
+    deps = ["//pkg/safemem"],
 )
diff --git a/pkg/buffer/buffer.go b/pkg/buffer/buffer.go
index d5f64609b..c6d089fd9 100644
--- a/pkg/buffer/buffer.go
+++ b/pkg/buffer/buffer.go
@@ -13,6 +13,10 @@
 // limitations under the License.
 
 // Package buffer provides the implementation of a buffer view.
+//
+// A view is an flexible buffer, backed by a pool, supporting the safecopy
+// operations natively as well as the ability to grow via either prepend or
+// append, as well as shrink.
 package buffer
 
 import (
@@ -21,7 +25,7 @@ import (
 
 const bufferSize = 8144 // See below.
 
-// Buffer encapsulates a queueable byte buffer.
+// buffer encapsulates a queueable byte buffer.
 //
 // Note that the total size is slightly less than two pages. This is done
 // intentionally to ensure that the buffer object aligns with runtime
@@ -30,38 +34,61 @@ const bufferSize = 8144 // See below.
 // large enough chunk to limit excessive segmentation.
 //
 // +stateify savable
-type Buffer struct {
+type buffer struct {
 	data  [bufferSize]byte
 	read  int
 	write int
 	bufferEntry
 }
 
-// Reset resets internal data.
+// reset resets internal data.
 //
-// This must be called before use.
-func (b *Buffer) Reset() {
+// This must be called before returning the buffer to the pool.
+func (b *buffer) Reset() {
 	b.read = 0
 	b.write = 0
 }
 
-// Empty indicates the buffer is empty.
-//
-// This indicates there is no data left to read.
-func (b *Buffer) Empty() bool {
-	return b.read == b.write
-}
-
 // Full indicates the buffer is full.
 //
 // This indicates there is no capacity left to write.
-func (b *Buffer) Full() bool {
+func (b *buffer) Full() bool {
 	return b.write == len(b.data)
 }
 
+// ReadSize returns the number of bytes available for reading.
+func (b *buffer) ReadSize() int {
+	return b.write - b.read
+}
+
+// ReadMove advances the read index by the given amount.
+func (b *buffer) ReadMove(n int) {
+	b.read += n
+}
+
+// ReadSlice returns the read slice for this buffer.
+func (b *buffer) ReadSlice() []byte {
+	return b.data[b.read:b.write]
+}
+
+// WriteSize returns the number of bytes available for writing.
+func (b *buffer) WriteSize() int {
+	return len(b.data) - b.write
+}
+
+// WriteMove advances the write index by the given amount.
+func (b *buffer) WriteMove(n int) {
+	b.write += n
+}
+
+// WriteSlice returns the write slice for this buffer.
+func (b *buffer) WriteSlice() []byte {
+	return b.data[b.write:]
+}
+
 // bufferPool is a pool for buffers.
 var bufferPool = sync.Pool{
 	New: func() interface{} {
-		return new(Buffer)
+		return new(buffer)
 	},
 }
diff --git a/pkg/buffer/safemem.go b/pkg/buffer/safemem.go
index 071aaa488..0e5b86344 100644
--- a/pkg/buffer/safemem.go
+++ b/pkg/buffer/safemem.go
@@ -15,19 +15,17 @@
 package buffer
 
 import (
-	"io"
-
 	"gvisor.dev/gvisor/pkg/safemem"
 )
 
 // WriteBlock returns this buffer as a write Block.
-func (b *Buffer) WriteBlock() safemem.Block {
-	return safemem.BlockFromSafeSlice(b.data[b.write:])
+func (b *buffer) WriteBlock() safemem.Block {
+	return safemem.BlockFromSafeSlice(b.WriteSlice())
 }
 
 // ReadBlock returns this buffer as a read Block.
-func (b *Buffer) ReadBlock() safemem.Block {
-	return safemem.BlockFromSafeSlice(b.data[b.read:b.write])
+func (b *buffer) ReadBlock() safemem.Block {
+	return safemem.BlockFromSafeSlice(b.ReadSlice())
 }
 
 // WriteFromBlocks implements safemem.Writer.WriteFromBlocks.
@@ -47,21 +45,21 @@ func (v *View) WriteFromBlocks(srcs safemem.BlockSeq) (uint64, error) {
 	// Need at least one buffer.
 	firstBuf := v.data.Back()
 	if firstBuf == nil {
-		firstBuf = bufferPool.Get().(*Buffer)
+		firstBuf = bufferPool.Get().(*buffer)
 		v.data.PushBack(firstBuf)
 	}
 
 	// Does the last block have sufficient capacity alone?
-	if l := len(firstBuf.data) - firstBuf.write; l >= need {
+	if l := firstBuf.WriteSize(); l >= need {
 		dst = safemem.BlockSeqOf(firstBuf.WriteBlock())
 	} else {
 		// Append blocks until sufficient.
 		need -= l
 		blocks = append(blocks, firstBuf.WriteBlock())
 		for need > 0 {
-			emptyBuf := bufferPool.Get().(*Buffer)
+			emptyBuf := bufferPool.Get().(*buffer)
 			v.data.PushBack(emptyBuf)
-			need -= len(emptyBuf.data) // Full block.
+			need -= emptyBuf.WriteSize()
 			blocks = append(blocks, emptyBuf.WriteBlock())
 		}
 		dst = safemem.BlockSeqFromSlice(blocks)
@@ -73,11 +71,11 @@ func (v *View) WriteFromBlocks(srcs safemem.BlockSeq) (uint64, error) {
 
 	// Update all indices.
 	for left := int(n); left > 0; firstBuf = firstBuf.Next() {
-		if l := len(firstBuf.data) - firstBuf.write; left >= l {
-			firstBuf.write += l // Whole block.
+		if l := firstBuf.WriteSize(); left >= l {
+			firstBuf.WriteMove(l) // Whole block.
 			left -= l
 		} else {
-			firstBuf.write += left // Partial block.
+			firstBuf.WriteMove(left) // Partial block.
 			left = 0
 		}
 	}
@@ -103,18 +101,18 @@ func (v *View) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
 
 	firstBuf := v.data.Front()
 	if firstBuf == nil {
-		return 0, io.EOF
+		return 0, nil // No EOF.
 	}
 
 	// Is all the data in a single block?
-	if l := firstBuf.write - firstBuf.read; l >= need {
+	if l := firstBuf.ReadSize(); l >= need {
 		src = safemem.BlockSeqOf(firstBuf.ReadBlock())
 	} else {
 		// Build a list of all the buffers.
 		need -= l
 		blocks = append(blocks, firstBuf.ReadBlock())
 		for buf := firstBuf.Next(); buf != nil && need > 0; buf = buf.Next() {
-			need -= buf.write - buf.read
+			need -= buf.ReadSize()
 			blocks = append(blocks, buf.ReadBlock())
 		}
 		src = safemem.BlockSeqFromSlice(blocks)
diff --git a/pkg/buffer/safemem_test.go b/pkg/buffer/safemem_test.go
new file mode 100644
index 000000000..47f357e0c
--- /dev/null
+++ b/pkg/buffer/safemem_test.go
@@ -0,0 +1,170 @@
+// Copyright 2020 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 (
+	"bytes"
+	"strings"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/safemem"
+)
+
+func TestSafemem(t *testing.T) {
+	testCases := []struct {
+		name    string
+		input   string
+		output  string
+		readLen int
+		op      func(*View)
+	}{
+		// Basic coverage.
+		{
+			name:   "short",
+			input:  "010",
+			output: "010",
+		},
+		{
+			name:   "long",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: "0" + strings.Repeat("1", bufferSize) + "0",
+		},
+		{
+			name:    "short-read",
+			input:   "0",
+			readLen: 100, // > size.
+			output:  "0",
+		},
+		{
+			name:   "zero-read",
+			input:  "0",
+			output: "",
+		},
+		{
+			name:    "read-empty",
+			input:   "",
+			readLen: 1, // > size.
+			output:  "",
+		},
+
+		// Ensure offsets work.
+		{
+			name:   "offsets-short",
+			input:  "012",
+			output: "2",
+			op: func(v *View) {
+				v.TrimFront(2)
+			},
+		},
+		{
+			name:   "offsets-long0",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: strings.Repeat("1", bufferSize) + "0",
+			op: func(v *View) {
+				v.TrimFront(1)
+			},
+		},
+		{
+			name:   "offsets-long1",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: strings.Repeat("1", bufferSize-1) + "0",
+			op: func(v *View) {
+				v.TrimFront(2)
+			},
+		},
+		{
+			name:   "offsets-long2",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: "10",
+			op: func(v *View) {
+				v.TrimFront(bufferSize)
+			},
+		},
+
+		// Ensure truncation works.
+		{
+			name:   "truncate-short",
+			input:  "012",
+			output: "01",
+			op: func(v *View) {
+				v.Truncate(2)
+			},
+		},
+		{
+			name:   "truncate-long0",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: "0" + strings.Repeat("1", bufferSize),
+			op: func(v *View) {
+				v.Truncate(bufferSize + 1)
+			},
+		},
+		{
+			name:   "truncate-long1",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: "0" + strings.Repeat("1", bufferSize-1),
+			op: func(v *View) {
+				v.Truncate(bufferSize)
+			},
+		},
+		{
+			name:   "truncate-long2",
+			input:  "0" + strings.Repeat("1", bufferSize) + "0",
+			output: "01",
+			op: func(v *View) {
+				v.Truncate(2)
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Construct the new view.
+			var view View
+			bs := safemem.BlockSeqOf(safemem.BlockFromSafeSlice([]byte(tc.input)))
+			n, err := view.WriteFromBlocks(bs)
+			if err != nil {
+				t.Errorf("expected err nil, got %v", err)
+			}
+			if n != uint64(len(tc.input)) {
+				t.Errorf("expected %d bytes, got %d", len(tc.input), n)
+			}
+
+			// Run the operation.
+			if tc.op != nil {
+				tc.op(&view)
+			}
+
+			// Read and validate.
+			readLen := tc.readLen
+			if readLen == 0 {
+				readLen = len(tc.output) // Default.
+			}
+			out := make([]byte, readLen)
+			bs = safemem.BlockSeqOf(safemem.BlockFromSafeSlice(out))
+			n, err = view.ReadToBlocks(bs)
+			if err != nil {
+				t.Errorf("expected nil, got %v", err)
+			}
+			if n != uint64(len(tc.output)) {
+				t.Errorf("expected %d bytes, got %d", len(tc.output), n)
+			}
+
+			// Ensure the contents are correct.
+			if !bytes.Equal(out[:n], []byte(tc.output[:n])) {
+				t.Errorf("contents are wrong: expected %q, got %q", tc.output, string(out))
+			}
+		})
+	}
+}
diff --git a/pkg/buffer/view.go b/pkg/buffer/view.go
index 00fc11e9c..e6901eadb 100644
--- a/pkg/buffer/view.go
+++ b/pkg/buffer/view.go
@@ -38,14 +38,6 @@ func (v *View) TrimFront(count int64) {
 	}
 }
 
-// Read implements io.Reader.Read.
-//
-// Note that reading does not advance the read index. This must be done
-// manually using TrimFront or other methods.
-func (v *View) Read(p []byte) (int, error) {
-	return v.ReadAt(p, 0)
-}
-
 // ReadAt implements io.ReaderAt.ReadAt.
 func (v *View) ReadAt(p []byte, offset int64) (int, error) {
 	var (
@@ -54,54 +46,46 @@ func (v *View) ReadAt(p []byte, offset int64) (int, error) {
 	)
 	for buf := v.data.Front(); buf != nil && done < int64(len(p)); buf = buf.Next() {
 		needToSkip := int(offset - skipped)
-		if l := buf.write - buf.read; l <= needToSkip {
-			skipped += int64(l)
+		if sz := buf.ReadSize(); sz <= needToSkip {
+			skipped += int64(sz)
 			continue
 		}
 
 		// Actually read data.
-		n := copy(p[done:], buf.data[buf.read+needToSkip:buf.write])
+		n := copy(p[done:], buf.ReadSlice()[needToSkip:])
 		skipped += int64(needToSkip)
 		done += int64(n)
 	}
-	if int(done) < len(p) {
+	if int(done) < len(p) || offset+done == v.size {
 		return int(done), io.EOF
 	}
 	return int(done), nil
 }
 
-// Write implements io.Writer.Write.
-func (v *View) Write(p []byte) (int, error) {
-	v.Append(p) // Does not fail.
-	return len(p), nil
-}
-
 // advanceRead advances the view's read index.
 //
 // Precondition: there must be sufficient bytes in the buffer.
 func (v *View) advanceRead(count int64) {
 	for buf := v.data.Front(); buf != nil && count > 0; {
-		l := int64(buf.write - buf.read)
-		if l > count {
+		sz := int64(buf.ReadSize())
+		if sz > count {
 			// There is still data for reading.
-			buf.read += int(count)
+			buf.ReadMove(int(count))
 			v.size -= count
 			count = 0
 			break
 		}
 
-		// Read from this buffer.
-		buf.read += int(l)
-		count -= l
-		v.size -= l
-
-		// When all data has been read from a buffer, we push
-		// it into the empty buffer pool for reuse.
+		// Consume the whole buffer.
 		oldBuf := buf
 		buf = buf.Next() // Iterate.
 		v.data.Remove(oldBuf)
 		oldBuf.Reset()
 		bufferPool.Put(oldBuf)
+
+		// Update counts.
+		count -= sz
+		v.size -= sz
 	}
 	if count > 0 {
 		panic(fmt.Sprintf("advanceRead still has %d bytes remaining", count))
@@ -109,37 +93,39 @@ func (v *View) advanceRead(count int64) {
 }
 
 // Truncate truncates the view to the given bytes.
+//
+// This will not grow the view, only shrink it. If a length is passed that is
+// greater than the current size of the view, then nothing will happen.
+//
+// Precondition: length must be >= 0.
 func (v *View) Truncate(length int64) {
-	if length < 0 || length >= v.size {
+	if length < 0 {
+		panic("negative length provided")
+	}
+	if length >= v.size {
 		return // Nothing to do.
 	}
 	for buf := v.data.Back(); buf != nil && v.size > length; buf = v.data.Back() {
-		l := int64(buf.write - buf.read) // Local bytes.
-		switch {
-		case v.size-l >= length:
-			// Drop the buffer completely; see above.
-			v.data.Remove(buf)
-			v.size -= l
-			buf.Reset()
-			bufferPool.Put(buf)
-
-		case v.size > length && v.size-l < length:
-			// Just truncate the buffer locally.
-			delta := (length - (v.size - l))
-			buf.write = buf.read + int(delta)
+		sz := int64(buf.ReadSize())
+		if after := v.size - sz; after < length {
+			// Truncate the buffer locally.
+			left := (length - after)
+			buf.write = buf.read + int(left)
 			v.size = length
-
-		default:
-			// Should never happen.
-			panic("invalid buffer during truncation")
+			break
 		}
+
+		// Drop the buffer completely; see above.
+		v.data.Remove(buf)
+		buf.Reset()
+		bufferPool.Put(buf)
+		v.size -= sz
 	}
-	v.size = length // Save the new size.
 }
 
-// Grow grows the given view to the number of bytes. If zero
-// is true, all these bytes will be zero. If zero is false,
-// then this is the caller's responsibility.
+// Grow grows the given view to the number of bytes, which will be appended. If
+// zero is true, all these bytes will be zero. If zero is false, then this is
+// the caller's responsibility.
 //
 // Precondition: length must be >= 0.
 func (v *View) Grow(length int64, zero bool) {
@@ -149,29 +135,29 @@ func (v *View) Grow(length int64, zero bool) {
 	for v.size < length {
 		buf := v.data.Back()
 
-		// Is there at least one buffer?
+		// Is there some space in the last buffer?
 		if buf == nil || buf.Full() {
-			buf = bufferPool.Get().(*Buffer)
+			buf = bufferPool.Get().(*buffer)
 			v.data.PushBack(buf)
 		}
 
 		// Write up to length bytes.
-		l := len(buf.data) - buf.write
-		if int64(l) > length-v.size {
-			l = int(length - v.size)
+		sz := buf.WriteSize()
+		if int64(sz) > length-v.size {
+			sz = int(length - v.size)
 		}
 
 		// Zero the written section; note that this pattern is
 		// specifically recognized and optimized by the compiler.
 		if zero {
-			for i := buf.write; i < buf.write+l; i++ {
+			for i := buf.write; i < buf.write+sz; i++ {
 				buf.data[i] = 0
 			}
 		}
 
 		// Advance the index.
-		buf.write += l
-		v.size += int64(l)
+		buf.WriteMove(sz)
+		v.size += int64(sz)
 	}
 }
 
@@ -181,31 +167,40 @@ func (v *View) Prepend(data []byte) {
 	if buf := v.data.Front(); buf != nil && buf.read > 0 {
 		// Fill up before the first write.
 		avail := buf.read
-		copy(buf.data[0:], data[len(data)-avail:])
-		data = data[:len(data)-avail]
-		v.size += int64(avail)
+		bStart := 0
+		dStart := len(data) - avail
+		if avail > len(data) {
+			bStart = avail - len(data)
+			dStart = 0
+		}
+		n := copy(buf.data[bStart:], data[dStart:])
+		data = data[:dStart]
+		v.size += int64(n)
+		buf.read -= n
 	}
 
 	for len(data) > 0 {
 		// Do we need an empty buffer?
-		buf := bufferPool.Get().(*Buffer)
+		buf := bufferPool.Get().(*buffer)
 		v.data.PushFront(buf)
 
 		// The buffer is empty; copy last chunk.
-		start := len(data) - len(buf.data)
-		if start < 0 {
-			start = 0 // Everything.
+		avail := len(buf.data)
+		bStart := 0
+		dStart := len(data) - avail
+		if avail > len(data) {
+			bStart = avail - len(data)
+			dStart = 0
 		}
 
 		// We have to put the data at the end of the current
 		// buffer in order to ensure that the next prepend will
 		// correctly fill up the beginning of this buffer.
-		bStart := len(buf.data) - len(data[start:])
-		n := copy(buf.data[bStart:], data[start:])
-		buf.read = bStart
-		buf.write = len(buf.data)
-		data = data[:start]
+		n := copy(buf.data[bStart:], data[dStart:])
+		data = data[:dStart]
 		v.size += int64(n)
+		buf.read = len(buf.data) - n
+		buf.write = len(buf.data)
 	}
 }
 
@@ -214,16 +209,16 @@ func (v *View) Append(data []byte) {
 	for done := 0; done < len(data); {
 		buf := v.data.Back()
 
-		// Find the first empty buffer.
+		// Ensure there's a buffer with space.
 		if buf == nil || buf.Full() {
-			buf = bufferPool.Get().(*Buffer)
+			buf = bufferPool.Get().(*buffer)
 			v.data.PushBack(buf)
 		}
 
 		// Copy in to the given buffer.
-		n := copy(buf.data[buf.write:], data[done:])
+		n := copy(buf.WriteSlice(), data[done:])
 		done += n
-		buf.write += n
+		buf.WriteMove(n)
 		v.size += int64(n)
 	}
 }
@@ -232,52 +227,52 @@ func (v *View) Append(data []byte) {
 //
 // This method should not be used in any performance-sensitive paths. It may
 // allocate a fresh byte slice sufficiently large to contain all the data in
-// the buffer.
+// the buffer. This is principally for debugging.
 //
 // N.B. Tee data still belongs to this view, as if there is a single buffer
 // present, then it will be returned directly. This should be used for
 // temporary use only, and a reference to the given slice should not be held.
 func (v *View) Flatten() []byte {
-	if buf := v.data.Front(); buf.Next() == nil {
-		return buf.data[buf.read:buf.write] // Only one buffer.
+	if buf := v.data.Front(); buf == nil {
+		return nil // No data at all.
+	} else if buf.Next() == nil {
+		return buf.ReadSlice() // Only one buffer.
 	}
 	data := make([]byte, 0, v.size) // Need to flatten.
 	for buf := v.data.Front(); buf != nil; buf = buf.Next() {
 		// Copy to the allocated slice.
-		data = append(data, buf.data[buf.read:buf.write]...)
+		data = append(data, buf.ReadSlice()...)
 	}
 	return data
 }
 
 // Size indicates the total amount of data available in this view.
-func (v *View) Size() (sz int64) {
-	sz = v.size // Pre-calculated.
-	return sz
+func (v *View) Size() int64 {
+	return v.size
 }
 
 // Copy makes a strict copy of this view.
 func (v *View) Copy() (other View) {
 	for buf := v.data.Front(); buf != nil; buf = buf.Next() {
-		other.Append(buf.data[buf.read:buf.write])
+		other.Append(buf.ReadSlice())
 	}
-	return other
+	return
 }
 
 // Apply applies the given function across all valid data.
 func (v *View) Apply(fn func([]byte)) {
 	for buf := v.data.Front(); buf != nil; buf = buf.Next() {
-		if l := int64(buf.write - buf.read); l > 0 {
-			fn(buf.data[buf.read:buf.write])
-		}
+		fn(buf.ReadSlice())
 	}
 }
 
 // Merge merges the provided View with this one.
 //
-// The other view will be empty after this operation.
+// The other view will be appended to v, and other will be empty after this
+// operation completes.
 func (v *View) Merge(other *View) {
 	// Copy over all buffers.
-	for buf := other.data.Front(); buf != nil && !buf.Empty(); buf = other.data.Front() {
+	for buf := other.data.Front(); buf != nil; buf = other.data.Front() {
 		other.data.Remove(buf)
 		v.data.PushBack(buf)
 	}
@@ -288,6 +283,9 @@ func (v *View) Merge(other *View) {
 }
 
 // WriteFromReader writes to the buffer from an io.Reader.
+//
+// A minimum read size equal to unsafe.Sizeof(unintptr) is enforced,
+// provided that count is greater than or equal to unsafe.Sizeof(uintptr).
 func (v *View) WriteFromReader(r io.Reader, count int64) (int64, error) {
 	var (
 		done int64
@@ -297,17 +295,17 @@ func (v *View) WriteFromReader(r io.Reader, count int64) (int64, error) {
 	for done < count {
 		buf := v.data.Back()
 
-		// Find the first empty buffer.
+		// Ensure we have an empty buffer.
 		if buf == nil || buf.Full() {
-			buf = bufferPool.Get().(*Buffer)
+			buf = bufferPool.Get().(*buffer)
 			v.data.PushBack(buf)
 		}
 
 		// Is this less than the minimum batch?
-		if len(buf.data[buf.write:]) < minBatch && (count-done) >= int64(minBatch) {
+		if buf.WriteSize() < minBatch && (count-done) >= int64(minBatch) {
 			tmp := make([]byte, minBatch)
 			n, err = r.Read(tmp)
-			v.Write(tmp[:n])
+			v.Append(tmp[:n])
 			done += int64(n)
 			if err != nil {
 				break
@@ -316,14 +314,14 @@ func (v *View) WriteFromReader(r io.Reader, count int64) (int64, error) {
 		}
 
 		// Limit the read, if necessary.
-		end := len(buf.data)
-		if int64(end-buf.write) > (count - done) {
-			end = buf.write + int(count-done)
+		sz := buf.WriteSize()
+		if left := count - done; int64(sz) > left {
+			sz = int(left)
 		}
 
 		// Pass the relevant portion of the buffer.
-		n, err = r.Read(buf.data[buf.write:end])
-		buf.write += n
+		n, err = r.Read(buf.WriteSlice()[:sz])
+		buf.WriteMove(n)
 		done += int64(n)
 		v.size += int64(n)
 		if err == io.EOF {
@@ -340,6 +338,9 @@ func (v *View) WriteFromReader(r io.Reader, count int64) (int64, error) {
 //
 // N.B. This does not consume the bytes read. TrimFront should
 // be called appropriately after this call in order to do so.
+//
+// A minimum write size equal to unsafe.Sizeof(unintptr) is enforced,
+// provided that count is greater than or equal to unsafe.Sizeof(uintptr).
 func (v *View) ReadToWriter(w io.Writer, count int64) (int64, error) {
 	var (
 		done int64
@@ -348,15 +349,22 @@ func (v *View) ReadToWriter(w io.Writer, count int64) (int64, error) {
 	)
 	offset := 0 // Spill-over for batching.
 	for buf := v.data.Front(); buf != nil && done < count; buf = buf.Next() {
-		l := buf.write - buf.read - offset
+		// Has this been consumed? Skip it.
+		sz := buf.ReadSize()
+		if sz <= offset {
+			offset -= sz
+			continue
+		}
+		sz -= offset
 
 		// Is this less than the minimum batch?
-		if l < minBatch && (count-done) >= int64(minBatch) && (v.size-done) >= int64(minBatch) {
+		left := count - done
+		if sz < minBatch && left >= int64(minBatch) && (v.size-done) >= int64(minBatch) {
 			tmp := make([]byte, minBatch)
 			n, err = v.ReadAt(tmp, done)
 			w.Write(tmp[:n])
 			done += int64(n)
-			offset = n - l // Reset below.
+			offset = n - sz // Reset below.
 			if err != nil {
 				break
 			}
@@ -364,12 +372,12 @@ func (v *View) ReadToWriter(w io.Writer, count int64) (int64, error) {
 		}
 
 		// Limit the write if necessary.
-		if int64(l) >= (count - done) {
-			l = int(count - done)
+		if int64(sz) >= left {
+			sz = int(left)
 		}
 
 		// Perform the actual write.
-		n, err = w.Write(buf.data[buf.read+offset : buf.read+offset+l])
+		n, err = w.Write(buf.ReadSlice()[offset : offset+sz])
 		done += int64(n)
 		if err != nil {
 			break
diff --git a/pkg/buffer/view_test.go b/pkg/buffer/view_test.go
index 37e652f16..3db1bc6ee 100644
--- a/pkg/buffer/view_test.go
+++ b/pkg/buffer/view_test.go
@@ -16,218 +16,452 @@ package buffer
 
 import (
 	"bytes"
+	"io"
 	"strings"
 	"testing"
 )
 
+func fillAppend(v *View, data []byte) {
+	v.Append(data)
+}
+
+func fillAppendEnd(v *View, data []byte) {
+	v.Grow(bufferSize-1, false)
+	v.Append(data)
+	v.TrimFront(bufferSize - 1)
+}
+
+func fillWriteFromReader(v *View, data []byte) {
+	b := bytes.NewBuffer(data)
+	v.WriteFromReader(b, int64(len(data)))
+}
+
+func fillWriteFromReaderEnd(v *View, data []byte) {
+	v.Grow(bufferSize-1, false)
+	b := bytes.NewBuffer(data)
+	v.WriteFromReader(b, int64(len(data)))
+	v.TrimFront(bufferSize - 1)
+}
+
+var fillFuncs = map[string]func(*View, []byte){
+	"append":             fillAppend,
+	"appendEnd":          fillAppendEnd,
+	"writeFromReader":    fillWriteFromReader,
+	"writeFromReaderEnd": fillWriteFromReaderEnd,
+}
+
+func testReadAt(t *testing.T, v *View, offset int64, n int, wantStr string, wantErr error) {
+	t.Helper()
+	d := make([]byte, n)
+	n, err := v.ReadAt(d, offset)
+	if n != len(wantStr) {
+		t.Errorf("got %d, want %d", n, len(wantStr))
+	}
+	if err != wantErr {
+		t.Errorf("got err %v, want %v", err, wantErr)
+	}
+	if !bytes.Equal(d[:n], []byte(wantStr)) {
+		t.Errorf("got %q, want %q", string(d[:n]), wantStr)
+	}
+}
+
 func TestView(t *testing.T) {
 	testCases := []struct {
 		name   string
 		input  string
 		output string
-		ops    []func(*View)
+		op     func(*testing.T, *View)
 	}{
-		// Prepend.
+		// Preconditions.
+		{
+			name:   "truncate-check",
+			input:  "hello",
+			output: "hello", // Not touched.
+			op: func(t *testing.T, v *View) {
+				defer func() {
+					if r := recover(); r == nil {
+						t.Errorf("Truncate(-1) did not panic")
+					}
+				}()
+				v.Truncate(-1)
+			},
+		},
+		{
+			name:   "grow-check",
+			input:  "hello",
+			output: "hello", // Not touched.
+			op: func(t *testing.T, v *View) {
+				defer func() {
+					if r := recover(); r == nil {
+						t.Errorf("Grow(-1) did not panic")
+					}
+				}()
+				v.Grow(-1, false)
+			},
+		},
 		{
-			name:  "prepend",
-			input: "world",
-			ops: []func(*View){
-				func(v *View) {
-					v.Prepend([]byte("hello "))
-				},
+			name:   "advance-check",
+			input:  "hello",
+			output: "", // Consumed.
+			op: func(t *testing.T, v *View) {
+				defer func() {
+					if r := recover(); r == nil {
+						t.Errorf("advanceRead(Size()+1) did not panic")
+					}
+				}()
+				v.advanceRead(v.Size() + 1)
 			},
+		},
+
+		// Prepend.
+		{
+			name:   "prepend",
+			input:  "world",
 			output: "hello world",
+			op: func(t *testing.T, v *View) {
+				v.Prepend([]byte("hello "))
+			},
 		},
 		{
-			name:  "prepend fill",
-			input: strings.Repeat("1", bufferSize-1),
-			ops: []func(*View){
-				func(v *View) {
-					v.Prepend([]byte("0"))
-				},
+			name:   "prepend-backfill-full",
+			input:  "hello world",
+			output: "jello world",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(1)
+				v.Prepend([]byte("j"))
 			},
-			output: "0" + strings.Repeat("1", bufferSize-1),
 		},
 		{
-			name:  "prepend overflow",
-			input: strings.Repeat("1", bufferSize),
-			ops: []func(*View){
-				func(v *View) {
-					v.Prepend([]byte("0"))
-				},
+			name:   "prepend-backfill-under",
+			input:  "hello world",
+			output: "hola world",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(5)
+				v.Prepend([]byte("hola"))
 			},
-			output: "0" + strings.Repeat("1", bufferSize),
 		},
 		{
-			name:  "prepend multiple buffers",
-			input: strings.Repeat("1", bufferSize-1),
-			ops: []func(*View){
-				func(v *View) {
-					v.Prepend([]byte(strings.Repeat("0", bufferSize*3)))
-				},
+			name:   "prepend-backfill-over",
+			input:  "hello world",
+			output: "smello world",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(1)
+				v.Prepend([]byte("sm"))
 			},
+		},
+		{
+			name:   "prepend-fill",
+			input:  strings.Repeat("1", bufferSize-1),
+			output: "0" + strings.Repeat("1", bufferSize-1),
+			op: func(t *testing.T, v *View) {
+				v.Prepend([]byte("0"))
+			},
+		},
+		{
+			name:   "prepend-overflow",
+			input:  strings.Repeat("1", bufferSize),
+			output: "0" + strings.Repeat("1", bufferSize),
+			op: func(t *testing.T, v *View) {
+				v.Prepend([]byte("0"))
+			},
+		},
+		{
+			name:   "prepend-multiple-buffers",
+			input:  strings.Repeat("1", bufferSize-1),
 			output: strings.Repeat("0", bufferSize*3) + strings.Repeat("1", bufferSize-1),
+			op: func(t *testing.T, v *View) {
+				v.Prepend([]byte(strings.Repeat("0", bufferSize*3)))
+			},
 		},
 
-		// Append.
+		// Append and write.
 		{
-			name:  "append",
-			input: "hello",
-			ops: []func(*View){
-				func(v *View) {
-					v.Append([]byte(" world"))
-				},
-			},
+			name:   "append",
+			input:  "hello",
 			output: "hello world",
+			op: func(t *testing.T, v *View) {
+				v.Append([]byte(" world"))
+			},
 		},
 		{
-			name:  "append fill",
-			input: strings.Repeat("1", bufferSize-1),
-			ops: []func(*View){
-				func(v *View) {
-					v.Append([]byte("0"))
-				},
-			},
+			name:   "append-fill",
+			input:  strings.Repeat("1", bufferSize-1),
 			output: strings.Repeat("1", bufferSize-1) + "0",
+			op: func(t *testing.T, v *View) {
+				v.Append([]byte("0"))
+			},
 		},
 		{
-			name:  "append overflow",
-			input: strings.Repeat("1", bufferSize),
-			ops: []func(*View){
-				func(v *View) {
-					v.Append([]byte("0"))
-				},
-			},
+			name:   "append-overflow",
+			input:  strings.Repeat("1", bufferSize),
 			output: strings.Repeat("1", bufferSize) + "0",
+			op: func(t *testing.T, v *View) {
+				v.Append([]byte("0"))
+			},
 		},
 		{
-			name:  "append multiple buffers",
-			input: strings.Repeat("1", bufferSize-1),
-			ops: []func(*View){
-				func(v *View) {
-					v.Append([]byte(strings.Repeat("0", bufferSize*3)))
-				},
-			},
+			name:   "append-multiple-buffers",
+			input:  strings.Repeat("1", bufferSize-1),
 			output: strings.Repeat("1", bufferSize-1) + strings.Repeat("0", bufferSize*3),
+			op: func(t *testing.T, v *View) {
+				v.Append([]byte(strings.Repeat("0", bufferSize*3)))
+			},
 		},
 
 		// Truncate.
 		{
-			name:  "truncate",
-			input: "hello world",
-			ops: []func(*View){
-				func(v *View) {
-					v.Truncate(5)
-				},
-			},
+			name:   "truncate",
+			input:  "hello world",
 			output: "hello",
+			op: func(t *testing.T, v *View) {
+				v.Truncate(5)
+			},
 		},
 		{
-			name:  "truncate multiple buffers",
-			input: strings.Repeat("1", bufferSize*2),
-			ops: []func(*View){
-				func(v *View) {
-					v.Truncate(bufferSize*2 - 1)
-				},
+			name:   "truncate-noop",
+			input:  "hello world",
+			output: "hello world",
+			op: func(t *testing.T, v *View) {
+				v.Truncate(v.Size() + 1)
 			},
-			output: strings.Repeat("1", bufferSize*2-1),
 		},
 		{
-			name:  "truncate multiple buffers to one buffer",
-			input: strings.Repeat("1", bufferSize*2),
-			ops: []func(*View){
-				func(v *View) {
-					v.Truncate(5)
-				},
+			name:   "truncate-multiple-buffers",
+			input:  strings.Repeat("1", bufferSize*2),
+			output: strings.Repeat("1", bufferSize*2-1),
+			op: func(t *testing.T, v *View) {
+				v.Truncate(bufferSize*2 - 1)
 			},
+		},
+		{
+			name:   "truncate-multiple-buffers-to-one",
+			input:  strings.Repeat("1", bufferSize*2),
 			output: "11111",
+			op: func(t *testing.T, v *View) {
+				v.Truncate(5)
+			},
 		},
 
 		// TrimFront.
 		{
-			name:  "trim",
-			input: "hello world",
-			ops: []func(*View){
-				func(v *View) {
-					v.TrimFront(6)
-				},
-			},
+			name:   "trim",
+			input:  "hello world",
 			output: "world",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(6)
+			},
 		},
 		{
-			name:  "trim multiple buffers",
-			input: strings.Repeat("1", bufferSize*2),
-			ops: []func(*View){
-				func(v *View) {
-					v.TrimFront(1)
-				},
+			name:   "trim-too-large",
+			input:  "hello world",
+			output: "",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(v.Size() + 1)
 			},
-			output: strings.Repeat("1", bufferSize*2-1),
 		},
 		{
-			name:  "trim multiple buffers to one buffer",
-			input: strings.Repeat("1", bufferSize*2),
-			ops: []func(*View){
-				func(v *View) {
-					v.TrimFront(bufferSize*2 - 1)
-				},
+			name:   "trim-multiple-buffers",
+			input:  strings.Repeat("1", bufferSize*2),
+			output: strings.Repeat("1", bufferSize*2-1),
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(1)
 			},
+		},
+		{
+			name:   "trim-multiple-buffers-to-one-buffer",
+			input:  strings.Repeat("1", bufferSize*2),
 			output: "1",
+			op: func(t *testing.T, v *View) {
+				v.TrimFront(bufferSize*2 - 1)
+			},
 		},
 
 		// Grow.
 		{
-			name:  "grow",
-			input: "hello world",
-			ops: []func(*View){
-				func(v *View) {
-					v.Grow(1, true)
-				},
-			},
+			name:   "grow",
+			input:  "hello world",
 			output: "hello world",
+			op: func(t *testing.T, v *View) {
+				v.Grow(1, true)
+			},
 		},
 		{
-			name: "grow from zero",
-			ops: []func(*View){
-				func(v *View) {
-					v.Grow(1024, true)
-				},
-			},
+			name:   "grow-from-zero",
 			output: strings.Repeat("\x00", 1024),
+			op: func(t *testing.T, v *View) {
+				v.Grow(1024, true)
+			},
 		},
 		{
-			name:  "grow from non-zero",
-			input: strings.Repeat("1", bufferSize),
-			ops: []func(*View){
-				func(v *View) {
-					v.Grow(bufferSize*2, true)
-				},
-			},
+			name:   "grow-from-non-zero",
+			input:  strings.Repeat("1", bufferSize),
 			output: strings.Repeat("1", bufferSize) + strings.Repeat("\x00", bufferSize),
+			op: func(t *testing.T, v *View) {
+				v.Grow(bufferSize*2, true)
+			},
+		},
+
+		// Copy.
+		{
+			name:   "copy",
+			input:  "hello",
+			output: "hello",
+			op: func(t *testing.T, v *View) {
+				other := v.Copy()
+				bs := other.Flatten()
+				want := []byte("hello")
+				if !bytes.Equal(bs, want) {
+					t.Errorf("expected %v, got %v", want, bs)
+				}
+			},
+		},
+		{
+			name:   "copy-large",
+			input:  strings.Repeat("1", bufferSize+1),
+			output: strings.Repeat("1", bufferSize+1),
+			op: func(t *testing.T, v *View) {
+				other := v.Copy()
+				bs := other.Flatten()
+				want := []byte(strings.Repeat("1", bufferSize+1))
+				if !bytes.Equal(bs, want) {
+					t.Errorf("expected %v, got %v", want, bs)
+				}
+			},
+		},
+
+		// Merge.
+		{
+			name:   "merge",
+			input:  "hello",
+			output: "hello world",
+			op: func(t *testing.T, v *View) {
+				var other View
+				other.Append([]byte(" world"))
+				v.Merge(&other)
+				if sz := other.Size(); sz != 0 {
+					t.Errorf("expected 0, got %d", sz)
+				}
+			},
+		},
+		{
+			name:   "merge-large",
+			input:  strings.Repeat("1", bufferSize+1),
+			output: strings.Repeat("1", bufferSize+1) + strings.Repeat("0", bufferSize+1),
+			op: func(t *testing.T, v *View) {
+				var other View
+				other.Append([]byte(strings.Repeat("0", bufferSize+1)))
+				v.Merge(&other)
+				if sz := other.Size(); sz != 0 {
+					t.Errorf("expected 0, got %d", sz)
+				}
+			},
+		},
+
+		// ReadAt.
+		{
+			name:   "readat",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 0, 6, "hello", io.EOF) },
+		},
+		{
+			name:   "readat-long",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 0, 8, "hello", io.EOF) },
+		},
+		{
+			name:   "readat-short",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 0, 3, "hel", nil) },
+		},
+		{
+			name:   "readat-offset",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 2, 3, "llo", io.EOF) },
+		},
+		{
+			name:   "readat-long-offset",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 2, 8, "llo", io.EOF) },
+		},
+		{
+			name:   "readat-short-offset",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, 2, 2, "ll", nil) },
+		},
+		{
+			name:   "readat-skip-all",
+			input:  "hello",
+			output: "hello",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, bufferSize+1, 1, "", io.EOF) },
+		},
+		{
+			name:   "readat-second-buffer",
+			input:  strings.Repeat("0", bufferSize+1) + "12",
+			output: strings.Repeat("0", bufferSize+1) + "12",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, bufferSize+1, 1, "1", nil) },
+		},
+		{
+			name:   "readat-second-buffer-end",
+			input:  strings.Repeat("0", bufferSize+1) + "12",
+			output: strings.Repeat("0", bufferSize+1) + "12",
+			op:     func(t *testing.T, v *View) { testReadAt(t, v, bufferSize+1, 2, "12", io.EOF) },
 		},
 	}
 
 	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			// Construct the new view.
-			var view View
-			view.Append([]byte(tc.input))
-
-			// Run all operations.
-			for _, op := range tc.ops {
-				op(&view)
-			}
-
-			// Flatten and validate.
-			out := view.Flatten()
-			if !bytes.Equal([]byte(tc.output), out) {
-				t.Errorf("expected %q, got %q", tc.output, string(out))
-			}
-
-			// Ensure the size is correct.
-			if len(out) != int(view.Size()) {
-				t.Errorf("size is wrong: expected %d, got %d", len(out), view.Size())
-			}
-		})
+		for fillName, fn := range fillFuncs {
+			t.Run(fillName+"/"+tc.name, func(t *testing.T) {
+				// Construct & fill the view.
+				var view View
+				fn(&view, []byte(tc.input))
+
+				// Run the operation.
+				if tc.op != nil {
+					tc.op(t, &view)
+				}
+
+				// Flatten and validate.
+				out := view.Flatten()
+				if !bytes.Equal([]byte(tc.output), out) {
+					t.Errorf("expected %q, got %q", tc.output, string(out))
+				}
+
+				// Ensure the size is correct.
+				if len(out) != int(view.Size()) {
+					t.Errorf("size is wrong: expected %d, got %d", len(out), view.Size())
+				}
+
+				// Calculate contents via apply.
+				var appliedOut []byte
+				view.Apply(func(b []byte) {
+					appliedOut = append(appliedOut, b...)
+				})
+				if len(appliedOut) != len(out) {
+					t.Errorf("expected %d, got %d", len(out), len(appliedOut))
+				}
+				if !bytes.Equal(appliedOut, out) {
+					t.Errorf("expected %v, got %v", out, appliedOut)
+				}
+
+				// Calculate contents via ReadToWriter.
+				var b bytes.Buffer
+				n, err := view.ReadToWriter(&b, int64(len(out)))
+				if n != int64(len(out)) {
+					t.Errorf("expected %d, got %d", len(out), n)
+				}
+				if err != nil {
+					t.Errorf("expected nil, got %v", err)
+				}
+				if !bytes.Equal(b.Bytes(), out) {
+					t.Errorf("expected %v, got %v", out, b.Bytes())
+				}
+			})
+		}
 	}
 }