ring: add tests/benchmark for WriterTo path and fix SIGSEGV error

Also add a call to ioutil.ReadAll to mimic the use in revid. There is
not a great deal of difference on the RPi3 in terms of throughput,
though the allocations obviously increase.

```
$ go test -run ^$ -bench . -benchmem -benchtime 30s
goos: linux
goarch: arm
BenchmarkRoundTrip-4           	    2000    20184818 ns/op    1.49 MB/s    104646 B/op    4 allocs/op
BenchmarkRoundTripWriterTo-4   	    2000    20175948 ns/op    1.49 MB/s       175 B/op    3 allocs/op
PASS
```

ring/ring.go

@@ -147,6 +147,8 @@ func (b *Buffer) Close() error {
 // Next gets the next element from the queue ready for reading, returning ErrTimeout if no
 // element is available within the timeout. If the Buffer has been closed Next returns io.EOF.
 //
+// Is it the responsibility of the caller to close the returned Chunk.
+//
 // Next is safe to use concurrently with write operations, but may not be used concurrently with
 // another Read call or Next call. A goroutine calling Next must not call Flush or Close.
 func (b *Buffer) Next(timeout time.Duration) (*Chunk, error) {
@@ -266,8 +268,9 @@ func (b *Chunk) Close() error {
 		panic("ring: invalid use of ring buffer chunk")
 	}
 	b.reset()
-	b.owner = nil
 	b.owner.tail = nil
-	b.owner.empty <- b
+	empty := b.owner.empty
+	b.owner = nil
+	empty <- b
 	return nil
 }

ring/ring_test.go

@@ -29,7 +29,9 @@ LICENSE
 package ring
 
 import (
+	"bytes"
 	"io"
+	"io/ioutil"
 	"reflect"
 	"strings"
 	"sync"
@@ -206,6 +208,112 @@ func TestRoundTrip(t *testing.T) {
 	}
 }
 
+func TestRoundTripWriterTo(t *testing.T) {
+	const maxTimeouts = 100
+	for _, test := range roundTripTests {
+		b := NewBuffer(test.len, test.size, test.timeout)
+		var wg sync.WaitGroup
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			for _, c := range test.data {
+				var dropped int
+				for _, f := range c {
+					time.Sleep(test.writeDelay) // Simulate slow data capture.
+					_, err := b.Write([]byte(f))
+					switch err {
+					case nil:
+						dropped = 0
+					case ErrDropped:
+						if dropped > maxTimeouts {
+							t.Errorf("too many write drops for %q", test.name)
+							return
+						}
+						dropped++
+					default:
+						t.Errorf("unexpected write error for %q: %v", test.name, err)
+						return
+					}
+				}
+				b.Flush()
+			}
+			b.Close()
+		}()
+		go func() {
+			var buf bytes.Buffer
+			defer wg.Done()
+			var got []string
+			var timeouts int
+		elements:
+			for {
+				chunk, err := b.Next(test.nextTimeout)
+				switch err {
+				case nil:
+					timeouts = 0
+				case ErrTimeout:
+					if timeouts > maxTimeouts {
+						t.Errorf("too many timeouts for %q", test.name)
+						return
+					}
+					timeouts++
+					continue
+				case io.EOF:
+					break elements
+				default:
+					t.Errorf("unexpected read error for %q: %v", test.name, err)
+					return
+				}
+
+				n, err := chunk.WriteTo(&buf)
+				if n != 0 {
+					time.Sleep(test.readDelay) // Simulate slow data processing.
+					got = append(got, buf.String())
+					buf.Reset()
+				}
+				if err != nil {
+					t.Errorf("unexpected writeto error for %q: %v", test.name, err)
+					return
+				}
+				err = chunk.Close()
+				if err != nil {
+					t.Errorf("unexpected close error for %q: %v", test.name, err)
+					return
+				}
+			}
+			var want []string
+			for _, c := range test.data {
+				want = append(want, strings.Join(c, ""))
+			}
+			if test.readDelay == 0 {
+				if !reflect.DeepEqual(got, want) {
+					t.Errorf("unexpected round-trip result for %q:\ngot: %#v\nwant:%#v", test.name, got, want)
+				}
+			} else {
+				// We may have dropped writes in this case.
+				// So just check that we can consume every
+				// received element with reference to what
+				// was sent.
+				// TODO(kortschak): Check that the number of
+				// missing elements matches the number of
+				// dropped writes.
+				var sidx, ridx int
+				var recd string
+				for ridx, recd = range got {
+					for ; sidx < len(want); sidx++ {
+						if recd == want[sidx] {
+							break
+						}
+					}
+				}
+				if ridx != len(got)-1 {
+					t.Errorf("unexpected round-trip result for %q (unexplained element received):\ngot: %#v\nwant:%#v", test.name, got, want)
+				}
+			}
+		}()
+		wg.Wait()
+	}
+}
+
 func BenchmarkRoundTrip(b *testing.B) {
 	const (
 		maxTimeouts = 100
@@ -224,11 +332,6 @@ func BenchmarkRoundTrip(b *testing.B) {
 	// amortised cost.
 	rb := NewBuffer(len, size, timeout)
 
-	// This is hoisted here to ensure the allocation
-	// is not counted since this is outside the control
-	// of the ring buffer.
-	buf := make([]byte, size+1)
-
 	var wg sync.WaitGroup
 	wg.Add(1)
 	go func() {
@@ -252,22 +355,108 @@ func BenchmarkRoundTrip(b *testing.B) {
 				b.Errorf("unexpected read error: %v", err)
 				return
 			}
-		reads:
-			for {
-				n, err := rb.Read(buf)
-				if n != 0 {
+
+			_, err = ioutil.ReadAll(rb)
 			time.Sleep(readDelay) // Simulate slow data processing.
-				}
-				switch err {
-				case nil:
-				case io.EOF:
-					break reads
-				default:
+			if err != nil {
 				b.Errorf("unexpected read error: %v", err)
 				return
 			}
 		}
-		}
+	}()
+
+	data := make([]byte, frameLen)
+
+	b.ResetTimer()
+	b.SetBytes(frameLen)
+
+	var dropped int
+	for i := 0; i < b.N; i++ {
+		time.Sleep(writeDelay) // Simulate slow data capture.
+		_, err := rb.Write(data)
+		switch err {
+		case nil:
+			dropped = 0
+		case ErrDropped:
+			if dropped > maxTimeouts {
+				b.Error("too many write drops")
+				return
+			}
+			dropped++
+		default:
+			b.Errorf("unexpected write error: %v", err)
+			return
+		}
+	}
+
+	rb.Close()
+
+	wg.Wait()
+}
+
+func BenchmarkRoundTripWriterTo(b *testing.B) {
+	const (
+		maxTimeouts = 100
+
+		len     = 50
+		size    = 150e3
+		timeout = 10 * time.Millisecond
+
+		frameLen = 30e3
+
+		writeDelay = 20 * time.Millisecond
+		readDelay  = 50 * time.Millisecond
+	)
+
+	// Allocated prior to timer reset since it is an
+	// amortised cost.
+	rb := NewBuffer(len, size, timeout)
+
+	// This is hoisted here to ensure the allocation
+	// is not counted since this is outside the control
+	// of the ring buffer.
+	buf := bytes.NewBuffer(make([]byte, 0, size+1))
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		var timeouts int
+	elements:
+		for {
+			chunk, err := rb.Next(timeout)
+			switch err {
+			case nil:
+				timeouts = 0
+			case ErrTimeout:
+				if timeouts > maxTimeouts {
+					b.Error("too many timeouts")
+					return
+				}
+				timeouts++
+				continue
+			case io.EOF:
+				break elements
+			default:
+				b.Errorf("unexpected read error: %v", err)
+				return
+			}
+
+			n, err := chunk.WriteTo(buf)
+			if n != 0 {
+				time.Sleep(readDelay) // Simulate slow data processing.
+				buf.Reset()
+			}
+			if err != nil {
+				b.Errorf("unexpected writeto error: %v", err)
+				return
+			}
+			err = chunk.Close()
+			if err != nil {
+				b.Errorf("unexpected close error: %v", err)
+				return
+			}
+		}
 	}()
 
 	data := make([]byte, frameLen)