blob: pass through reader/writer to WriteTo/ReadFrom if available

The blob `Reader` and `Writer` implement `WriterTo` and `ReaderFrom`,
respectively, which is meant to avoid intermediary allocations and
copies (passing readers and readers through all the way down).
Unfortunately, the current implementation falls short and implements
those interfaces by making a local copy ala io.Copy.

Instead, reproduce the strategy in newer versions of `io.Copy`: reflect
on the reader/writer provided by the user, and if they implement these
interface, too, then call into that.

Local benchmarks show significant performance improvements:

```
                         │   /tmp/old   │              /tmp/new               │
                         │    sec/op    │   sec/op     vs base                │
Memblob/BenchmarkRead-10   3.449µ ± 15%   1.919µ ± 2%  -44.37% (p=0.000 n=10)
```

blob/blob.go

@@ -233,6 +233,12 @@ func (r *Reader) As(i interface{}) bool {
 //
 // It implements the io.WriterTo interface.
 func (r *Reader) WriteTo(w io.Writer) (int64, error) {
+	// If the writer has a ReaderFrom method, use it to do the copy.
+	// Avoids an allocation and a copy.
+	if rt, ok := w.(io.ReaderFrom); ok {
+		return rt.ReadFrom(r)
+	}
+
 	_, nw, err := readFromWriteTo(r, w)
 	return nw, err
 }
@@ -476,6 +482,12 @@ func (w *Writer) write(p []byte) (int, error) {
 //
 // It implements the io.ReaderFrom interface.
 func (w *Writer) ReadFrom(r io.Reader) (int64, error) {
+	// If the reader has a WriteTo method, use it to do the copy.
+	// Avoids an allocation and a copy.
+	if wt, ok := r.(io.WriterTo); ok {
+		return wt.WriteTo(w)
+	}
+
 	nr, _, err := readFromWriteTo(r, w)
 	return nr, err
 }

blob/blob_reader_test.go

@@ -15,7 +15,9 @@
 package blob_test
 
 import (
+	"bytes"
 	"context"
+	"io"
 	"testing"
 	"testing/iotest"
 
@@ -41,12 +43,29 @@ func TestReader(t *testing.T) {
 	bucket.WriteAll(ctx, myKey, data, nil)
 
 	// Create a blob.Reader.
-	r, err := bucket.NewReader(ctx, myKey, nil)
+	r1, err := bucket.NewReader(ctx, myKey, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer r.Close()
-	if err := iotest.TestReader(r, data); err != nil {
+	r1.Close()
+	if err := iotest.TestReader(r1, data); err != nil {
 		t.Error(err)
 	}
+
+	// Create another blob.Reader to exercise the ReadFrom code path
+	r2, err := bucket.NewReader(ctx, myKey, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r2.Close()
+
+	var buffer bytes.Buffer
+	n, err := io.Copy(&buffer, r2)
+	if err != nil {
+		t.Fatal(err)
+	} else if n != int64(len(data)) {
+		t.Fatal("wrote fewer bytes than expected")
+	} else if !bytes.Equal(buffer.Bytes(), data) {
+		t.Fatal("wrote invalid bytes")
+	}
 }

blob/drivertest/drivertest.go

@@ -2624,12 +2624,21 @@ func benchmarkRead(b *testing.B, bkt *blob.Bucket) {
 
 	b.ResetTimer()
 	b.RunParallel(func(pb *testing.PB) {
+		var buffer bytes.Buffer
+		buffer.Grow(len(content))
+
 		for pb.Next() {
-			buf, err := bkt.ReadAll(ctx, key)
+			buffer.Reset()
+			r, err := bkt.NewReader(ctx, key, nil)
 			if err != nil {
 				b.Error(err)
 			}
-			if !bytes.Equal(buf, content) {
+
+			if _, err = io.Copy(&buffer, r); err != nil {
+				b.Error(err)
+			}
+			r.Close()
+			if !bytes.Equal(buffer.Bytes(), content) {
 				b.Error("read didn't match write")
 			}
 		}