Use io.MultiReader to inject kopia file version (#1767)

## Description

Instead of rolling our own logic for injecting a version, use a
MultiReader to concatenate the streams. Handling Close() is now more
complex though.

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [x] 🐹 Trivial/Minor

## Issue(s)

* closes #1766 

## Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/upload.go

@@ -1,9 +1,11 @@
 package kopia
 
 import (
+	"bytes"
 	"context"
 	"encoding/binary"
 	"io"
+	"os"
 	"runtime/trace"
 	"sync"
 	"sync/atomic"
@@ -25,35 +27,47 @@ import (
 
 var versionSize = int(unsafe.Sizeof(serializationVersion))
 
+func newBackupStreamReader(version uint32, reader io.ReadCloser) *backupStreamReader {
+	buf := make([]byte, versionSize)
+	binary.BigEndian.PutUint32(buf, version)
+	bufReader := io.NopCloser(bytes.NewReader(buf))
+
+	return &backupStreamReader{
+		readers:  []io.ReadCloser{bufReader, reader},
+		combined: io.NopCloser(io.MultiReader(bufReader, reader)),
+	}
+}
+
 // backupStreamReader is a wrapper around the io.Reader that other Corso
 // components return when backing up information. It injects a version number at
 // the start of the data stream. Future versions of Corso may not need this if
 // they use more complex serialization logic as serialization/version injection
 // will be handled by other components.
 type backupStreamReader struct {
-	io.ReadCloser
-	version   uint32
-	readBytes int
+	readers  []io.ReadCloser
+	combined io.ReadCloser
 }
 
 func (rw *backupStreamReader) Read(p []byte) (n int, err error) {
-	if rw.readBytes < versionSize {
-		marshalled := make([]byte, versionSize)
-
-		toCopy := len(marshalled) - rw.readBytes
-		if len(p) < toCopy {
-			toCopy = len(p)
-		}
-
-		binary.BigEndian.PutUint32(marshalled, rw.version)
-
-		copy(p, marshalled[rw.readBytes:rw.readBytes+toCopy])
-		rw.readBytes += toCopy
-
-		return toCopy, nil
+	if rw.combined == nil {
+		return 0, os.ErrClosed
 	}
 
-	return rw.ReadCloser.Read(p)
+	return rw.combined.Read(p)
+}
+
+func (rw *backupStreamReader) Close() error {
+	if rw.combined == nil {
+		return nil
+	}
+
+	rw.combined = nil
+
+	for _, r := range rw.readers {
+		r.Close()
+	}
+
+	return nil
 }
 
 // restoreStreamReader is a wrapper around the io.Reader that kopia returns when
@@ -169,11 +183,11 @@ func (cp *corsoProgress) FinishedFile(relativePath string, err error) {
 }
 
 // Kopia interface function used as a callback when kopia finishes hashing a file.
-func (cp *corsoProgress) FinishedHashingFile(fname string, bytes int64) {
+func (cp *corsoProgress) FinishedHashingFile(fname string, bs int64) {
 	// Pass the call through as well so we don't break expected functionality.
-	defer cp.UploadProgress.FinishedHashingFile(fname, bytes)
+	defer cp.UploadProgress.FinishedHashingFile(fname, bs)
 
-	atomic.AddInt64(&cp.totalBytes, bytes)
+	atomic.AddInt64(&cp.totalBytes, bs)
 }
 
 func (cp *corsoProgress) put(k string, v *itemDetails) {
@@ -275,10 +289,7 @@ func collectionEntries(
 			entry := virtualfs.StreamingFileWithModTimeFromReader(
 				encodeAsPath(e.UUID()),
 				modTime,
-				&backupStreamReader{
-					version:    serializationVersion,
-					ReadCloser: e.ToReader(),
-				},
+				newBackupStreamReader(serializationVersion, e.ToReader()),
 			)
 			if err := cb(ctx, entry); err != nil {
 				// Kopia's uploader swallows errors in most cases, so if we see

src/internal/kopia/upload_test.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	stdpath "path"
 	"testing"
-	"unsafe"
 
 	"github.com/kopia/kopia/fs"
 	"github.com/kopia/kopia/snapshot/snapshotfs"
@@ -116,10 +115,10 @@ func (suite *VersionReadersUnitSuite) TestWriteAndRead() {
 
 			reversible := &restoreStreamReader{
 				expectedVersion: test.readVersion,
-				ReadCloser: &backupStreamReader{
-					version:    test.writeVersion,
-					ReadCloser: io.NopCloser(baseReader),
-				},
+				ReadCloser: newBackupStreamReader(
+					test.writeVersion,
+					io.NopCloser(baseReader),
+				),
 			}
 
 			defer reversible.Close()
@@ -165,11 +164,8 @@ func (suite *VersionReadersUnitSuite) TestWriteHandlesShortReads() {
 	inputData := []byte("This is some data for the reader to test with")
 	version := uint32(42)
 	baseReader := bytes.NewReader(inputData)
-	versioner := &backupStreamReader{
-		version:    version,
-		ReadCloser: io.NopCloser(baseReader),
-	}
-	expectedToWrite := len(inputData) + int(unsafe.Sizeof(versioner.version))
+	versioner := newBackupStreamReader(version, io.NopCloser(baseReader))
+	expectedToWrite := len(inputData) + int(versionSize)
 
 	// "Write" all the data.
 	versionedData, writtenLen := readAllInParts(t, 1, versioner)