Ensure bufferWriter is always closed in Migration.Buffer and propagate close errors (#1308)

* Ensure bufferWriter is always closed and error is returned in Buffer()

* Ensure bufferWriter is always closed and error is returned in Buffer()

* addressed review comments. used named error and joined error to return final error

* Fixed defer block

---------

Co-authored-by: Chandra kant <chandra.kant@cohesity.com>

migration.go

@@ -2,6 +2,7 @@ package migrate
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"time"
@@ -118,7 +119,7 @@ func (m *Migration) LogString() string {
 
 // Buffer buffers Body up to BufferSize.
 // Calling this function blocks. Call with goroutine.
-func (m *Migration) Buffer() error {
+func (m *Migration) Buffer() (berr error) {
 	if m.Body == nil {
 		return nil
 	}
@@ -127,6 +128,21 @@ func (m *Migration) Buffer() error {
 
 	b := bufio.NewReaderSize(m.Body, int(m.BufferSize))
 
+	// defer closing buffer writer and body.
+	defer func() {
+		// close bufferWriter so Buffer knows that there is no
+		// more data coming.
+		if err := m.bufferWriter.Close(); err != nil {
+			berr = errors.Join(berr, err)
+		}
+
+		// it's safe to close the Body too.
+		if err := m.Body.Close(); err != nil {
+			berr = errors.Join(berr, err)
+		}
+
+	}()
+
 	// start reading from body, peek won't move the read pointer though
 	// poor man's solution?
 	if _, err := b.Peek(int(m.BufferSize)); err != nil && err != io.EOF {
@@ -145,16 +161,5 @@ func (m *Migration) Buffer() error {
 	m.FinishedReading = time.Now()
 	m.BytesRead = n
 
-	// close bufferWriter so Buffer knows that there is no
-	// more data coming
-	if err := m.bufferWriter.Close(); err != nil {
-		return err
-	}
-
-	// it's safe to close the Body too
-	if err := m.Body.Close(); err != nil {
-		return err
-	}
-
 	return nil
 }