Add staticcheck linter

Fix issues with sync.Pool being passed an array and not a pointer.
See https://github.com/dominikh/go-tools/blob/master/cmd/staticcheck/docs/checks/SA6002

Add missing tests for content.Copy

Fix T.Fatal being called in a goroutine

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

archive/tar.go

@@ -19,13 +19,12 @@ import (
 	"github.com/pkg/errors"
 )
 
-var (
-	bufferPool = &sync.Pool{
-		New: func() interface{} {
-			return make([]byte, 32*1024)
-		},
-	}
-)
+var bufferPool = &sync.Pool{
+	New: func() interface{} {
+		buffer := make([]byte, 32*1024)
+		return &buffer
+	},
+}
 
 // Diff returns a tar stream of the computed filesystem
 // difference between the provided directories.
@@ -404,8 +403,8 @@ func (cw *changeWriter) HandleChange(k fs.ChangeKind, p string, f os.FileInfo, e
 			}
 			defer file.Close()
 
-			buf := bufferPool.Get().([]byte)
-			n, err := io.CopyBuffer(cw.tw, file, buf)
+			buf := bufferPool.Get().(*[]byte)
+			n, err := io.CopyBuffer(cw.tw, file, *buf)
 			bufferPool.Put(buf)
 			if err != nil {
 				return errors.Wrap(err, "failed to copy")
@@ -529,7 +528,7 @@ func createTarFile(ctx context.Context, path, extractDir string, hdr *tar.Header
 }
 
 func copyBuffered(ctx context.Context, dst io.Writer, src io.Reader) (written int64, err error) {
-	buf := bufferPool.Get().([]byte)
+	buf := bufferPool.Get().(*[]byte)
 	defer bufferPool.Put(buf)
 
 	for {
@@ -540,9 +539,9 @@ func copyBuffered(ctx context.Context, dst io.Writer, src io.Reader) (written in
 		default:
 		}
 
-		nr, er := src.Read(buf)
+		nr, er := src.Read(*buf)
 		if nr > 0 {
-			nw, ew := dst.Write(buf[0:nr])
+			nw, ew := dst.Write((*buf)[0:nr])
 			if nw > 0 {
 				written += int64(nw)
 			}