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>

content/helpers_test.go

@@ -0,0 +1,112 @@
+package content
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/containerd/containerd/errdefs"
+	"github.com/opencontainers/go-digest"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type copySource struct {
+	reader io.Reader
+	size   int64
+	digest digest.Digest
+}
+
+func TestCopy(t *testing.T) {
+	defaultSource := newCopySource("this is the source to copy")
+
+	var testcases = []struct {
+		name     string
+		source   copySource
+		writer   fakeWriter
+		expected string
+	}{
+		{
+			name:     "copy no offset",
+			source:   defaultSource,
+			writer:   fakeWriter{},
+			expected: "this is the source to copy",
+		},
+		{
+			name:     "copy with offset from seeker",
+			source:   defaultSource,
+			writer:   fakeWriter{status: Status{Offset: 8}},
+			expected: "the source to copy",
+		},
+		{
+			name:     "copy with offset from unseekable source",
+			source:   copySource{reader: bytes.NewBufferString("foo"), size: 3},
+			writer:   fakeWriter{status: Status{Offset: 8}},
+			expected: "foo",
+		},
+		{
+			name:   "commit already exists",
+			source: defaultSource,
+			writer: fakeWriter{commitFunc: func() error {
+				return errdefs.ErrAlreadyExists
+			}},
+		},
+	}
+
+	for _, testcase := range testcases {
+		t.Run(testcase.name, func(t *testing.T) {
+			err := Copy(context.Background(),
+				&testcase.writer,
+				testcase.source.reader,
+				testcase.source.size,
+				testcase.source.digest)
+
+			require.NoError(t, err)
+			assert.Equal(t, testcase.source.digest, testcase.writer.commitedDigest)
+			assert.Equal(t, testcase.expected, testcase.writer.String())
+		})
+	}
+}
+
+func newCopySource(raw string) copySource {
+	return copySource{
+		reader: strings.NewReader(raw),
+		size:   int64(len(raw)),
+		digest: digest.FromBytes([]byte(raw)),
+	}
+}
+
+type fakeWriter struct {
+	bytes.Buffer
+	commitedDigest digest.Digest
+	status         Status
+	commitFunc     func() error
+}
+
+func (f *fakeWriter) Close() error {
+	f.Buffer.Reset()
+	return nil
+}
+
+func (f *fakeWriter) Commit(ctx context.Context, size int64, expected digest.Digest, opts ...Opt) error {
+	f.commitedDigest = expected
+	if f.commitFunc == nil {
+		return nil
+	}
+	return f.commitFunc()
+}
+
+func (f *fakeWriter) Digest() digest.Digest {
+	return f.commitedDigest
+}
+
+func (f *fakeWriter) Status() (Status, error) {
+	return f.status, nil
+}
+
+func (f *fakeWriter) Truncate(size int64) error {
+	f.Buffer.Truncate(int(size))
+	return nil
+}