Fix spurious error from "docker load"

"docker load" validates parent links by comparing image histories, and the
History struct has a time.Time member "Created". Time.UnmarshalJSON can read
RFC3339 timestamps with offset "+00:00", but t.MarshalJSON writes them with
offset "Z". Equivalent times in these two formats are not equal when compared
with the == operator.

This causes checkValidParent to incorrectly return false when the parent image
history contains times using offset "+00:00". In that case the history copied
to the child image will have been converted into "Z" form when marshaled out.

This patch adds an "Equal" method to History, which compares "Created" times
with t.Equal. This is used instead of reflect.DeepEqual in checkValidParent.

Signed-off-by: Rob Cowsill <42620235+rcowsill@users.noreply.github.com>

image/image.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
+	"reflect"
 	"runtime"
 	"strings"
 	"time"
@@ -218,6 +219,16 @@ func NewHistory(author, comment, createdBy string, isEmptyLayer bool) History {
 	}
 }
 
+// Equal compares two history structs for equality
+func (h History) Equal(i History) bool {
+	if !h.Created.Equal(i.Created) {
+		return false
+	}
+	i.Created = h.Created
+
+	return reflect.DeepEqual(h, i)
+}
+
 // Exporter provides interface for loading and saving images
 type Exporter interface {
 	Load(io.ReadCloser, io.Writer, bool) error

image/image_test.go

@@ -56,6 +56,29 @@ func TestMarshalKeyOrder(t *testing.T) {
 	}
 }
 
+const sampleHistoryJSON = `{
+	"created": "2021-01-13T09:35:56Z",
+	"created_by": "image_test.go"
+}`
+
+func TestHistoryEqual(t *testing.T) {
+	h := historyFromJSON(t, sampleHistoryJSON)
+	hCopy := h
+	assert.Check(t, h.Equal(hCopy))
+
+	hUTC := historyFromJSON(t, `{"created": "2021-01-13T14:00:00Z"}`)
+	hOffset0 := historyFromJSON(t, `{"created": "2021-01-13T14:00:00+00:00"}`)
+	assert.Check(t, hUTC.Created != hOffset0.Created)
+	assert.Check(t, hUTC.Equal(hOffset0))
+}
+
+func historyFromJSON(t *testing.T, historyJSON string) History {
+	var h History
+	err := json.Unmarshal([]byte(historyJSON), &h)
+	assert.Check(t, err)
+	return h
+}
+
 func TestImage(t *testing.T) {
 	cid := "50a16564e727"
 	config := &container.Config{

image/tarexport/load.go

@@ -8,7 +8,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"reflect"
 	"runtime"
 
 	"github.com/containerd/containerd/platforms"
@@ -406,7 +405,7 @@ func checkValidParent(img, parent *image.Image) bool {
 		return false
 	}
 	for i, h := range parent.History {
-		if !reflect.DeepEqual(h, img.History[i]) {
+		if !h.Equal(img.History[i]) {
 			return false
 		}
 	}