br: fix get snapshot response pointer deref panic (#54510) (#54712)

close #54511

br/pkg/aws/ebs.go

@@ -252,7 +252,7 @@ func (e *EC2Session) WaitSnapshotsCreated(snapIDMap map[string]string, progress
 				log.Info("snapshot completed", zap.String("id", *s.SnapshotId))
 				totalVolumeSize += *s.VolumeSize
 			} else if *s.State == ec2.SnapshotStateError {
-				log.Error("snapshot failed", zap.String("id", *s.SnapshotId), zap.String("error", (*s.StateMessage)))
+				log.Error("snapshot failed", zap.String("id", *s.SnapshotId), zap.String("error", utils.GetOrZero(s.StateMessage)))
 				return 0, errors.Errorf("snapshot %s failed", *s.SnapshotId)
 			} else {
 				log.Debug("snapshot creating...", zap.Stringer("snap", s))

br/pkg/aws/ebs_test.go

@@ -145,6 +145,25 @@ func TestWaitSnapshotsCreated(t *testing.T) {
 			expectedSize: 0,
 			expectErr:    true,
 		},
+		{
+			desc: "snapshot failed w/out state message",
+			snapshotsOutput: ec2.DescribeSnapshotsOutput{
+				Snapshots: []*ec2.Snapshot{
+					{
+						SnapshotId: awsapi.String("snap-1"),
+						VolumeSize: awsapi.Int64(1),
+						State:      awsapi.String(ec2.SnapshotStateCompleted),
+					},
+					{
+						SnapshotId:   awsapi.String("snap-2"),
+						State:        awsapi.String(ec2.SnapshotStateError),
+						StateMessage: nil,
+					},
+				},
+			},
+			expectedSize: 0,
+			expectErr:    true,
+		},
 		{
 			desc: "snapshots pending",
 			snapshotsOutput: ec2.DescribeSnapshotsOutput{

br/pkg/utils/BUILD.bazel

@@ -14,6 +14,7 @@ go_library(
         "math.go",
         "misc.go",
         "permission.go",
+        "pointer.go",
         "pprof.go",
         "progress.go",
         "retry.go",

br/pkg/utils/pointer.go

@@ -0,0 +1,13 @@
+// Copyright 2022 PingCAP, Inc. Licensed under Apache-2.0.
+
+package utils
+
+// GetOrZero returns the value pointed to by p, or a zero value of
+// its type if p is nil.
+func GetOrZero[T any](p *T) T {
+	var zero T
+	if p == nil {
+		return zero
+	}
+	return *p
+}