Flaky unit test: TestCopyGraph_WithOptions/MountFrom_error

[Wwwsylvia]

The unit test TestCopyGraph_WithOptions/MountFrom_error in copy_test.go has been flaky in recent builds.
Here is an instance: https://github.com/oras-project/oras-go/actions/runs/13639868900/job/38127074239?pr=906

=== RUN TestCopyGraph_WithOptions/MountFrom_error
copy_test.go:1810: count(Exists()) = 3, want 2
copy_test.go:1819: count(MountFrom()) = 2, want 1
=== RUN TestCopyGraph_WithOptions/MountFrom_OnMounted_error
--- FAIL: TestCopyGraph_WithOptions (0.01s)
--- PASS: TestCopyGraph_WithOptions/SkipNode (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_mounted (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_copied (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_mounted_second_try (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_copied_dst_not_a_Mounter (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_empty_sourceRepositories (0.00s)
--- FAIL: TestCopyGraph_WithOptions/MountFrom_error (0.00s)
--- PASS: TestCopyGraph_WithOptions/MountFrom_OnMounted_error (0.00s)

oras-go/copy_test.go

Lines 1792 to 1821 in f3f718b

	t.Run("MountFrom error", func(t *testing.T) {
	root = descs[3]
	dst := &countingStorage{storage: cas.NewMemory()}
	opts = oras.CopyGraphOptions{
	// to make the run result deterministic, we limit concurrency to 1
	Concurrency: 1,
	}
	var numMountFrom atomic.Int64
	e := errors.New("mountFrom error")
	opts.MountFrom = func(ctx context.Context, desc ocispec.Descriptor) ([]string, error) {
	numMountFrom.Add(1)
	return nil, e
	}
	if err := oras.CopyGraph(ctx, src, dst, root, opts); !errors.Is(err, e) {
	t.Fatalf("CopyGraph() error = %v, wantErr %v", err, e)
	}
	if got, expected := dst.numExists.Load(), int64(2); got != expected {
	t.Errorf("count(Exists()) = %d, want %d", got, expected)
	}
	if got, expected := dst.numFetch.Load(), int64(0); got != expected {
	t.Errorf("count(Fetch()) = %d, want %d", got, expected)
	}
	if got, expected := dst.numPush.Load(), int64(0); got != expected {
	t.Errorf("count(Push()) = %d, want %d", got, expected)
	}
	if got, expected := numMountFrom.Load(), int64(1); got != expected {
	t.Errorf("count(MountFrom()) = %d, want %d", got, expected)
	}
	})