Test image with a backing file

We cannot have useful tests for raw backing file since we have only a
stub implementation reporting that the entire backing file is allocated,
so we test only qcow2 image backing file.

We test backing file in qcow2 and qcow2 compressed. When both images are
uncompressed, allocated extents from top and base are merged. When base
is compressed, allocated extents from top and based cannot be merged,
since all base clusters are compressed.

Compressed image with backing file may be possible to crate with
qemu-io, but is not real use case so we don't test it.

Signed-off-by: Nir Soffer <nirsof@gmail.com>

qcow2reader_test.go

@@ -431,6 +431,215 @@ func TestExtentsZero(t *testing.T) {
 	})
 }
 
+func TestExtentsBackingFile(t *testing.T) {
+	// Create an image with some clusters in the backing file, and some cluasters
+	// in the image. Accessing extents should present a unified view using both
+	// image and backing file.
+	tmpDir := t.TempDir()
+	baseExtents := []image.Extent{
+		{
+			Start:     0 * clusterSize,
+			Length:    1 * clusterSize,
+			Allocated: true,
+		},
+		{
+			Start:  1 * clusterSize,
+			Length: 9 * clusterSize,
+			Zero:   true,
+		},
+		{
+			Start:     10 * clusterSize,
+			Length:    2 * clusterSize,
+			Allocated: true,
+		},
+		{
+			Start:  12 * clusterSize,
+			Length: 88 * clusterSize,
+			Zero:   true,
+		},
+		{
+			Start:     100 * clusterSize,
+			Length:    1 * clusterSize,
+			Allocated: true,
+		},
+		{
+			Start:  101 * clusterSize,
+			Length: 899 * clusterSize,
+			Zero:   true,
+		},
+	}
+	topExtents := []image.Extent{
+		{
+			Start:  0 * clusterSize,
+			Length: 1 * clusterSize,
+			Zero:   true,
+		},
+		{
+			Start:     1 * clusterSize,
+			Length:    1 * clusterSize,
+			Allocated: true,
+		},
+		{
+			Start:  2 * clusterSize,
+			Length: 9 * clusterSize,
+			Zero:   true,
+		},
+		{
+			Start:     11 * clusterSize,
+			Length:    2 * clusterSize,
+			Allocated: true,
+		},
+		{
+			Start:  13 * clusterSize,
+			Length: 986 * clusterSize,
+			Zero:   true,
+		},
+		{
+			Start:     999 * clusterSize,
+			Length:    1 * clusterSize,
+			Allocated: true,
+		},
+	}
+	baseRaw := filepath.Join(tmpDir, "base.raw")
+	if err := createTestImageWithExtents(baseRaw, qemuimg.FormatRaw, baseExtents, "", ""); err != nil {
+		t.Fatal(err)
+	}
+
+	t.Run("qcow2", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		baseQcow2 := filepath.Join(tmpDir, "base.qcow2")
+		if err := qemuimg.Convert(baseRaw, baseQcow2, qemuimg.FormatQcow2, qemuimg.CompressionNone); err != nil {
+			t.Fatal(err)
+		}
+		top := filepath.Join(tmpDir, "top.qcow2")
+		if err := createTestImageWithExtents(top, qemuimg.FormatQcow2, topExtents, baseQcow2, qemuimg.FormatQcow2); err != nil {
+			t.Fatal(err)
+		}
+		// When top and base are uncompressed, extents from to and based are merged.
+		expected := []image.Extent{
+			{
+				Start:     0 * clusterSize,
+				Length:    2 * clusterSize,
+				Allocated: true,
+			},
+			{
+				Start:  2 * clusterSize,
+				Length: 8 * clusterSize,
+				Zero:   true,
+			},
+			{
+				Start:     10 * clusterSize,
+				Length:    3 * clusterSize,
+				Allocated: true,
+			},
+			{
+				Start:  13 * clusterSize,
+				Length: 87 * clusterSize,
+				Zero:   true,
+			},
+			{
+				Start:     100 * clusterSize,
+				Length:    1 * clusterSize,
+				Allocated: true,
+			},
+			{
+				Start:  101 * clusterSize,
+				Length: 898 * clusterSize,
+				Zero:   true,
+			},
+			{
+				Start:     999 * clusterSize,
+				Length:    1 * clusterSize,
+				Allocated: true,
+			},
+		}
+		actual, err := listExtents(top)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !slices.Equal(expected, actual) {
+			t.Fatalf("expected %v, got %v", expected, actual)
+		}
+	})
+	t.Run("qcow2 zlib", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		baseQcow2Zlib := filepath.Join(tmpDir, "base.qcow2")
+		if err := qemuimg.Convert(baseRaw, baseQcow2Zlib, qemuimg.FormatQcow2, qemuimg.CompressionZlib); err != nil {
+			t.Fatal(err)
+		}
+		top := filepath.Join(tmpDir, "top.qcow2")
+		if err := createTestImageWithExtents(top, qemuimg.FormatQcow2, topExtents, baseQcow2Zlib, qemuimg.FormatQcow2); err != nil {
+			t.Fatal(err)
+		}
+		// When base is compressed, extents from to and based cannot be merged since
+		// allocated extents from base are compressed. When copying we can merge
+		// extents with different types that read as zero.
+		expected := []image.Extent{
+			// From base
+			{
+				Start:      0 * clusterSize,
+				Length:     1 * clusterSize,
+				Allocated:  true,
+				Compressed: true,
+			},
+			// From top
+			{
+				Start:     1 * clusterSize,
+				Length:    1 * clusterSize,
+				Allocated: true,
+			},
+			{
+				Start:  2 * clusterSize,
+				Length: 8 * clusterSize,
+				Zero:   true,
+			},
+			// From base
+			{
+				Start:      10 * clusterSize,
+				Length:     1 * clusterSize,
+				Allocated:  true,
+				Compressed: true,
+			},
+			// From top (top clusters hide base clusters)
+			{
+				Start:     11 * clusterSize,
+				Length:    2 * clusterSize,
+				Allocated: true,
+			},
+			{
+				Start:  13 * clusterSize,
+				Length: 87 * clusterSize,
+				Zero:   true,
+			},
+			// From base
+			{
+				Start:      100 * clusterSize,
+				Length:     1 * clusterSize,
+				Allocated:  true,
+				Compressed: true,
+			},
+			{
+				Start:  101 * clusterSize,
+				Length: 898 * clusterSize,
+				Zero:   true,
+			},
+			// From top
+			{
+				Start:     999 * clusterSize,
+				Length:    1 * clusterSize,
+				Allocated: true,
+			},
+		}
+		actual, err := listExtents(top)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !slices.Equal(expected, actual) {
+			t.Fatalf("expected %v, got %v", expected, actual)
+		}
+	})
+}
+
 func compressed(extents []image.Extent) []image.Extent {
 	var res []image.Extent
 	for _, extent := range extents {