Reverts bc1239b, no longer needed to conform to legacy

This fixes the tests and verifies that the fix from bc1239b, which
broke the tests, is no longer needed to conform to legacy compression.

internal/lz4block/blocks.go

@@ -8,12 +8,9 @@ const (
 	Block256Kb
 	Block1Mb
 	Block4Mb
+	Block8Mb = 2 * Block4Mb
 )
 
-// In legacy mode all blocks are compressed regardless
-// of the compressed size: use the bound size.
-var Block8Mb = uint32(CompressBlockBound(8 << 20))
-
 var (
 	BlockPool64K  = sync.Pool{New: func() interface{} { return make([]byte, Block64Kb) }}
 	BlockPool256K = sync.Pool{New: func() interface{} { return make([]byte, Block256Kb) }}

internal/lz4stream/block.go

@@ -224,9 +224,7 @@ func (b *FrameDataBlock) Close(f *Frame) {
 func (b *FrameDataBlock) Compress(f *Frame, src []byte, level lz4block.CompressionLevel) *FrameDataBlock {
 	data := b.data
 	if f.isLegacy() {
-		// In legacy mode, the buffer is sized according to CompressBlockBound,
-		// but only 8Mb is buffered for compression.
-		src = src[:8<<20]
+		data = data[:cap(data)]
 	} else {
 		data = data[:len(src)] // trigger the incompressible flag in CompressBlock
 	}

writer_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"reflect"
 	"strings"
 	"testing"
@@ -285,13 +286,69 @@ func TestWriterLegacy(t *testing.T) {
 			if _, err := io.Copy(out2, zr); err != nil {
 				t.Fatal(err)
 			}
+
+			if len(src) != out2.Len() {
+				t.Fatalf("uncompressed output not correct size. %d != %d", len(src), out2.Len())
+			}
+
 			if !bytes.Equal(out2.Bytes(), src) {
 				t.Fatal("uncompressed compressed output different from source")
 			}
 		})
 	}
 }
 
+func TestWriterLegacyCommand(t *testing.T) {
+	_, err := exec.LookPath("lz4")
+	if err != nil {
+		t.Skip("no lz4 binary to test against")
+	}
+
+	goldenFiles := []string{
+		"testdata/vmlinux_LZ4_19377",
+		"testdata/bzImage_lz4_isolated",
+	}
+
+	for _, fname := range goldenFiles {
+		t.Run(fname, func(t *testing.T) {
+			fname := fname
+			t.Parallel()
+
+			src, err := ioutil.ReadFile(fname)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			out := new(bytes.Buffer)
+			zw := lz4.NewWriter(out)
+			if err := zw.Apply(lz4.LegacyOption(true), lz4.CompressionLevelOption(lz4.Fast)); err != nil {
+				t.Fatal(err)
+			}
+			if _, err := io.Copy(zw, bytes.NewReader(src)); err != nil {
+				t.Fatal(err)
+			}
+			if err := zw.Close(); err != nil {
+				t.Fatal(err)
+			}
+
+			// write to filesystem for further checking
+			tmp, err := ioutil.TempFile("", "")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer os.Remove(tmp.Name())
+			if _, err := tmp.Write(out.Bytes()); err != nil {
+				t.Fatal(err)
+			}
+
+			cmd := exec.Command("lz4", "--test", tmp.Name())
+			if _, err := cmd.Output(); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
 func TestWriterConcurrency(t *testing.T) {
 	const someGiantFile = "testdata/vmlinux_LZ4_19377"