sstable: refactor sstable layout facilities, add decodeLayout

This commit refactors some of the code surrounding the parsing and construction
of a sstable.Layout. It also adds a new decodeLayout helper that parses an
in-memory sstable's Layout without opening a Reader. Future work will use
decodeLayout to describe the binary layout of a sstable with columnar blocks in
tests.

sstable/block/block.go

@@ -175,6 +175,8 @@ type DataBlockIterator interface {
 // iterators over index blocks. It's currently satisifed by the
 // *rowblk.IndexIter type.
 type IndexBlockIterator interface {
+	// Init initializes the block iterator from the provided block.
+	Init(base.Compare, base.Split, []byte, IterTransforms) error
 	// InitHandle initializes an iterator from the provided block handle.
 	InitHandle(base.Compare, base.Split, BufferHandle, IterTransforms) error
 	// Valid returns true if the iterator is currently positioned at a valid

sstable/colblk/index_block.go

@@ -163,9 +163,19 @@ type IndexIter struct {
 // Assert that IndexIter satisfies the block.IndexBlockIterator interface.
 var _ block.IndexBlockIterator = (*IndexIter)(nil)
 
-// Init initializes an index iterator from the provided reader.
-func (i *IndexIter) Init(r *IndexReader) {
-	*i = IndexIter{r: r, n: int(r.br.header.Rows)}
+// InitReader initializes an index iterator from the provided reader.
+func (i *IndexIter) InitReader(r *IndexReader) {
+	*i = IndexIter{r: r, n: int(r.br.header.Rows), allocReader: i.allocReader}
+}
+
+// Init initializes an iterator from the provided block data slice.
+func (i *IndexIter) Init(
+	cmp base.Compare, split base.Split, blk []byte, transforms block.IterTransforms,
+) error {
+	// TODO(jackson): Handle the transforms.
+	i.allocReader.Init(blk)
+	i.InitReader(&i.allocReader)
+	return nil
 }
 
 // InitHandle initializes an iterator from the provided block handle.
@@ -181,9 +191,7 @@ func (i *IndexIter) InitHandle(
 	// overhead can be material.)
 	i.h.Release()
 	i.h = block
-	i.allocReader.Init(i.h.Get())
-	i.r = &i.allocReader
-	return nil
+	return i.Init(cmp, split, i.h.Get(), transforms)
 }
 
 // RowIndex returns the index of the block entry at the iterator's current

sstable/colblk/index_block_test.go

@@ -46,7 +46,7 @@ func TestIndexBlock(t *testing.T) {
 			return buf.String()
 		case "iter":
 			var it IndexIter
-			it.Init(&r)
+			it.InitReader(&r)
 			for _, line := range strings.Split(d.Input, "\n") {
 				fields := strings.Fields(line)
 				var valid bool

sstable/data_test.go

@@ -66,15 +66,15 @@ func optsFromArgs(td *datadriven.TestData, writerOpts *WriterOptions) error {
 	return nil
 }
 
-func runBuildCmd(
-	td *datadriven.TestData, writerOpts *WriterOptions, cacheSize int,
-) (*WriterMetadata, *Reader, error) {
-	f0 := &objstorage.MemObj{}
+func runBuildMemObjCmd(
+	td *datadriven.TestData, writerOpts *WriterOptions,
+) (*WriterMetadata, *objstorage.MemObj, error) {
+	obj := &objstorage.MemObj{}
 	if err := optsFromArgs(td, writerOpts); err != nil {
 		return nil, nil, err
 	}
 
-	w := NewRawWriter(f0, *writerOpts)
+	w := NewRawWriter(obj, *writerOpts)
 	defer func() {
 		if w != nil {
 			_ = w.Close()
@@ -122,7 +122,10 @@ func runBuildCmd(
 	if err != nil {
 		return nil, nil, err
 	}
+	return meta, obj, nil
+}
 
+func openReader(obj *objstorage.MemObj, writerOpts *WriterOptions, cacheSize int) (*Reader, error) {
 	readerOpts := ReaderOptions{Comparer: writerOpts.Comparer}
 	if writerOpts.FilterPolicy != nil {
 		readerOpts.Filters = map[string]FilterPolicy{
@@ -138,7 +141,17 @@ func runBuildCmd(
 			},
 		})
 	}
-	r, err := NewMemReader(f0.Data(), readerOpts)
+	return NewMemReader(obj.Data(), readerOpts)
+}
+
+func runBuildCmd(
+	td *datadriven.TestData, writerOpts *WriterOptions, cacheSize int,
+) (*WriterMetadata, *Reader, error) {
+	meta, obj, err := runBuildMemObjCmd(td, writerOpts)
+	if err != nil {
+		return nil, nil, err
+	}
+	r, err := openReader(obj, writerOpts, cacheSize)
 	if err != nil {
 		return nil, nil, err
 	}