colblk: add IndexBlockWriter.UnsafeSeparator

Add an API for retrieving a separator contained in an entry written to a
pending index block. This will be used by the sstable writer when finishing an
index block in a sstable with a two-level index.

sstable/colblk/data_block.go

@@ -181,7 +181,8 @@ func (w *defaultKeyWriter) ComparePrev(key []byte) KeyComparison {
 	// CommonPrefixLen, it would sort after [key].)
 	if cmpv.CommonPrefixLen == cmpv.PrefixLen {
 		// The keys share the same MVCC prefix. Compare the suffixes.
-		cmpv.UserKeyComparison = int32(w.comparer.CompareSuffixes(key[cmpv.PrefixLen:], w.suffixes.LastSlice()))
+		cmpv.UserKeyComparison = int32(w.comparer.CompareSuffixes(key[cmpv.PrefixLen:],
+			w.suffixes.UnsafeGet(w.suffixes.rows-1)))
 		if invariants.Enabled {
 			if !w.comparer.Equal(lp, key[:cmpv.PrefixLen]) {
 				panic(errors.AssertionFailedf("keys have different logical prefixes: %q != %q", lp, key[:cmpv.PrefixLen]))
@@ -206,7 +207,8 @@ func (w *defaultKeyWriter) ComparePrev(key []byte) KeyComparison {
 		// so the UserKeyComparison should be equal to the result of comparing
 		// the prefixes and nonzero.
 		if cmpv.UserKeyComparison == 0 {
-			panic(errors.AssertionFailedf("user keys should not be equal: %q+%q, %q", lp, w.suffixes.LastSlice(), key))
+			panic(errors.AssertionFailedf("user keys should not be equal: %q+%q, %q",
+				lp, w.suffixes.UnsafeGet(w.suffixes.rows-1), key))
 		}
 		if v := w.comparer.Compare(key, lp); v != int(cmpv.UserKeyComparison) {
 			panic(errors.AssertionFailedf("user key comparison mismatch: Compare(%q, %q) = %d ≠ %d",

sstable/colblk/index_block.go

@@ -69,6 +69,11 @@ func (w *IndexBlockWriter) Reset() {
 	w.enc.reset()
 }
 
+// Rows returns the number of entries in the index block so far.
+func (w *IndexBlockWriter) Rows() int {
+	return w.rows
+}
+
 // AddBlockHandle adds a new separator and end offset of a data block to the
 // index block.  Add returns the index of the row.
 //
@@ -85,6 +90,11 @@ func (w *IndexBlockWriter) AddBlockHandle(
 	return idx
 }
 
+// UnsafeSeparator returns the separator of the i'th entry.
+func (w *IndexBlockWriter) UnsafeSeparator(i int) []byte {
+	return w.separators.UnsafeGet(i)
+}
+
 // Size returns the size of the pending index block.
 func (w *IndexBlockWriter) Size() int {
 	off := blockHeaderSize(indexBlockColumnCount, indexBlockCustomHeaderSize)

sstable/colblk/index_block_test.go

@@ -39,9 +39,11 @@ func TestIndexBlock(t *testing.T) {
 				}
 				w.AddBlockHandle([]byte(fields[0]), h, bp)
 			}
+			fmt.Fprintf(&buf, "UnsafeSeparator(Rows()-1) = %q\n", w.UnsafeSeparator(w.Rows()-1))
 			data := w.Finish()
 			r.Init(data)
-			return r.DebugString()
+			fmt.Fprint(&buf, r.DebugString())
+			return buf.String()
 		case "iter":
 			var it IndexIter
 			it.Init(&r)

sstable/colblk/raw_bytes.go

@@ -165,13 +165,18 @@ func (b *RawBytesBuilder) PutConcat(s1, s2 []byte) {
 	b.offsets.Set(b.rows, uint64(len(b.data)))
 }
 
-// LastSlice returns the last slice added to the builder. The returned slice is
+// Rows returns the count of slices that have been added to the builder.
+func (b *RawBytesBuilder) Rows() int {
+	return b.rows
+}
+
+// UnsafeGet returns the i'th slice added to the builder. The returned slice is
 // owned by the builder and must not be mutated.
-func (b *RawBytesBuilder) LastSlice() []byte {
+func (b *RawBytesBuilder) UnsafeGet(i int) []byte {
 	if b.rows == 0 {
 		return nil
 	}
-	return b.data[b.offsets.array.elems.At(b.rows-1):b.offsets.array.elems.At(b.rows)]
+	return b.data[b.offsets.array.elems.At(i):b.offsets.array.elems.At(i+1)]
 }
 
 // Finish writes the serialized byte slices to buf starting at offset. The buf

sstable/colblk/testdata/index_block

@@ -6,6 +6,7 @@ bacitracin 412  212
 banana     632  215    bp5
 bonifide   963  326    bp6
 ----
+UnsafeSeparator(Rows()-1) = "bonifide"
 # index block header
 # columnar block header
 000-001: x 01                   # version 1
@@ -130,6 +131,7 @@ catatonic   102422  20442
 cephalopod  122864   9104   bp4
 coat        293128  32104
 ----
+UnsafeSeparator(Rows()-1) = "coat"
 # index block header
 # columnar block header
 000-001: x 01                   # version 1

sstable/colblk/unsafe_slice.go

@@ -59,7 +59,7 @@ var _ Array[uint64] = UnsafeUints{}
 func DecodeUnsafeUints(b []byte, off uint32, rows int) (_ UnsafeUints, endOffset uint32) {
 	encoding := UintEncoding(b[off])
 	if !encoding.IsValid() {
-		panic(errors.AssertionFailedf("invalid encoding 0x%x", b))
+		panic(errors.AssertionFailedf("invalid encoding 0x%x", b[off:off+1]))
 	}
 	off++
 	var base uint64