feat(tsm1): Add Seek API to TSMIndexIterator

Permits random access of the iterator, correctly maintaining state,
so that Next may be called to iterator from a given key.

This API will be used by the schema APIs when a predicate is specified,
typically requiring random access.

tsdb/tsm1/reader_index_iterator.go

@@ -59,6 +59,39 @@ func (t *TSMIndexIterator) Next() bool {
 	return true
 }
 
+// Seek points the iterator at the smallest key greater than or equal to the
+// given key, returning true if it was an exact match. It returns false for
+// ok if the key does not exist.
+func (t *TSMIndexIterator) Seek(key []byte) (exact, ok bool) {
+	t.d.mu.RLock()
+	if n := len(t.d.ro.offsets); t.n != n {
+		t.err, t.ok = fmt.Errorf("Key count changed during iteration"), false
+	}
+	if t.err != nil {
+		t.d.mu.RUnlock()
+		return false, false
+	}
+
+	t.peeked = false
+	t.first = false
+
+	exact, t.ok = t.iter.Seek(key, t.b)
+	if !t.ok {
+		t.d.mu.RUnlock()
+		return false, false
+	}
+
+	t.offset = t.iter.Offset()
+	t.eoffset = t.iter.EntryOffset(t.b)
+	t.d.mu.RUnlock()
+
+	// reset lazy loaded state
+	t.key = nil
+	t.typ = 0
+	t.entries = t.entries[:0]
+	return exact, true
+}
+
 // Peek reports the next key or nil if there is not one or an error happened.
 func (t *TSMIndexIterator) Peek() []byte {
 	if !t.ok || t.err != nil {

tsdb/tsm1/reader_index_iterator_test.go

@@ -51,6 +51,38 @@ func TestIndirectIndexIterator(t *testing.T) {
 	checkEqual(t, iter.Next(), false)
 	checkEqual(t, iter.Err(), error(nil))
 
+	// check can seek and iterate index
+	iter = ind.Iterator(nil)
+	exact, ok := iter.Seek([]byte("cpu2"))
+	checkEqual(t, exact, true)
+	checkEqual(t, ok, true)
+	checkEqual(t, iter.Key(), []byte("cpu2"))
+	checkEqual(t, iter.Type(), BlockInteger)
+	checkEqual(t, iter.Entries(), []IndexEntry{
+		{0, 10, 10, 20},
+		{10, 20, 10, 20},
+	})
+	checkEqual(t, iter.Next(), true)
+	checkEqual(t, iter.Key(), []byte("mem"))
+	checkEqual(t, iter.Next(), false)
+	exact, ok = iter.Seek([]byte("cpu1"))
+	checkEqual(t, exact, true)
+	checkEqual(t, ok, true)
+	checkEqual(t, iter.Key(), []byte("cpu1"))
+	exact, ok = iter.Seek([]byte("cpu3"))
+	checkEqual(t, exact, false)
+	checkEqual(t, ok, true)
+	checkEqual(t, iter.Key(), []byte("mem"))
+	exact, ok = iter.Seek([]byte("cpu0"))
+	checkEqual(t, exact, false)
+	checkEqual(t, ok, true)
+	checkEqual(t, iter.Key(), []byte("cpu1"))
+	exact, ok = iter.Seek([]byte("zzz"))
+	checkEqual(t, exact, false)
+	checkEqual(t, ok, false)
+	checkEqual(t, iter.Next(), false)
+	checkEqual(t, iter.Err(), error(nil))
+
 	// delete the cpu2 key and make sure it's skipped
 	ind.Delete([][]byte{[]byte("cpu2")})
 	iter = ind.Iterator(nil)