indexers: add ResetFlatFileState()

ResetFlatFileState() allows the caller to reset the flatfile state so
that all the data can be removed and the flatfile state restored to the
initial state.

blockchain/indexers/flatfile.go

@@ -387,6 +387,26 @@ func (ff *FlatFileState) BestHeight() int32 {
 	return ff.currentHeight
 }
 
+// ResetFlatFileState removes all of the data in the files and resets the
+// state to 0.
+func (ff *FlatFileState) ResetFlatFileState() error {
+	ff.mtx.Lock()
+	defer ff.mtx.Unlock()
+
+	ff.currentHeight = 0
+	ff.currentOffset = 0
+	ff.offsets = ff.offsets[:1]
+
+	err := ff.dataFile.Truncate(0)
+	if err != nil {
+		return err
+	}
+
+	// We truncate to 8 because each offset is 8 bytes and we reserve the
+	// space for the genesis block.
+	return ff.offsetFile.Truncate(8)
+}
+
 // deleteFileFile removes the flat file state directory and all the contents
 // in it.
 func deleteFlatFile(path string) error {

blockchain/indexers/flatfile_test.go

@@ -846,3 +846,62 @@ func TestRecover(t *testing.T) {
 		}
 	}
 }
+
+func TestReset(t *testing.T) {
+	t.Parallel()
+
+	rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	ff, tmpDir, err := initFF("TestReset")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir) // clean up. Always runs
+
+	for i := 0; i < 100; i++ {
+		data, err := createRandByteSlice(rnd)
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = ff.StoreData(int32(i)+1, data)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	err = ff.ResetFlatFileState()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Check that we've successfully reset the flat file.
+	if ff.currentHeight != 0 {
+		t.Fatalf("expected 0 but got %v", ff.currentHeight)
+	}
+
+	if ff.currentOffset != 0 {
+		t.Fatalf("expected 0 but got %v", ff.currentOffset)
+	}
+
+	if len(ff.offsets) != 1 {
+		t.Fatalf("expected 1 but got %v", len(ff.offsets))
+	}
+
+	dataFileSize, err := ff.dataFile.Seek(0, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if dataFileSize != 0 {
+		t.Fatalf("expected 0 but got %v", dataFileSize)
+	}
+
+	offsetFileSize, err := ff.offsetFile.Seek(0, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if offsetFileSize != 8 {
+		t.Fatalf("expected 8 but got %v", offsetFileSize)
+	}
+}