quad subject nil after transaction

[schwankl]

Description

quad subject nil after removing 2 quads with same subject and adding a new one the same transaction.

starting with empty store:

2019/11/06 09:18:12 after first transaction:
2019/11/06 09:18:12 (quad.Quad) "a" -- "to" -> "b"
2019/11/06 09:18:12 (quad.Quad) "a" -- "updated" -> "2019-11-06T11:45:26Z"^^<schema:DateTime>
2019/11/06 09:18:12 after second transaction:
2019/11/06 09:18:12 (quad.Quad) <nil> -- "updated" -> "2019-11-06T11:46:26Z"^^<schema:DateTime>
2019/11/06 09:18:12 expected:
2019/11/06 09:18:12 (quad.Quad) "a" -- "updated" -> "2019-11-06T11:46:26Z"^^<schema:DateTime>

Steps to reproduce the issue:

package main

import (
	"log"
	"os"
	"time"

	"github.com/cayleygraph/cayley"
	"github.com/cayleygraph/cayley/graph"
	_ "github.com/cayleygraph/cayley/graph/kv/all"
	"github.com/cayleygraph/quad"

	"github.com/davecgh/go-spew/spew"
)

const (
	dbpath = ".db/leveldb"
	dbtype = "leveldb"
//	dbpath = ".db/bolt"
//	dbtype = "bolt"
)

func main() {
	dbExists, err := exists(dbpath)
	if err != nil {
		log.Fatal(err)
	}

	if !dbExists {
		// initialize the database
		err := graph.InitQuadStore(dbtype, dbpath, nil)
		if err != nil {
			if err != graph.ErrDatabaseExists {
				log.Fatal(err)
			}
		}
	}

	store, err := cayley.NewGraph(dbtype, dbpath, nil)
	if err != nil {
		log.Fatal(err)
	}

	t0, err := time.Parse(time.RFC3339, "2019-11-06T11:45:26.371Z")
	t1, err := time.Parse(time.RFC3339, "2019-11-06T11:46:26.371Z")

	addQuads := []quad.Quad{
		quad.Make("a", "to", "b", nil),
		quad.Make("a", "updated", t0, nil),
	}

	t := cayley.NewTransaction()
	for _, q := range addQuads {
		t.AddQuad(q)
	}
	err = store.ApplyTransaction(t)
	if err != nil {
		log.Fatal(err)
	}

	log.Printf("after first transaction:")
	it := store.QuadsAllIterator()
	for it.Next(nil) {
		qu := store.Quad(it.Result())
		log.Printf(spew.Sdump(qu))
	}

	rmQuads := []quad.Quad{
		quad.Make("a", "to", "b", nil),
		quad.Make("a", "updated", t0, nil),
	}

	addQuads = []quad.Quad{
		quad.Make("a", "updated", t1, nil),
	}

	t = cayley.NewTransaction()
	for _, q := range rmQuads {
		t.RemoveQuad(q)
	}
	for _, q := range addQuads {
		t.AddQuad(q)
	}
	err = store.ApplyTransaction(t)
	if err != nil {
		log.Fatal(err)
	}

	log.Printf("after second transaction:")
	it = store.QuadsAllIterator()
	for it.Next(nil) {
		qu := store.Quad(it.Result())
		log.Printf(spew.Sdump(qu))
	}

	log.Printf("expected:")
	for _, q := range addQuads {
		log.Printf(spew.Sdump(q))
	}
}


// exists returns whether the given file or directory exists
func exists(path string) (bool, error) {
	_, err := os.Stat(path)
	if err == nil {
		return true, nil
	}
	if os.IsNotExist(err) {
		return false, nil
	}
	return true, err
}

Received results:

2019/11/06 09:18:12 after first transaction:
2019/11/06 09:18:12 (quad.Quad) "a" -- "to" -> "b"
2019/11/06 09:18:12 (quad.Quad) "a" -- "updated" -> "2019-11-06T11:45:26Z"^^schema:DateTime
2019/11/06 09:18:12 after second transaction:
2019/11/06 09:18:12 (quad.Quad) -- "updated" -> "2019-11-06T11:46:26Z"^^schema:DateTime

also dump panics/NPE:
➜ txTest cayley dump -o - -d leveldb -a .db/leveldb
I1106 09:12:15.961568 9765 command.go:805] Cayley version: v0.8.x-dev (dev snapshot)
I1106 09:12:15.961795 9765 database.go:146] using backend "leveldb" (.db/leveldb)
I1106 09:12:16.029981 9765 dump.go:73] writing quads to stdout
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x43dbce6]

goroutine 1 [running]:
github.com/cayleygraph/quad/nquads.(*Writer).writeValue(0xc000448120, 0x0, 0x0, 0x5231378, 0x1)
/Users/mike/zo/pkg/mod/github.com/cayleygraph/quad@v1.1.0/nquads/nquads.go:303 +0x36
github.com/cayleygraph/quad/nquads.(*Writer).WriteQuad(0xc000448120, 0x0, 0x0, 0x54e7c80, 0xc0004500e0, 0x54e7cc0, 0xc0001463a0, 0x0, 0x0, 0x1, ...)
/Users/mike/zo/pkg/mod/github.com/cayleygraph/quad@v1.1.0/nquads/nquads.go:316 +0x58
github.com/cayleygraph/quad/nquads.(*Writer).WriteQuads(0xc000448120, 0xc0057e4000, 0x1, 0x2710, 0x1, 0x54d24a0, 0xc0000da050)
/Users/mike/zo/pkg/mod/github.com/cayleygraph/quad@v1.1.0/nquads/nquads.go:329 +0x10d
...

Expected results:

2019/11/06 09:18:12 after first transaction:
2019/11/06 09:18:12 (quad.Quad) "a" -- "to" -> "b"
2019/11/06 09:18:12 (quad.Quad) "a" -- "updated" -> "2019-11-06T11:45:26Z"^^schema:DateTime
2019/11/06 09:18:12 after second transaction:
2019/11/06 09:18:12 (quad.Quad) "a" -- "updated" -> "2019-11-06T11:46:26Z"^^schema:DateTime

Output of cayley version or commit hash:
v0.7.7

Environment details:
go1.13.3 darwin/amd64
bolt and leveldb both affected