leveldb: possible iterator leaks causes assertion failure on index close

[tmm1]

I recently upgraded leveldb and forgot to build it with -DNDEBUG, which silences internal assertions.

I've noticed intermittent assertion failures on both Windows and Linux when closing a bleve search index:

indexer: leveldb/db/version_set.cc:776: leveldb::VersionSet::~VersionSet(): Assertion `dummy_versions_.next_ == &dummy_versions_' failed.
SIGABRT: abort
PC=0x1112c9b m=13 sigcode=18446744073709551610

goroutine 0 [idle]:
runtime: unknown pc 0x1112c9b
stack: frame={sp:0x7ffb89ffaa08, fp:0x0} stack=[0x7ffb897fb108,0x7ffb89ffad08)

goroutine 65 [syscall]:
runtime.cgocall(0x106f414, 0xc0038bdc18, 0xc00003e070)
	/home/vagrant/go/src/runtime/cgocall.go:128 +0x5b fp=0xc0038bdbe8 sp=0xc0038bdbb0 pc=0x40560b
github.com/jmhodges/levigo._Cfunc_leveldb_close(0x7ffb9007f950)
	_cgo_gotypes.go:181 +0x41 fp=0xc0038bdc18 sp=0xc0038bdbe8 pc=0xc40211
github.com/jmhodges/levigo.(*DB).Close.func1(0xc0003bfe80)
	/home/vagrant/gopath/pkg/mod/github.com/jmhodges/levigo@v1.0.0/db.go:387 +0x5e fp=0xc0038bdc58 sp=0xc0038bdc18 pc=0xc456de
github.com/jmhodges/levigo.(*DB).Close(0xc0003bfe80)
	/home/vagrant/gopath/pkg/mod/github.com/jmhodges/levigo@v1.0.0/db.go:387 +0x3f fp=0xc0038bdc70 sp=0xc0038bdc58 pc=0xc436df
github.com/tmm1/blevex/leveldb.(*Store).Close(0xc00018ac00, 0x14, 0xc0001e6848)
	/home/vagrant/gopath/pkg/mod/github.com/tmm1/blevex@v0.0.0-20170301185655-58782622cfc6/leveldb/store.go:61 +0x2f fp=0xc0038bdc88 sp=0xc0038bdc70 pc=0xc483bf
github.com/blevesearch/bleve/index/upsidedown.(*UpsideDownCouch).Close(0xc000117280, 0x1731640, 0xc00017e150)
	/home/vagrant/gopath/pkg/mod/github.com/blevesearch/bleve@v0.7.0/index/upsidedown/upsidedown.go:412 +0x34 fp=0xc0038bdcb0 sp=0xc0038bdc88 pc=0xb189b4
github.com/blevesearch/bleve.(*indexImpl).Close(0xc00017e150, 0x0, 0x0)
	/home/vagrant/gopath/pkg/mod/github.com/blevesearch/bleve@v0.7.0/index_impl.go:665 +0xa9 fp=0xc0038bdce0 sp=0xc0038bdcb0 pc=0xc2b239

According to google/leveldb#708, this assertion means an iterator was still alive when leveldb_close() was called.

[tmm1]

Note that this was observed on bleve v0.7.0, so its possible the leak is no longer present.