Chained batch should be persistent

[vweevers]

A segfault can happen when a chained batch is GC-ed in between batch.write(callback) and BatchWriteWorker->DoExecute().

[vweevers]

Wrote a test and ran it in v4 as well. Only v5 has the bug.

In v4 we do:

leveldown/src/batch.cc

Lines 137 to 139 in 0cb52da

	// persist to prevent accidental GC
	v8::Local<v8::Object> _this = info.This();
	worker->SaveToPersistent("batch", _this);

[peakji]

I've obtained a core dump file on segfault 11/SEGV from a server running v5.0.0 (been waiting for a week!).

Back trace shows the segment fault happened on a uv worker pool thread, but not sure if it is running BatchWriteWorker->DoExecute() or not.

However, I found one unwritten batch which means the software crashed while writing batches. Since the software does not perform other operations during this phase, I think it is safe to say this PR fixed the segfault described in #601. 🎉

I'll deploy v5.0.3 today and see if it crashes.

---

Back trace generated by lldb & llnode:

Objects:

[vweevers]

@peakji that's good! Your analysis makes sense. I assume you're running parallel batches? The presence of that unwritten batch seems to indicate GC did not collect it yet, so the segfault must have been caused by another batch (perhaps those "Invalid values"?).

I also deployed 5.0.3 a few hours ago. Ended up using chained batch. Low volume atm, increasing on monday. In a week or so, our use cases combined should give us all the confidence we need 💯

[vweevers]

I've since recorded 204M operations without any problems. 🎈

[peakji]

Same here! I've been using v5.0.3 in production for two weeks (wrote ~400GB to disk), everything worked perfectly. 🎉