Panic on invalid back-reference.

[levirak]

I'm running:

• Which Parity version?: v1.10.3-stable-b9ceda38a-20180507
• Which operating system?: Linux (Arch Linux)
• How installed?: via pacman (local package manager, main repository)
• Are you fully synchronized?: yes
• Which network are you connected to?: ethereum
• Did you try to restart the node?: yes

In the process of using Web3 to inspect the Ethereum main chain, I feed the Parity light client an RPC call that crashed it. Specifically, I asked for for the block 0x1784ac (full hash given below), which is as an uncle of 0x248fe7. Asking for 0x248fe7 successfully returns the block, including any requests for an arbitrary depth of ancestors.

My naive assessment of the situation is that because I am running a light client, the requested block was not recorded on my machine, leading to a chain of events which caused the client to panic. I propose this with a high degree of confidence, as this event only occurred after I first asked for an uncle block while asking for main-chain blocks continues to work.

I presume that the correct behavior would be to either retrieve the block from the Ethereum network or answer with some standard "no such block" kind of RPC response. What follows is the complete transcript of the event for the Parity client:

$ parity --light
2018-05-21 16:10:43  Starting Parity/v1.10.3-stable-b9ceda38a-20180507/x86_64-linux-gnu/rustc1.25.0
2018-05-21 16:10:43  Keys path /home/levi/.local/share/io.parity.ethereum/keys/Foundation
2018-05-21 16:10:43  DB path /home/levi/.local/share/io.parity.ethereum/chains/ethereum/db/906a34e69aec8c0d
2018-05-21 16:10:43  Path to dapps /home/levi/.local/share/io.parity.ethereum/dapps
2018-05-21 16:10:43  Running in experimental Light Client mode.
2018-05-21 16:10:44  Removed existing file '/home/levi/.local/share/io.parity.ethereum/jsonrpc.ipc'.
2018-05-21 16:10:49  Public node URL: [REDACTED]

====================

stack backtrace:
   0:     0x56248f3ed93c - <unknown>
   1:     0x56248f3ec1f2 - <unknown>
   2:     0x56248ea357cb - <unknown>

Thread '<unnamed>' panicked at 'Fails only on invalid back-references; back-references here known to be valid; qed: NoSuchOutput', libcore/result.rs:945

This is a bug. Please report it at:

    https://github.com/paritytech/parity/issues/new

Aborted (core dumped)

Steps to reproduce:

The following steps reliably allow me to reproduce the above error.

First, start Parity as such:

$ parity --light

Second, via NodeJS, ask for 0x1784ac:

$ node
> const Web3 = require('web3');
> var web3 = new Web3(new Web3.providers.HttpProvider('http://localhost:8545'));
> web3.eth.getBlock('0x1784ac7715ffe762a730f1dc07aca04138f0a028422b89d700e38717de5fa4bb')

The result is the following error on Node's end:

Error: Invalid JSON RPC response: undefined
    at Object.InvalidResponse (/home/levi/Desktop/test/node.js/node_modules/web3/lib/web3/errors.js:38:16)
    at HttpProvider.send (/home/levi/Desktop/test/node.js/node_modules/web3/lib/web3/httpprovider.js:101:18)
    at RequestManager.send (/home/levi/Desktop/test/node.js/node_modules/web3/lib/web3/requestmanager.js:58:32)
    at Eth.send [as getBlock] (/home/levi/Desktop/test/node.js/node_modules/web3/lib/web3/method.js:145:58)

At this time the instance of Parity has crashed with the previously given output.

[dvdplm]

Confirmed. The panic comes from here. Investigating.