Parity stops; Issue: all back-references to answered requests have been filled

[Shachindra]

I'm running:

• Which Parity version?: v1.8.5-beta-54bae9a-20171228/x86_64-linux-gnu/rustc1.22.1
• Which operating system?: VM- 4.11.0-1016-azure BlockChain #16-Ubuntu
• How installed?: via installer
• Are you fully synchronized?: no
• Did you try to restart the node?: yes (After restart no issues so far as it's still syncing)

Error: While syncing with the kovan testnet, parity abruptly gets stopped.

2018-01-01 06:39:03 UTC Syncing #4973397 5101…5fb5 0 hdr/s 0+ 0 Qed #4973397 24/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 3 req/s, 53 µs
2018-01-01 06:39:08 UTC Syncing #4973397 5101…5fb5 0 hdr/s 0+ 0 Qed #4973397 24/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 1 req/s, 53 µs
2018-01-01 06:39:13 UTC Syncing #4973397 5101…5fb5 0 hdr/s 0+ 0 Qed #4973397 23/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 3 req/s, 53 µs
2018-01-01 06:39:18 UTC Syncing #4973397 5101…5fb5 0 hdr/s 0+ 0 Qed #4973397 23/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 2 req/s, 53 µs
2018-01-01 06:39:23 UTC Syncing #4973397 5101…5fb5 0 hdr/s 0+ 0 Qed #4973397 22/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 0 req/s, 53 µs

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

stack backtrace:
0: 0x267f7a3d9c -

Thread '' panicked at 'all back-references to answered requests have been filled; qed: NoSuchOutput', /checkout/src/libcore/result.rs:906

This is a bug. Please report it at:

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

Aborted (core dumped)

Run Command:
parity --chain kovan --light --jsonrpc-apis web3,eth,net,parity,traces,rpc,personal --jsonrpc-port 8545 --jsonrpc-interface 0.0.0.0 --jsonrpc-cors all --jsonrpc-hosts all --force-ui --ui-no-validation

[Shachindra]

Happened again even after full sync was completed:
2018-01-01 09:30:32 UTC Syncing #5194214 9123…a74c 0 hdr/s 0+ 0 Qed #5194214 25/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 0 req/s, 96 µs
2018-01-01 09:31:02 UTC 25/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 0 req/s, 97 µs
2018-01-01 09:31:32 UTC 25/50 peers 10 MiB cache 0 bytes queue RPC: 0 conn, 0 req/s, 97 µs

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

stack backtrace:
0: 0xcf25413d9c -

Thread '' panicked at 'all back-references to answered requests have been filled; qed: NoSuchOutput', /checkout/src/libcore/result.rs:906

This is a bug. Please report it at:

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

Aborted (core dumped)

[debris]

@rphmeier any tips?

https://github.com/paritytech/parity/blob/594716dc1ec822feb0230c4d57e99c4810e4b813/ethcore/light/src/on_demand/mod.rs#L161-L166

[rphmeier]

I think the issue is here: https://github.com/paritytech/parity/blob/594716dc1ec822feb0230c4d57e99c4810e4b813/ethcore/light/src/on_demand/mod.rs#L413-L419

Answering requests doesn't automatically fill back-references, so if we got something from the cache the outputs will not be filled and the expect will fail. The simple fix is to call fill_unanswered just before making the new net request builder instead of before invoking update_net_requests.