Light client does not answer RPC calls

[Tbaut]

Before filing a new issue, please provide the following information.

I'm running:

• Which Parity version?: Parity/v1.12.0-unstable-458afcd-20180620/x86_64-linux-gnu/rustc1.26.1
• How installed?: from source
• Are you fully synchronized?: yes
• Which network are you connected to?: kovan / mainnet
• Did you try to restart the node?: yes

When running Fether, the wallet sometimes seems not to get any answer from the node.
Any RPC such as getBalance don't get answered.
Here is a

• gist of 1min of logs when it happens with -l pip,rpc,network
• gist of the same 1min of logs when it happens with network filtered out for readability
• gist of another time it happens with -l less,light_fetch,on_demand,pip_provider

Note that the last one, the problem seems to be the peer discovery.. oscillating between 1 and 0.

[Tbaut]

This seems to be a peer issue. In both cases, I end up having 1 or 2 peers, but they might not serve light clients as the logs do not show it. It could be related to #8974.

[Tbaut]

This seems to not be bound to external calls as calling:
curl --data '{"method":"eth_estimateGas","params":[{"from":"0x004702bdcc3c7dbffd943136107e70b827028600", "gas":"0xcf08"}],"id":1,"jsonrpc":"2.0"}' -H "Content-Type: application/json" -X POST localhost:8545 ^C
did not resolve (both while syncing or being synced)

[Tbaut]

Must be a duplicate: #8918