light-client: Verification failure

[thanethomson]

Follows from this comment in issue 583. The issue is reproducible in the code currently on master, specifically this integration test.

To reproduce locally, in one terminal, run a Tendermint kvstore accessible at tcp://127.0.0.1:26657:

docker run -it --rm -v "/tmp/tendermint:/tendermint" tendermint/tendermint init
docker run -it --rm -v "/tmp/tendermint:/tendermint" -p 26657:26657 tendermint/tendermint node --proxy_app=kvstore

Then set this variable to a value arbitrarily higher than 1 (e.g. 10).

Then, from the root of the repository:

cd light-client
cargo test --all-features sync -- --nocapture --ignored

The test fails with a result along the lines of:

running 1 test
[info ] - iteration 1/10
[info ] synced to block 219
[info ] - iteration 2/10
[error] sync failed: no witness left: invalid light block: invalid commit value: header_hash=1735EC2938A2377C3C0EB730C9DF28FA22EBEC88A6C01EB6201BBC377E636D23 commit_hash=142DBB6210610A1349163B6611A1338A00E186DE586783F3F16DD964604E812E
thread 'sync' panicked at 'failed to sync to highest: no witness left: invalid light block: invalid commit value: header_hash=1735EC2938A2377C3C0EB730C9DF28FA22EBEC88A6C01EB6201BBC377E636D23 commit_hash=142DBB6210610A1349163B6611A1338A00E186DE586783F3F16DD964604E812E', light-client/tests/integration.rs:111:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test sync ... FAILED

[romac]

Will be fixed in #592 by 007bb09.