panic in node_exec_state

[davepacheco]

See #16 and oxidecomputer/omicron#279 for details.

We saw this panic:

thread 'test_disks' panicked at 'assertion failed: self.node_errors.contains_key(&node_id)', /home/dap/.cargo/git/checkouts/steno-adfbaf4765a4f9e1/8da3eda/src/saga_exec.rs:1351:13

This is pretty reproducible in that issue and we have a core file, but it'll take some time to wade through the state and figure out what's wrong here. omicron was on commit 8da3eda at this point so that failure is here:

steno/src/saga_exec.rs

Line 1351 in 8da3eda

assert!(self.node_errors.contains_key(&node_id));