Function simulated_cgroup_memory_environment 

pub fn simulated_cgroup_memory_environment(
    host_available_bytes: u64,
    host_total_bytes: u64,
    cgroup_limit_bytes: u64,
    cgroup_current_bytes: u64,
) -> MemoryAvailability

Replay a cgroup-constrained memory environment without needing the machine to be in it.

The two knobs are deliberately the two that #2684 conflated:

• cgroup_limit_bytes is the job’s hard ceiling — the CAPACITY. It is a property of how the process was launched and does not move while it runs.
• cgroup_current_bytes is how much of that ceiling is charged RIGHT NOW — the load. It moves continuously, and on any cgroup that has done file I/O it sits near the ceiling because page cache fills whatever is free.

Holding the first fixed while sweeping the second is what lets a test ask “does this decision depend on ambient load?” as an assertion instead of an argument. The returned observation is built by the same constructor the live probe uses, so its derived working_set/available arithmetic is the shipped arithmetic and not a second implementation that could agree with the first only by luck.

The host numbers are taken as parameters too, because the host’s own free memory is the other half of the conflation: the incident this exists for had hundreds of GB free on the host while the job’s cgroup reported kilobytes.