Spack environment recovery after a failed upgrade: lockfile re-concretization vs. preserving the old environment view
I manage a Spack environment (spack.yaml + spack.lock under version control) and want a reliable recovery path when an upgrade of a core spec (for example a new compiler or MPI) fails concretization or build. Spack has no built-in rollback command, so the plan depends on installs being immutable and the old DAG remaining usable. The constraint I cannot resol