There was an error introduced in #19209 where `full_hash()` and
`build_hash()` are called on older specs that we've read in from the DB;
older specs may not be able to compute these hashes (e.g. if they have
removed patches used in computing the full_hash).
When serializing a Spec, we want to generate the full/build hash when
possible, but we need a mechanism to skip it for Specs that have
themselves been read from YAML (and may not support this).
To get around this ambiguity and to fix the issue, we:
- Add an attribute to the spec called `_hashes_final`, that is `True`
if we can't lazily compute `build_hash` and `full_hash`.
- Set `_hashes_final` to `False` for new specs (i.e., lazily
computing hashes is ok)
- Set `_hashes_final` to `True` for concrete specs read in via
`from_node_dict`, as it may be too late to recompute hashes.
- Compute and write out all hashes in `node_dict_with_hashes` *if
possible*.
Effectively what this means is that we can round-trip specs that are
missing `_build_hash` and `_full_hash` without recomputing them, but for
all new specs, we'll compute them and store them. So Spack should work
fine with old DBs now.
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