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This PR introduces a new build cache layout and package format, with improvements for
both efficiency and security.
## Old Format
Currently a binary package consists of a `spec.json` file at the root and a `.spack` file,
which is a `tar` archive containing a copy of the `spec.json` format, possibly a detached
signature (`.asc`) file, and a tar-gzip compressed archive containing the install tree.
```
build_cache/
# metadata (for indexing)
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spec.json
<arch>/
<compiler>/
<name>-<ver>/
# tar archive
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spack
# tar archive contents:
# metadata (contains sha256 of internal .tar.gz)
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spec.json
# signature
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spec.json.asc
# tar.gz-compressed prefix
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.tar.gz
```
After this change, the nesting has been removed so that the `.spack` file is the
compressed archive of the install tree. Now signed binary packages, will take the
form of a clearsigned `spec.json` file (a `spec.json.sig`) at the root, while unsigned
binary packages will contain a `spec.json` at the root.
## New Format
```
build_cache/
# metadata (for indexing, contains sha256 of .spack file)
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spec.json
# clearsigned spec.json metadata
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spec.json.sig
<arch>/
<compiler>/
<name>-<ver>/
# tar.gz-compressed prefix (may support more compression formats later)
<arch>-<compiler>-<name>-<ver>-24zvipcqgg2wyjpvdq2ajy5jnm564hen.spack
```
## Benefits
The major benefit of this change is that the signatures on binary packages can be
verified without:
1. Having to download the tarball, or
2. having to extract an unknown tarball.
(1) is an improvement in efficiency; (2) is a security fix: we now ensure that we trust the
binary before we try to run it through `tar`, which avoids potential attacks.
## Backward compatibility
Also after this change, spack should still be able to handle the previous buildcache
structure and binary mirrors with mixed layouts.
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