move binary indices are stored into the misc_cache (#22500)

Remote buildcache indices need to be stored in a place that does not
require writing to the Spack prefix. Move them from the install_tree to
the misc_cache.

lib/spack/spack/binary_distribution.py

@@ -37,13 +37,11 @@
 import spack.mirror
 import spack.util.url as url_util
 import spack.util.web as web_util
+from spack.caches import misc_cache_location
 from spack.spec import Spec
 from spack.stage import Stage
 
 
-#: default root, relative to the Spack install path
-default_binary_index_root = os.path.join(spack.paths.opt_path, 'spack')
-
 _build_cache_relative_path = 'build_cache'
 _build_cache_keys_relative_path = '_pgp'
 
@@ -67,9 +65,8 @@ class BinaryCacheIndex(object):
     mean we should have paid the price to update the cache earlier?
     """
 
-    def __init__(self, cache_root=None):
-        self._cache_root = cache_root or default_binary_index_root
-        self._index_cache_root = os.path.join(self._cache_root, 'indices')
+    def __init__(self, cache_root):
+        self._index_cache_root = cache_root
 
         # the key associated with the serialized _local_index_cache
         self._index_contents_key = 'contents.json'
@@ -440,13 +437,15 @@ def _fetch_and_cache_index(self, mirror_url, expect_hash=None):
         return True
 
 
+def binary_index_location():
+    """Set up a BinaryCacheIndex for remote buildcache dbs in the user's homedir."""
+    cache_root = os.path.join(misc_cache_location(), 'indices')
+    return spack.util.path.canonicalize_path(cache_root)
+
+
 def _binary_index():
     """Get the singleton store instance."""
-    cache_root = spack.config.get(
-        'config:binary_index_root', default_binary_index_root)
-    cache_root = spack.util.path.canonicalize_path(cache_root)
-
-    return BinaryCacheIndex(cache_root)
+    return BinaryCacheIndex(binary_index_location())
 
 
 #: Singleton binary_index instance

lib/spack/spack/caches.py

@@ -17,7 +17,7 @@
 import spack.util.path
 
 
-def _misc_cache():
+def misc_cache_location():
     """The ``misc_cache`` is Spack's cache for small data.
 
     Currently the ``misc_cache`` stores indexes for virtual dependency
@@ -27,7 +27,11 @@ def _misc_cache():
     if not path:
         path = os.path.join(spack.paths.user_config_path, 'cache')
     path = spack.util.path.canonicalize_path(path)
+    return path
 
+
+def _misc_cache():
+    path = misc_cache_location()
     return spack.util.file_cache.FileCache(path)
 
 
@@ -35,7 +39,7 @@ def _misc_cache():
 misc_cache = llnl.util.lang.Singleton(_misc_cache)
 
 
-def _fetch_cache():
+def fetch_cache_location():
     """Filesystem cache of downloaded archives.
 
     This prevents Spack from repeatedly fetch the same files when
@@ -45,7 +49,11 @@ def _fetch_cache():
     if not path:
         path = os.path.join(spack.paths.var_path, "cache")
     path = spack.util.path.canonicalize_path(path)
+    return path
 
+
+def _fetch_cache():
+    path = fetch_cache_location()
     return spack.fetch_strategy.FsCache(path)