vfs: flatten resolution cache key

This will require more storage if there are a lot of lookups with the
same parent and differing paths, but otherwise, without more
intentional structure sharing among paths, this should be better, and
we can always revisit the arrangement later.

Serializing the parent path segments should also make sure the same
parent (semantically-speaking) contributes the same hash to the key.
Previously, Metadata objects could prevent that, given their trivial,
pointer-based hashes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>

lib/bup/vfs.py

@@ -253,20 +253,20 @@ def clear_cache():
 def is_valid_cache_key(x):
     """Return logically true if x looks like it could be a valid cache key
     (with respect to structure).  Current valid cache entries:
-      (repo-id, path, parent, want_meta, dref) -> resolution
+      res:... -> resolution
       commit_oid -> commit
       commit_oid + ':r' -> rev-list
          i.e. rev-list -> {'.', commit, '2012...', next_commit, ...}
     """
     # Suspect we may eventually add "(container_oid, name) -> ...", and others.
     x_t = type(x)
-    if x_t is tuple:
-        return len(x) == 5
     if x_t is bytes:
         if len(x) == 20:
             return True
         if len(x) == 22 and x.endswith(b':r'):
             return True
+        if x.startswith('res:'):
+            return True
 
 def cache_get(key):
     global _cache
@@ -759,7 +759,10 @@ def contents(repo, item, names=None, want_meta=True):
         yield x
 
 def _resolve_path(repo, path, parent=None, want_meta=True, deref=False):
-    cache_key = (repo.id(), tuple(path), parent, bool(want_meta), bool(deref))
+    cache_key = b'res:%d%d%d:%s\0%s' \
+                % (bool(want_meta), bool(deref), repo.id(),
+                   ('/'.join(x[0] for x in parent) if parent else ''),
+                   '/'.join(path))
     resolution = cache_get(cache_key)
     if resolution:
         return resolution