Clean up store hierarchy with IndirectRootStore

See the API doc comments for details.

src/libstore/gc-store.hh

@@ -71,19 +71,36 @@ struct GCResults
 };
 
 
+/**
+ * Mix-in class for \ref Store "stores" which expose a notion of garbage
+ * collection.
+ *
+ * Garbage collection will allow deleting paths which are not
+ * transitively "rooted".
+ *
+ * The notion of GC roots actually not part of this class.
+ *
+ *  - The base `Store` class has `Store::addTempRoot()` because for a store
+ *    that doesn't support garbage collection at all, a temporary GC root is
+ *    safely implementable as no-op.
+ *
+ *    @todo actually this is not so good because stores are *views*.
+ *    Some views have only a no-op temp roots even though others to the
+ *    same store allow triggering GC. For instance one can't add a root
+ *    over ssh, but that doesn't prevent someone from gc-ing that store
+ *    accesed via SSH locally).
+ *
+ *  - The derived `LocalFSStore` class has `LocalFSStore::addPermRoot`,
+ *    which is not part of this class because it relies on the notion of
+ *    an ambient file system. There are stores (`ssh-ng://`, for one),
+ *    that *do* support garbage collection but *don't* expose any file
+ *    system, and `LocalFSStore::addPermRoot` thus does not make sense
+ *    for them.
+ */
 struct GcStore : public virtual Store
 {
     inline static std::string operationName = "Garbage collection";
 
-    /**
-     * Add an indirect root, which is merely a symlink to `path` from
-     * `/nix/var/nix/gcroots/auto/<hash of path>`.  `path` is supposed
-     * to be a symlink to a store path.  The garbage collector will
-     * automatically remove the indirect root when it finds that
-     * `path` has disappeared.
-     */
-    virtual void addIndirectRoot(const Path & path) = 0;
-
     /**
      * Find the roots of the garbage collector.  Each root is a pair
      * `(link, storepath)` where `link` is the path of the symlink