* Move addTempRoot() to the store API, and add another function

syncWithGC() to allow clients to register GC roots without needing
  write access to the global roots directory or the GC lock.

src/libstore/store-api.hh

@@ -86,6 +86,31 @@ public:
        may be made valid by running a substitute (if defined for the
        path). */
     virtual void ensurePath(const Path & path) = 0;
+
+    /* Add a store path as a temporary root of the garbage collector.
+       The root disappears as soon as we exit. */
+    virtual void addTempRoot(const Path & path) = 0;
+
+    /* Acquire the global GC lock, then immediately release it.  This
+       function must be called after registering a new permanent root,
+       but before exiting.  Otherwise, it is possible that a running
+       garbage collector doesn't see the new root and deletes the
+       stuff we've just built.  By acquiring the lock briefly, we
+       ensure that either:
+
+       - The collector is already running, and so we block until the
+         collector is finished.  The collector will know about our
+         *temporary* locks, which should include whatever it is we
+         want to register as a permanent lock.
+
+       - The collector isn't running, or it's just started but hasn't
+         acquired the GC lock yet.  In that case we get and release
+         the lock right away, then exit.  The collector scans the
+         permanent root and sees our's.
+
+       In either case the permanent root is seen by the collector. */
+    virtual void syncWithGC() = 0;
+
 };