Add Store nesting to fix import-from-derivation within filterSource

src/libstore/remote-store.cc

@@ -414,7 +414,9 @@ Path RemoteStore::addToStore(const string & name, const Path & _srcPath,
     try {
         conn->to.written = 0;
         conn->to.warn = true;
+        connections->incCapacity();
         dumpPath(srcPath, conn->to, filter);
+        connections->decCapacity();
         conn->to.warn = false;
         conn->processStderr();
     } catch (SysError & e) {

src/libutil/pool.hh

@@ -68,6 +68,22 @@ public:
         state_->max = max;
     }
 
+    void incCapacity()
+    {
+        auto state_(state.lock());
+        state_->max++;
+        /* we could wakeup here, but this is only used when we're
+         * about to nest Pool usages, and we want to save the slot for
+         * the nested use if we can
+         */
+    }
+
+    void decCapacity()
+    {
+        auto state_(state.lock());
+        state_->max--;
+    }
+
     ~Pool()
     {
         auto state_(state.lock());