Hack to disable the fetcher cache in forked processes

Author: edolstra

src/libfetchers/cache.cc

@@ -54,6 +54,8 @@ struct CacheImpl : Cache
         const Key & key,
         const Attrs & value) override
     {
+        if (disabled()) return;
+
         _state.lock()->upsert.use()
             (key.first)
             (attrsToJSON(key.second).dump())
@@ -81,9 +83,20 @@ struct CacheImpl : Cache
         return {};
     }
 
+    pid_t originalPid = getpid();
+
+    bool disabled()
+    {
+        // FIXME: Temporary hack to disable the cache in
+        // builtin:fetch-tree builders.
+        return getpid() != originalPid;
+    }
+
     std::optional<Result> lookupExpired(
         const Key & key) override
     {
+        if (disabled()) return {};
+
         auto state(_state.lock());
 
         auto keyJSON = attrsToJSON(key.second).dump();
@@ -111,6 +124,8 @@ struct CacheImpl : Cache
         Attrs value,
         const StorePath & storePath) override
     {
+        if (disabled()) return;
+
         /* Add the store prefix to the cache key to handle multiple
            store prefixes. */
         key.second.insert_or_assign("store", store.storeDir);