Simplify the callback mechanism

src/libstore/s3-binary-cache-store.cc

@@ -365,10 +365,9 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
     }
 
     void getFile(const std::string & path,
-        std::function<void(std::shared_ptr<std::string>)> success,
-        std::function<void(std::exception_ptr exc)> failure) override
+        Callback<std::shared_ptr<std::string>> callback) override
     {
-        sync2async<std::shared_ptr<std::string>>(success, failure, [&]() {
+        try {
             stats.get++;
 
             auto res = s3Helper.getObject(bucketName, path);
@@ -380,8 +379,8 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
                 printTalkative("downloaded 's3://%s/%s' (%d bytes) in %d ms",
                     bucketName, path, res.data->size(), res.durationMs);
 
-            return res.data;
-        });
+            callback(std::move(res.data));
+        } catch (...) { callback.rethrow(); }
     }
 
     PathSet queryAllValidPaths() override