libutil: Add RestartableSource

This is necessary to make seeking work with libcurl.

src/libutil/include/nix/util/serialise.hh

@@ -230,10 +230,18 @@ struct StringSink : Sink
     void operator()(std::string_view data) override;
 };
 
+/**
+ * Source type that can be restarted.
+ */
+struct RestartableSource : Source
+{
+    virtual void restart() = 0;
+};
+
 /**
  * A source that reads data from a string.
  */
-struct StringSource : Source
+struct StringSource : RestartableSource
 {
     std::string_view s;
     size_t pos;
@@ -257,8 +265,22 @@ struct StringSource : Source
     size_t read(char * data, size_t len) override;
 
     void skip(size_t len) override;
+
+    void restart() override
+    {
+        pos = 0;
+    }
 };
 
+/**
+ * Create a restartable Source from a factory function.
+ *
+ * @param factory Factory function that returns a fresh instance of the Source. Gets
+ * called for each source restart.
+ * @pre factory must return an equivalent source for each invocation.
+ */
+std::unique_ptr<RestartableSource> restartableSourceFromFactory(std::function<std::unique_ptr<Source>()> factory);
+
 /**
  * A sink that writes all incoming data to two other sinks.
  */

src/libutil/serialise.cc

@@ -513,4 +513,41 @@ size_t ChainSource::read(char * data, size_t len)
     }
 }
 
+std::unique_ptr<RestartableSource> restartableSourceFromFactory(std::function<std::unique_ptr<Source>()> factory)
+{
+    struct RestartableSourceImpl : RestartableSource
+    {
+        RestartableSourceImpl(decltype(factory) factory_)
+            : factory_(std::move(factory_))
+            , impl(this->factory_())
+        {
+        }
+
+        decltype(factory) factory_;
+        std::unique_ptr<Source> impl = factory_();
+
+        size_t read(char * data, size_t len) override
+        {
+            return impl->read(data, len);
+        }
+
+        bool good() override
+        {
+            return impl->good();
+        }
+
+        void skip(size_t len) override
+        {
+            return impl->skip(len);
+        }
+
+        void restart() override
+        {
+            impl = factory_();
+        }
+    };
+
+    return std::make_unique<RestartableSourceImpl>(std::move(factory));
+}
+
 } // namespace nix