From 289b9b8dcf8dc651c2d245d9328911f7addd1626 Mon Sep 17 00:00:00 2001
From: Carlo Nucera <carlo.nucera@protonmail.com>
Date: Tue, 16 Jun 2020 15:14:11 -0400
Subject: [PATCH] Create a new TeeSink abstraction

This is a bit complex because we want to expose extra functionality the
wrapped class has. Perhaps there is some inheritancy trickery to do this
nicer, but I don't know it, and this is the first thing we tried after a
series of attempts that did build.

This design is kind of like that of Rust's Writer, Reader, or Iter
adapters, which impliment more traits based on what the inner type
implements.
---
 src/libutil/compression.hh | 12 ++++++++++++
 src/libutil/serialise.hh   | 22 ++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/src/libutil/compression.hh b/src/libutil/compression.hh
index dd666a4e1..1bd118b47 100644
--- a/src/libutil/compression.hh
+++ b/src/libutil/compression.hh
@@ -25,4 +25,16 @@ MakeError(UnknownCompressionMethod, Error);
 
 MakeError(CompressionError, Error);
 
+template<>
+struct TeeSink<ref<CompressionSink>> : CompressionSink
+{
+    MAKE_TEE_SINK(ref<CompressionSink>);
+    void finish() override {
+        orig->finish();
+    }
+    void write(const unsigned char * data, size_t len) override {
+        return orig->write(data, len);
+    }
+};
+
 }
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index a04118512..88a6b7ffe 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -181,6 +181,28 @@ struct TeeSource : Source
     }
 };
 
+#define MAKE_TEE_SINK(T) \
+	T orig; \
+    ref<std::string> data; \
+    TeeSink(T && orig) \
+        : orig(std::move(orig)), data(make_ref<std::string>()) { } \
+    void operator () (const unsigned char * data, size_t len) { \
+        this->data->append((const char *) data, len); \
+        (*this->orig)(data, len); \
+    } \
+    void operator () (const std::string & s) \
+    { \
+        *data += s; \
+        (*this->orig)(s); \
+    }
+
+template<typename T>
+struct TeeSink : Sink
+{
+    MAKE_TEE_SINK(T);
+};
+
+
 /* A reader that consumes the original Source until 'size'. */
 struct SizedSource : Source
 {