libutil: begin porting serialization to generators

generators are a better basis for serializers than streaming into sinks
as we do currently for many reasons, such as being usable as sources if
one wishes to (without requiring an intermediate sink to serialize full
data sets into memory, or boost coroutines to turn sinks into sources),
composing more naturally (as one can just yield a sub-generator instead
of being forced to wrap entire substreams into clunky functions or even
more clunky custom types to implement operator<< on), allowing wrappers
to transform data with clear ownership semantics (removing the need for
explicit memory allocations and Source wrappers), and many other things

Change-Id: I361d89ff556354f6930d9204f55117565f2f7f20

src/libstore/daemon.cc

@@ -160,8 +160,7 @@ struct TunnelSink : Sink
     TunnelSink(Sink & to) : to(to) { }
     void operator () (std::string_view data)
     {
-        to << STDERR_WRITE;
-        writeString(data, to);
+        to << STDERR_WRITE << data;
     }
 };
 

src/libstore/remote-store.cc

@@ -897,7 +897,7 @@ std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source *
             if (!source) throw Error("no source");
             size_t len = readNum<size_t>(from);
             auto buf = std::make_unique<char[]>(len);
-            writeString({(const char *) buf.get(), source->read(buf.get(), len)}, to);
+            to << std::string_view((const char *) buf.get(), source->read(buf.get(), len));
             to.flush();
         }
 

src/libutil/serialise.cc

@@ -330,55 +330,40 @@ void writePadding(size_t len, Sink & sink)
 }
 
 
-void writeString(std::string_view data, Sink & sink)
+WireFormatGenerator SerializingTransform::operator()(std::string_view s)
 {
-    sink << data.size();
-    sink(data);
-    writePadding(data.size(), sink);
+    co_yield s.size();
+    co_yield Bytes(s.begin(), s.size());
+    co_yield SerializingTransform::padding(s.size());
 }
 
-
-Sink & operator << (Sink & sink, std::string_view s)
+WireFormatGenerator SerializingTransform::operator()(const Strings & ss)
 {
-    writeString(s, sink);
-    return sink;
+    co_yield ss.size();
+    for (const auto & s : ss)
+        co_yield std::string_view(s);
 }
 
-
-template<class T> void writeStrings(const T & ss, Sink & sink)
+WireFormatGenerator SerializingTransform::operator()(const StringSet & ss)
 {
-    sink << ss.size();
-    for (auto & i : ss)
-        sink << i;
+    co_yield ss.size();
+    for (const auto & s : ss)
+        co_yield std::string_view(s);
 }
 
-Sink & operator << (Sink & sink, const Strings & s)
-{
-    writeStrings(s, sink);
-    return sink;
-}
-
-Sink & operator << (Sink & sink, const StringSet & s)
-{
-    writeStrings(s, sink);
-    return sink;
-}
-
-Sink & operator << (Sink & sink, const Error & ex)
+WireFormatGenerator SerializingTransform::operator()(const Error & ex)
 {
     auto & info = ex.info();
-    sink
-        << "Error"
-        << info.level
-        << "Error" // removed
-        << info.msg.str()
-        << 0 // FIXME: info.errPos
-        << info.traces.size();
+    co_yield "Error";
+    co_yield info.level;
+    co_yield "Error"; // removed
+    co_yield info.msg.str();
+    co_yield 0; // FIXME: info.errPos
+    co_yield info.traces.size();
     for (auto & trace : info.traces) {
-        sink << 0; // FIXME: trace.pos
-        sink << trace.hint.str();
+        co_yield 0; // FIXME: trace.pos
+        co_yield trace.hint.str();
     }
-    return sink;
 }
 
 

src/libutil/serialise.hh

@@ -350,33 +350,82 @@ inline Sink & operator<<(Sink & sink, Generator<Bytes> && g)
     return sink;
 }
 
-void writePadding(size_t len, Sink & sink);
-void writeString(std::string_view s, Sink & sink);
+struct SerializingTransform;
+using WireFormatGenerator = Generator<Bytes, SerializingTransform>;
 
-inline Sink & operator << (Sink & sink, uint64_t n)
+struct SerializingTransform
 {
-    unsigned char buf[8];
-    buf[0] = n & 0xff;
-    buf[1] = (n >> 8) & 0xff;
-    buf[2] = (n >> 16) & 0xff;
-    buf[3] = (n >> 24) & 0xff;
-    buf[4] = (n >> 32) & 0xff;
-    buf[5] = (n >> 40) & 0xff;
-    buf[6] = (n >> 48) & 0xff;
-    buf[7] = (unsigned char) (n >> 56) & 0xff;
-    sink({(char *) buf, sizeof(buf)});
-    return sink;
+    std::array<unsigned char, 8> buf;
+
+    Bytes operator()(uint64_t n)
+    {
+        buf[0] = n & 0xff;
+        buf[1] = (n >> 8) & 0xff;
+        buf[2] = (n >> 16) & 0xff;
+        buf[3] = (n >> 24) & 0xff;
+        buf[4] = (n >> 32) & 0xff;
+        buf[5] = (n >> 40) & 0xff;
+        buf[6] = (n >> 48) & 0xff;
+        buf[7] = (unsigned char) (n >> 56) & 0xff;
+        return {reinterpret_cast<const char *>(buf.begin()), 8};
+    }
+
+    static Bytes padding(size_t unpadded)
+    {
+        return Bytes("\0\0\0\0\0\0\0", unpadded % 8 ? 8 - unpadded % 8 : 0);
+    }
+
+    // opt in to generator chaining. without this co_yielding
+    // another generator of any type will cause a type error.
+    auto operator()(Generator<Bytes> && g)
+    {
+        return std::move(g);
+    }
+
+    // only choose this for *exactly* char spans, do not allow implicit
+    // conversions. this would cause ambiguities with strings literals,
+    // and resolving those with more string-like overloads needs a lot.
+    template<typename Span>
+        requires std::same_as<Span, std::span<char>> || std::same_as<Span, std::span<const char>>
+    Bytes operator()(Span s)
+    {
+        return s;
+    }
+    WireFormatGenerator operator()(std::string_view s);
+    WireFormatGenerator operator()(const Strings & s);
+    WireFormatGenerator operator()(const StringSet & s);
+    WireFormatGenerator operator()(const Error & s);
+};
+
+void writePadding(size_t len, Sink & sink);
+
+inline Sink & operator<<(Sink & sink, uint64_t u)
+{
+    return sink << [&]() -> WireFormatGenerator { co_yield u; }();
 }
 
-Sink & operator << (Sink & in, const Error & ex);
-Sink & operator << (Sink & sink, std::string_view s);
-Sink & operator << (Sink & sink, const Strings & s);
-Sink & operator << (Sink & sink, const StringSet & s);
+inline Sink & operator<<(Sink & sink, std::string_view s)
+{
+    return sink << [&]() -> WireFormatGenerator { co_yield s; }();
+}
 
+inline Sink & operator<<(Sink & sink, const Strings & s)
+{
+    return sink << [&]() -> WireFormatGenerator { co_yield s; }();
+}
+
+inline Sink & operator<<(Sink & sink, const StringSet & s)
+{
+    return sink << [&]() -> WireFormatGenerator { co_yield s; }();
+}
+
+inline Sink & operator<<(Sink & sink, const Error & ex)
+{
+    return sink << [&]() -> WireFormatGenerator { co_yield ex; }();
+}
 
 MakeError(SerialisationError, Error);
 
-
 template<typename T>
 T readNum(Source & source)
 {