diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 925ac25bfc5..dac7a50c4e0 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1036,11 +1036,13 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 StorePath LocalStore::addToStoreFromDump(Source & dump, const string & name,
     FileIngestionMethod method, HashType hashAlgo, RepairFlag repair)
 {
-    return addToStoreCommon(name, method, hashAlgo, repair, [&](auto & sink) {
+    return addToStoreCommon(name, method, hashAlgo, repair, [&](auto & sink, size_t & wanted) {
         while (1) {
-            uint8_t buf[1];
-            auto n = dump.read(buf, 1);
+            constexpr size_t bufSize = 1024;
+            uint8_t buf[bufSize];
+            auto n = dump.read(buf, std::min(wanted, bufSize));
             sink(buf, n);
+            // when control is yielded back to us wanted will be updated.
         }
     });
 }
@@ -1051,7 +1053,7 @@ StorePath LocalStore::addToStore(const string & name, const Path & _srcPath,
 {
     Path srcPath(absPath(_srcPath));
 
-    return addToStoreCommon(name, method, hashAlgo, repair, [&](auto & sink) {
+    return addToStoreCommon(name, method, hashAlgo, repair, [&](auto & sink, size_t & _) {
         if (method == FileIngestionMethod::Recursive)
             dumpPath(srcPath, sink, filter);
         else
@@ -1062,7 +1064,7 @@ StorePath LocalStore::addToStore(const string & name, const Path & _srcPath,
 
 StorePath LocalStore::addToStoreCommon(
     const string & name, FileIngestionMethod method, HashType hashAlgo, RepairFlag repair,
-    std::function<void(Sink &)> demux)
+    std::function<void(Sink &, size_t &)> demux)
 {
     /* For computing the NAR hash. */
     auto sha256Sink = std::make_unique<HashSink>(htSHA256);
@@ -1083,7 +1085,7 @@ StorePath LocalStore::addToStoreCommon(
     bool inMemory = true;
     std::string nar;
 
-    auto source = sinkToSource([&](Sink & sink) {
+    auto source = sinkToSource([&](Sink & sink, size_t & wanted) {
         LambdaSink sink2([&](const unsigned char * buf, size_t len) {
             (*sha256Sink)(buf, len);
             if (hashSink) (*hashSink)(buf, len);
@@ -1101,7 +1103,7 @@ StorePath LocalStore::addToStoreCommon(
 
             if (!inMemory) sink(buf, len);
         });
-        demux(sink2);
+        demux(sink2, wanted);
     });
 
     std::unique_ptr<AutoDelete> delTempDir;
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 215731f87d4..ae23004c474 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -292,7 +292,7 @@ private:
 
     StorePath addToStoreCommon(
         const string & name, FileIngestionMethod method, HashType hashAlgo, RepairFlag repair,
-        std::function<void(Sink &)> demux);
+        std::function<void(Sink &, size_t &)> demux);
 
     Path getRealStoreDir() override { return realStoreDir; }
 
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index c8b71188fe0..141e9e9767f 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -165,35 +165,43 @@ size_t StringSource::read(unsigned char * data, size_t len)
 #endif
 
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink &)> fun,
+    std::function<void(Sink &, size_t &)> fun,
     std::function<void()> eof)
 {
     struct SinkToSource : Source
     {
-        typedef boost::coroutines2::coroutine<std::string> coro_t;
+        typedef boost::coroutines2::coroutine<std::basic_string<uint8_t>> coro_t;
 
-        std::function<void(Sink &)> fun;
+        std::function<void(Sink &, size_t &)> fun;
         std::function<void()> eof;
         std::optional<coro_t::pull_type> coro;
         bool started = false;
 
-        SinkToSource(std::function<void(Sink &)> fun, std::function<void()> eof)
+        /* It would be nicer to have the co-routines have both args and a
+           return value, but unfortunately that was removed from Boost's
+           implementation for some reason, so we use some extra state instead.
+           */
+       size_t wanted = 0;
+
+        SinkToSource(std::function<void(Sink &, size_t &)> fun, std::function<void()> eof)
             : fun(fun), eof(eof)
         {
         }
 
-        std::string cur;
+        std::basic_string<uint8_t> cur;
         size_t pos = 0;
 
         size_t read(unsigned char * data, size_t len) override
         {
-            if (!coro)
+            wanted = len < cur.size() ? 0 : len - cur.size();
+            if (!coro) {
                 coro = coro_t::pull_type([&](coro_t::push_type & yield) {
-                    LambdaSink sink([&](const unsigned char * data, size_t len) {
-                            if (len) yield(std::string((const char *) data, len));
+                    LambdaSink sink([&](const uint8_t * data, size_t len) {
+                            if (len) yield(std::basic_string<uint8_t> { data, len });
                         });
-                    fun(sink);
+                    fun(sink, wanted);
                 });
+            }
 
             if (!*coro) { eof(); abort(); }
 
@@ -203,11 +211,10 @@ std::unique_ptr<Source> sinkToSource(
                 pos = 0;
             }
 
-            auto n = std::min(cur.size() - pos, len);
-            memcpy(data, (unsigned char *) cur.data() + pos, n);
-            pos += n;
+            auto numCopied = cur.copy(data, len, pos);
+            pos += numCopied;
 
-            return n;
+            return numCopied;
         }
     };
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 8386a499124..6cb9d1bf506 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -260,11 +260,20 @@ struct LambdaSource : Source
 /* Convert a function that feeds data into a Sink into a Source. The
    Source executes the function as a coroutine. */
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink &)> fun,
+    std::function<void(Sink &, size_t &)> fun,
     std::function<void()> eof = []() {
         throw EndOfFile("coroutine has finished");
     });
 
+static inline std::unique_ptr<Source> sinkToSource(
+    std::function<void(Sink &)> fun,
+    std::function<void()> eof = []() {
+        throw EndOfFile("coroutine has finished");
+    })
+{
+	return sinkToSource([fun](Sink & s, size_t & _) { fun(s); }, eof);
+}
+
 
 void writePadding(size_t len, Sink & sink);
 void writeString(const unsigned char * buf, size_t len, Sink & sink);