diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 3441b2472..acc0002ac 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -964,20 +964,11 @@ void LocalStore::invalidatePath(State & state, const Path & path)
 }
 
 
-void LocalStore::addToStore(const ValidPathInfo & info, const ref<std::string> & nar,
+void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
     RepairFlag repair, CheckSigsFlag checkSigs, std::shared_ptr<FSAccessor> accessor)
 {
     assert(info.narHash);
 
-    Hash h = hashString(htSHA256, *nar);
-    if (h != info.narHash)
-        throw Error("hash mismatch importing path '%s'; expected hash '%s', got '%s'",
-            info.path, info.narHash.to_string(), h.to_string());
-
-    if (nar->size() != info.narSize)
-        throw Error("size mismatch importing path '%s'; expected %s, got %s",
-            info.path, info.narSize, nar->size());
-
     if (requireSigs && checkSigs && !info.checkSignatures(*this, publicKeys))
         throw Error("cannot add path '%s' because it lacks a valid signature", info.path);
 
@@ -999,8 +990,27 @@ void LocalStore::addToStore(const ValidPathInfo & info, const ref<std::string> &
 
             deletePath(realPath);
 
-            StringSource source(*nar);
-            restorePath(realPath, source);
+            /* While restoring the path from the NAR, compute the hash
+               of the NAR. */
+            HashSink hashSink(htSHA256);
+
+            LambdaSource wrapperSource([&](unsigned char * data, size_t len) -> size_t {
+                size_t n = source.read(data, len);
+                hashSink(data, n);
+                return n;
+            });
+
+            restorePath(realPath, wrapperSource);
+
+            auto hashResult = hashSink.finish();
+
+            if (hashResult.first != info.narHash)
+                throw Error("hash mismatch importing path '%s'; expected hash '%s', got '%s'",
+                    info.path, info.narHash.to_string(), hashResult.first.to_string());
+
+            if (hashResult.second != info.narSize)
+                throw Error("size mismatch importing path '%s'; expected %s, got %s",
+                    info.path, info.narSize, hashResult.second);
 
             autoGC();
 
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index bbd50e1c1..0d6c17659 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -143,7 +143,7 @@ public:
     void querySubstitutablePathInfos(const PathSet & paths,
         SubstitutablePathInfos & infos) override;
 
-    void addToStore(const ValidPathInfo & info, const ref<std::string> & nar,
+    void addToStore(const ValidPathInfo & info, Source & source,
         RepairFlag repair, CheckSigsFlag checkSigs,
         std::shared_ptr<FSAccessor> accessor) override;
 
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 8830edcc3..64f9b8d68 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -590,32 +590,15 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 
     uint64_t total = 0;
 
-    auto progress = [&](size_t len) {
-        total += len;
-        act.progress(total, info->narSize);
-    };
-
-    struct MyStringSink : StringSink
-    {
-        typedef std::function<void(size_t)> Callback;
-        Callback callback;
-        MyStringSink(Callback callback) : callback(callback) { }
-        void operator () (const unsigned char * data, size_t len) override
-        {
-            StringSink::operator ()(data, len);
-            callback(len);
-        };
-    };
-
-    MyStringSink sink(progress);
-    srcStore->narFromPath({storePath}, sink);
-
+    // FIXME
+#if 0
     if (!info->narHash) {
         auto info2 = make_ref<ValidPathInfo>(*info);
         info2->narHash = hashString(htSHA256, *sink.s);
         if (!info->narSize) info2->narSize = sink.s->size();
         info = info2;
     }
+#endif
 
     if (info->ultimate) {
         auto info2 = make_ref<ValidPathInfo>(*info);
@@ -623,7 +606,16 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
         info = info2;
     }
 
-    dstStore->addToStore(*info, sink.s, repair, checkSigs);
+    auto source = sinkToSource([&](Sink & sink) {
+        LambdaSink wrapperSink([&](const unsigned char * data, size_t len) {
+            sink(data, len);
+            total += len;
+            act.progress(total, info->narSize);
+        });
+        srcStore->narFromPath({storePath}, wrapperSink);
+    });
+
+    dstStore->addToStore(*info, *source, repair, checkSigs);
 }
 
 
@@ -808,6 +800,21 @@ std::string makeFixedOutputCA(bool recursive, const Hash & hash)
 }
 
 
+void Store::addToStore(const ValidPathInfo & info, Source & narSource,
+    RepairFlag repair, CheckSigsFlag checkSigs,
+    std::shared_ptr<FSAccessor> accessor)
+{
+    addToStore(info, make_ref<std::string>(narSource.drain()), repair, checkSigs, accessor);
+}
+
+void Store::addToStore(const ValidPathInfo & info, const ref<std::string> & nar,
+    RepairFlag repair, CheckSigsFlag checkSigs,
+    std::shared_ptr<FSAccessor> accessor)
+{
+    StringSource source(*nar);
+    addToStore(info, source, repair, checkSigs, accessor);
+}
+
 }
 
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 563aa566b..ea259f07e 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -399,9 +399,14 @@ public:
     virtual bool wantMassQuery() { return false; }
 
     /* Import a path into the store. */
+    virtual void addToStore(const ValidPathInfo & info, Source & narSource,
+        RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs,
+        std::shared_ptr<FSAccessor> accessor = 0);
+
+    // FIXME: remove
     virtual void addToStore(const ValidPathInfo & info, const ref<std::string> & nar,
         RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs,
-        std::shared_ptr<FSAccessor> accessor = 0) = 0;
+        std::shared_ptr<FSAccessor> accessor = 0);
 
     /* Copy the contents of a path to the store and register the
        validity the resulting path.  The resulting path is returned.
diff --git a/src/libutil/local.mk b/src/libutil/local.mk
index 5fc2aab56..824f48fbf 100644
--- a/src/libutil/local.mk
+++ b/src/libutil/local.mk
@@ -6,7 +6,7 @@ libutil_DIR := $(d)
 
 libutil_SOURCES := $(wildcard $(d)/*.cc)
 
-libutil_LDFLAGS = $(LIBLZMA_LIBS) -lbz2 -pthread $(OPENSSL_LIBS) $(LIBBROTLI_LIBS)
+libutil_LDFLAGS = $(LIBLZMA_LIBS) -lbz2 -pthread $(OPENSSL_LIBS) $(LIBBROTLI_LIBS) -lboost_context
 
 libutil_LIBS = libformat
 
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 9e2a502af..6b7939225 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -5,6 +5,8 @@
 #include <cerrno>
 #include <memory>
 
+#include <boost/coroutine2/coroutine.hpp>
+
 
 namespace nix {
 
@@ -88,6 +90,23 @@ void Source::operator () (unsigned char * data, size_t len)
 }
 
 
+std::string Source::drain()
+{
+    std::string s;
+    std::vector<unsigned char> buf(8192);
+    while (true) {
+        size_t n;
+        try {
+            n = read(buf.data(), buf.size());
+            s.append((char *) buf.data(), n);
+        } catch (EndOfFile &) {
+            break;
+        }
+    }
+    return s;
+}
+
+
 size_t BufferedSource::read(unsigned char * data, size_t len)
 {
     if (!buffer) buffer = decltype(buffer)(new unsigned char[bufSize]);
@@ -138,6 +157,50 @@ size_t StringSource::read(unsigned char * data, size_t len)
 }
 
 
+std::unique_ptr<Source> sinkToSource(std::function<void(Sink &)> fun)
+{
+    struct SinkToSource : Source
+    {
+        typedef boost::coroutines2::coroutine<std::string> coro_t;
+
+        coro_t::pull_type coro;
+
+        SinkToSource(std::function<void(Sink &)> fun)
+            : coro([&](coro_t::push_type & yield) {
+                LambdaSink sink([&](const unsigned char * data, size_t len) {
+                    if (len) yield(std::string((const char *) data, len));
+                });
+                fun(sink);
+            })
+        {
+        }
+
+        std::string cur;
+        size_t pos = 0;
+
+        size_t read(unsigned char * data, size_t len) override
+        {
+            if (!coro)
+                throw EndOfFile("coroutine has finished");
+
+            if (pos == cur.size()) {
+                if (!cur.empty()) coro();
+                cur = std::move(coro.get());
+                pos = 0;
+            }
+
+            auto n = std::min(cur.size() - pos, len);
+            memcpy(data, (unsigned char *) cur.data() + pos, n);
+            pos += n;
+
+            return n;
+        }
+    };
+
+    return std::make_unique<SinkToSource>(fun);
+}
+
+
 void writePadding(size_t len, Sink & sink)
 {
     if (len % 8) {
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 103b05767..d0b4552e3 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -61,6 +61,8 @@ struct Source
     virtual size_t read(unsigned char * data, size_t len) = 0;
 
     virtual bool good() { return true; }
+
+    std::string drain();
 };
 
 
@@ -191,6 +193,27 @@ struct LambdaSink : Sink
 };
 
 
+/* Convert a function into a source. */
+struct LambdaSource : Source
+{
+    typedef std::function<size_t(unsigned char *, size_t)> lambda_t;
+
+    lambda_t lambda;
+
+    LambdaSource(const lambda_t & lambda) : lambda(lambda) { }
+
+    size_t read(unsigned char * data, size_t len) override
+    {
+        return lambda(data, len);
+    }
+};
+
+
+/* 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);
+
+
 void writePadding(size_t len, Sink & sink);
 void writeString(const unsigned char * buf, size_t len, Sink & sink);
 
diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index 30e2b5455..50258018a 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -695,7 +695,7 @@ static void performOp(TunnelLogger * logger, ref<LocalStore> store,
         parseDump(tee, tee.source);
 
         logger->startWork();
-        store->addToStore(info, tee.source.data, (RepairFlag) repair,
+        store.cast<Store>()->addToStore(info, tee.source.data, (RepairFlag) repair,
             dontCheckSigs ? NoCheckSigs : CheckSigs, nullptr);
         logger->stopWork();
         break;