diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 2519a2830..27ea621db 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -67,16 +67,18 @@ void BinaryCacheStore::upsertFile(const std::string & path,
     upsertFile(path, std::make_shared<std::stringstream>(std::move(data)), mimeType);
 }
 
-void BinaryCacheStore::getFile(const std::string & path, Sink & sink)
+box_ptr<Source> BinaryCacheStore::getFile(const std::string & path)
 {
-    sink(*getFileContents(path));
+    return make_box_ptr<GeneratorSource>([](std::string data) -> Generator<Bytes> {
+        co_yield std::span{data.data(), data.size()};
+    }(std::move(*getFileContents(path))));
 }
 
 std::optional<std::string> BinaryCacheStore::getFileContents(const std::string & path)
 {
     StringSink sink;
     try {
-        getFile(path, sink);
+        return getFile(path)->drain();
     } catch (NoSuchBinaryCacheFile &) {
         return std::nullopt;
     }
@@ -334,16 +336,15 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
     LengthSink narSize;
     TeeSink tee { sink, narSize };
 
-    auto decompressor = makeDecompressionSink(info->compression, tee);
 
     try {
-        getFile(info->url, *decompressor);
+        auto file = getFile(info->url);
+        auto decompressor = makeDecompressionSource(info->compression, *file);
+        decompressor->drainInto(tee);
     } catch (NoSuchBinaryCacheFile & e) {
         throw SubstituteGone(std::move(e.info()));
     }
 
-    decompressor->finish();
-
     stats.narRead++;
     //stats.narReadCompressedBytes += nar->size(); // FIXME
     stats.narReadBytes += narSize.length;
diff --git a/src/libstore/binary-cache-store.hh b/src/libstore/binary-cache-store.hh
index cd963fbf9..6c1f1348c 100644
--- a/src/libstore/binary-cache-store.hh
+++ b/src/libstore/binary-cache-store.hh
@@ -83,7 +83,7 @@ public:
     /**
      * Dump the contents of the specified file to a sink.
      */
-    virtual void getFile(const std::string & path, Sink & sink);
+    virtual box_ptr<Source> getFile(const std::string & path);
 
     virtual std::optional<std::string> getFileContents(const std::string & path);
 
diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index 06297e2eb..7ceea716a 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -150,12 +150,12 @@ protected:
 
     }
 
-    void getFile(const std::string & path, Sink & sink) override
+    box_ptr<Source> getFile(const std::string & path) override
     {
         checkEnabled();
         auto request(makeRequest(path));
         try {
-            getFileTransfer()->download(std::move(request))->drainInto(sink);
+            return getFileTransfer()->download(std::move(request));
         } catch (FileTransferError & e) {
             if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
                 throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());
diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
index 5ba5bd802..99dd0e0f6 100644
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@@ -68,10 +68,10 @@ protected:
         del.cancel();
     }
 
-    void getFile(const std::string & path, Sink & sink) override
+    box_ptr<Source> getFile(const std::string & path) override
     {
         try {
-            sink << readFileSource(binaryCacheDir + "/" + path);
+            return make_box_ptr<GeneratorSource>(readFileSource(binaryCacheDir + "/" + path));
         } catch (SysError & e) {
             if (e.errNo == ENOENT)
                 throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache", path);
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index 4aed791ce..4683f00f9 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -455,7 +455,7 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStoreConfig, public virtual
             uploadFile(path, istream, mimeType, "");
     }
 
-    void getFile(const std::string & path, Sink & sink) override
+    box_ptr<Source> getFile(const std::string & path) override
     {
         stats.get++;
 
@@ -469,7 +469,11 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStoreConfig, public virtual
             printTalkative("downloaded 's3://%s/%s' (%d bytes) in %d ms",
                 bucketName, path, res.data->size(), res.durationMs);
 
-            sink(*res.data);
+            return make_box_ptr<GeneratorSource>(
+                [](std::string data) -> Generator<Bytes> {
+                    co_yield std::span{data.data(), data.size()};
+                }(std::move(*res.data))
+            );
         } else
             throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());
     }
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index 13c2b27eb..278e5187c 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -294,7 +294,9 @@ Generator<Bytes> readFileSource(const Path & path)
     AutoCloseFD fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
     if (!fd)
         throw SysError("opening file '%s'", path);
-    co_yield drainFDSource(fd.get());
+    return [](AutoCloseFD fd) -> Generator<Bytes> {
+        co_yield drainFDSource(fd.get());
+    }(std::move(fd));
 }