From d7b6afecdb8b8ed4f714c15c55833e82b96b443d Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 25 Nov 2025 13:29:29 +0100
Subject: [PATCH 1/2] LambdaSink: Allow passing a destructor callback

---
 src/libutil/include/nix/util/serialise.hh | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/libutil/include/nix/util/serialise.hh b/src/libutil/include/nix/util/serialise.hh
index 09b33bf95..5db55c60d 100644
--- a/src/libutil/include/nix/util/serialise.hh
+++ b/src/libutil/include/nix/util/serialise.hh
@@ -447,18 +447,27 @@ struct LengthSource : Source
  */
 struct LambdaSink : Sink
 {
-    typedef std::function<void(std::string_view data)> lambda_t;
+    typedef std::function<void(std::string_view data)> data_t;
+    typedef std::function<void()> cleanup_t;
 
-    lambda_t lambda;
+    data_t dataFun;
+    cleanup_t cleanupFun;
 
-    LambdaSink(const lambda_t & lambda)
-        : lambda(lambda)
+    LambdaSink(
+        const data_t & dataFun, const cleanup_t & cleanupFun = []() {})
+        : dataFun(dataFun)
+        , cleanupFun(cleanupFun)
     {
     }
 
+    ~LambdaSink()
+    {
+        cleanupFun();
+    }
+
     void operator()(std::string_view data) override
     {
-        lambda(data);
+        dataFun(data);
     }
 };
 

From 7ba84437be28ef6bd7ada581e0edd0c4199f9d0c Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 25 Nov 2025 14:19:32 +0100
Subject: [PATCH 2/2] BinaryCacheStore::narFromPath(): Fix unreachable code

When this function is called as a coroutine (e.g. when it's called by
`copyStorePath()`), the code after `decompressor->finish()` is never
reached because the coroutine is destroyed when the caller reaches the
end of the NAR. So put that code in a `LambdaSink` destructor.
---
 src/libstore/binary-cache-store.cc | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index caae72479..e1f1d24c6 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -418,10 +418,20 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
 {
     auto info = queryPathInfo(storePath).cast<const NarInfo>();
 
-    LengthSink narSize;
-    TeeSink tee{sink, narSize};
+    uint64_t narSize = 0;
 
-    auto decompressor = makeDecompressionSink(info->compression, tee);
+    LambdaSink uncompressedSink{
+        [&](std::string_view data) {
+            narSize += data.size();
+            sink(data);
+        },
+        [&]() {
+            stats.narRead++;
+            // stats.narReadCompressedBytes += nar->size(); // FIXME
+            stats.narReadBytes += narSize;
+        }};
+
+    auto decompressor = makeDecompressionSink(info->compression, uncompressedSink);
 
     try {
         getFile(info->url, *decompressor);
@@ -431,9 +441,7 @@ void BinaryCacheStore::narFromPath(const StorePath & storePath, Sink & sink)
 
     decompressor->finish();
 
-    stats.narRead++;
-    // stats.narReadCompressedBytes += nar->size(); // FIXME
-    stats.narReadBytes += narSize.length;
+    // Note: don't do anything here because it's never reached if we're called as a coroutine.
 }
 
 void BinaryCacheStore::queryPathInfoUncached(