From d6f1e2de21b090ada8f658751ec1c528f13316bc Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 23 Oct 2025 21:17:09 +0200
Subject: [PATCH] Merge pull request #14323 from NixOS/skip-nar-parse

addToStore(): Don't parse the NAR

* StringSource: Implement skip()

This is slightly faster than doing a read() into a buffer just to
discard the data.

* LocalStore::addToStore(): Skip unnecessary NARs rather than parsing them

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
---
 src/libstore/local-store.cc               |  6 ++----
 src/libutil/include/nix/util/serialise.hh |  2 ++
 src/libutil/serialise.cc                  | 10 ++++++++++
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 59d5cc24f..3f108f97e 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1048,15 +1048,13 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source, RepairF
         /* In case we are not interested in reading the NAR: discard it. */
         bool narRead = false;
         Finally cleanup = [&]() {
-            if (!narRead) {
-                NullFileSystemObjectSink sink;
+            if (!narRead)
                 try {
-                    parseDump(sink, source);
+                    source.skip(info.narSize);
                 } catch (...) {
                     // TODO: should Interrupted be handled here?
                     ignoreExceptionInDestructor();
                 }
-            }
         };
 
         addTempRoot(info.path);
diff --git a/src/libutil/include/nix/util/serialise.hh b/src/libutil/include/nix/util/serialise.hh
index 8799e128f..d6845a494 100644
--- a/src/libutil/include/nix/util/serialise.hh
+++ b/src/libutil/include/nix/util/serialise.hh
@@ -255,6 +255,8 @@ struct StringSource : Source
     }
 
     size_t read(char * data, size_t len) override;
+
+    void skip(size_t len) override;
 };
 
 /**
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 47a00c8d6..ba153625e 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -242,6 +242,16 @@ size_t StringSource::read(char * data, size_t len)
     return n;
 }
 
+void StringSource::skip(size_t len)
+{
+    const size_t remain = s.size() - pos;
+    if (len > remain) {
+        pos = s.size();
+        throw EndOfFile("end of string reached");
+    }
+    pos += len;
+}
+
 std::unique_ptr<FinishSink> sourceToSink(std::function<void(Source &)> fun)
 {
     struct SourceToSink : FinishSink