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

@@ -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);

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;
 };
 
 /**

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