nario v1 optimizations

src/libstore/export-import.cc

@@ -84,11 +84,15 @@ StorePaths importPaths(Store & store, Source & source, CheckSigsFlag checkSigs)
         /* Empty version 1 nario, nothing to do. */
         break;
 
-    case 1:
+    case 1: {
+        /* Reuse a string buffer to avoid kernel overhead allocating
+           memory for large strings. */
+        StringSink saved;
+
         /* Non-empty version 1 nario. */
         while (true) {
             /* Extract the NAR from the source. */
-            StringSink saved;
+            saved.s.clear();
             TeeSource tee{source, saved};
             NullFileSystemObjectSink ether;
             parseDump(ether, tee);
@@ -101,23 +105,26 @@ StorePaths importPaths(Store & store, Source & source, CheckSigsFlag checkSigs)
 
             auto references = CommonProto::Serialise<StorePathSet>::read(store, CommonProto::ReadConn{.from = source});
             auto deriver = readString(source);
-            auto narHash = hashString(HashAlgorithm::SHA256, saved.s);
-
-            ValidPathInfo info{path, narHash};
-            if (deriver != "")
-                info.deriver = store.parseStorePath(deriver);
-            info.references = references;
-            info.narSize = saved.s.size();
 
             // Ignore optional legacy signature.
             if (readInt(source) == 1)
                 readString(source);
 
-            // Can't use underlying source, which would have been exhausted.
-            auto source2 = StringSource(saved.s);
-            store.addToStore(info, source2, NoRepair, checkSigs);
+            if (!store.isValidPath(path)) {
+                auto narHash = hashString(HashAlgorithm::SHA256, saved.s);
 
-            res.push_back(info.path);
+                ValidPathInfo info{path, narHash};
+                if (deriver != "")
+                    info.deriver = store.parseStorePath(deriver);
+                info.references = references;
+                info.narSize = saved.s.size();
+
+                // Can't use underlying source, which would have been exhausted.
+                auto source2 = StringSource(saved.s);
+                store.addToStore(info, source2, NoRepair, checkSigs);
+            }
+
+            res.push_back(path);
 
             auto n = readNum<uint64_t>(source);
             if (n == 0)
@@ -126,6 +133,7 @@ StorePaths importPaths(Store & store, Source & source, CheckSigsFlag checkSigs)
                 throw Error("input doesn't look like a nario");
         }
         break;
+    }
 
     case exportMagicV2:
         while (true) {

src/libstore/local-store.cc

@@ -1049,12 +1049,16 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source, RepairF
         bool narRead = false;
         Finally cleanup = [&]() {
             if (!narRead) {
-                NullFileSystemObjectSink sink;
-                try {
-                    parseDump(sink, source);
-                } catch (...) {
-                    // TODO: should Interrupted be handled here?
-                    ignoreExceptionInDestructor();
+                if (info.narSize)
+                    source.skip(info.narSize);
+                else {
+                    NullFileSystemObjectSink sink;
+                    try {
+                        parseDump(sink, source);
+                    } catch (...) {
+                        // TODO: should Interrupted be handled here?
+                        ignoreExceptionInDestructor();
+                    }
                 }
             }
         };

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