NullFileSystemObjectSink: Skip over file contents

src/libutil/archive.cc

@@ -132,6 +132,11 @@ static void parseContents(CreateRegularFileSink & sink, Source & source)
 
     sink.preallocateContents(size);
 
+    if (sink.skipContents) {
+        source.skip(size + (size % 8 ? 8 - (size % 8) : 0));
+        return;
+    }
+
     uint64_t left = size;
     std::array<char, 65536> buf;
 
@@ -166,7 +171,7 @@ static void parse(FileSystemObjectSink & sink, Source & source, const CanonPath
     auto expectTag = [&](std::string_view expected) {
         auto tag = getString();
         if (tag != expected)
-            throw badArchive("expected tag '%s', got '%s'", expected, tag);
+            throw badArchive("expected tag '%s', got '%s'", expected, tag.substr(0, 1024));
     };
 
     expectTag("(");

src/libutil/fs-sink.cc

@@ -196,6 +196,8 @@ void NullFileSystemObjectSink::createRegularFile(
         void isExecutable() override {}
     } crf;
 
+    crf.skipContents = true;
+
     // Even though `NullFileSystemObjectSink` doesn't do anything, it's important
     // that we call the function, to e.g. advance the parser using this
     // sink.

src/libutil/include/nix/util/fs-sink.hh

@@ -14,6 +14,14 @@ namespace nix {
  */
 struct CreateRegularFileSink : Sink
 {
+    /**
+     * If set to true, the sink will not be called with the contents
+     * of the file. `preallocateContents()` will still be called to
+     * convey the file size. Useful for sinks that want to efficiently
+     * discard the contents of the file.
+     */
+    bool skipContents = false;
+
     virtual void isExecutable() = 0;
 
     /**