libutil: Throw if str("contents") not found

This was broken in 7aa3e7e3a5 (since 2.25).

src/libutil-tests/archive.cc

@@ -0,0 +1,47 @@
+#include "nix/util/archive.hh"
+#include "nix/util/tests/characterization.hh"
+#include "nix/util/tests/gmock-matchers.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+namespace {
+
+class NarTest : public CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "nars";
+
+public:
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / (std::string(testStem) + ".nar");
+    }
+};
+
+class InvalidNarTest : public NarTest, public ::testing::WithParamInterface<std::tuple<std::string, std::string>>
+{};
+
+} // namespace
+
+TEST_P(InvalidNarTest, throwsErrorMessage)
+{
+    const auto & [name, message] = GetParam();
+    readTest(name, [&](const std::string & narContents) {
+        ASSERT_THAT(
+            [&]() {
+                StringSource source{narContents};
+                NullFileSystemObjectSink sink;
+                parseDump(sink, source);
+            },
+            ::testing::ThrowsMessage<SerialisationError>(testing::HasSubstrIgnoreANSIMatcher(message)));
+    });
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    NarTest,
+    InvalidNarTest,
+    ::testing::Values(
+        std::pair{"invalid-tag-instead-of-contents", "bad archive: expected tag 'contents', got 'AAAAAAAA'"}));
+
+} // namespace nix

src/libutil-tests/meson.build

@@ -45,6 +45,7 @@ subdir('nix-meson-build-support/common')
 subdir('nix-meson-build-support/asan-options')
 
 sources = files(
+  'archive.cc',
   'args.cc',
   'base-n.cc',
   'canon-path.cc',

src/libutil/archive.cc

@@ -187,8 +187,10 @@ static void parse(FileSystemObjectSink & sink, Source & source, const CanonPath
                 tag = getString();
             }
 
-            if (tag == "contents")
-                parseContents(crf, source);
+            if (tag != "contents")
+                throw badArchive("expected tag 'contents', got '%s'", tag);
+
+            parseContents(crf, source);
 
             expectTag(")");
         });