src: add utilities to help debugging reproducibility of snapshots

- Print offsets in blob serializer
- Add a special node:generate_default_snapshot ID to generate
  the built-in snapshot.
- Improve logging

src/blob_serializer_deserializer-inl.h

@@ -238,7 +238,8 @@ size_t BlobSerializer<Impl>::WriteVector(const std::vector<T>& data) {
   if (is_debug) {
     std::string str = std::is_arithmetic_v<T> ? "" : ToStr(data);
     std::string name = GetName<T>();
-    Debug("\nWriteVector<%s>() (%d-byte), count=%d: %s\n",
+    Debug("\nAt 0x%x: WriteVector<%s>() (%d-byte), count=%d: %s\n",
+          sink.size(),
           name.c_str(),
           sizeof(T),
           data.size(),
@@ -270,7 +271,10 @@ size_t BlobSerializer<Impl>::WriteVector(const std::vector<T>& data) {
 template <typename Impl>
 size_t BlobSerializer<Impl>::WriteStringView(std::string_view data,
                                              StringLogMode mode) {
-  Debug("WriteStringView(), length=%zu: %p\n", data.size(), data.data());
+  Debug("At 0x%x: WriteStringView(), length=%zu: %p\n",
+        sink.size(),
+        data.size(),
+        data.data());
   size_t written_total = WriteArithmetic<size_t>(data.size());
 
   size_t length = data.size();
@@ -294,17 +298,27 @@ size_t BlobSerializer<Impl>::WriteString(const std::string& data) {
   return WriteStringView(data, StringLogMode::kAddressAndContent);
 }
 
+static size_t kPreviewCount = 16;
+
 // Helper for writing an array of numeric types.
 template <typename Impl>
 template <typename T>
 size_t BlobSerializer<Impl>::WriteArithmetic(const T* data, size_t count) {
   static_assert(std::is_arithmetic_v<T>, "Arithmetic type");
   DCHECK_GT(count, 0);  // Should not write contents for vectors of size 0.
   if (is_debug) {
-    std::string str =
-        "{ " + std::to_string(data[0]) + (count > 1 ? ", ... }" : " }");
+    size_t preview_count = count < kPreviewCount ? count : kPreviewCount;
+    std::string str = "{ ";
+    for (size_t i = 0; i < preview_count; ++i) {
+      str += (std::to_string(data[i]) + ",");
+    }
+    if (count > preview_count) {
+      str += "...";
+    }
+    str += "}";
     std::string name = GetName<T>();
-    Debug("Write<%s>() (%zu-byte), count=%zu: %s",
+    Debug("At 0x%x: Write<%s>() (%zu-byte), count=%zu: %s",
+          sink.size(),
           name.c_str(),
           sizeof(T),
           count,

src/node.cc

@@ -1306,18 +1306,24 @@ ExitCode GenerateAndWriteSnapshotData(const SnapshotData** snapshot_data_ptr,
       return exit_code;
     }
   } else {
+    std::optional<std::string> builder_script_content;
     // Otherwise, load and run the specified builder script.
     std::unique_ptr<SnapshotData> generated_data =
         std::make_unique<SnapshotData>();
-    std::string builder_script_content;
-    int r = ReadFileSync(&builder_script_content, builder_script.c_str());
-    if (r != 0) {
-      FPrintF(stderr,
-              "Cannot read builder script %s for building snapshot. %s: %s",
-              builder_script,
-              uv_err_name(r),
-              uv_strerror(r));
-      return ExitCode::kGenericUserError;
+    if (builder_script != "node:generate_default_snapshot") {
+      builder_script_content = std::string();
+      int r = ReadFileSync(&(builder_script_content.value()),
+                           builder_script.c_str());
+      if (r != 0) {
+        FPrintF(stderr,
+                "Cannot read builder script %s for building snapshot. %s: %s\n",
+                builder_script,
+                uv_err_name(r),
+                uv_strerror(r));
+        return ExitCode::kGenericUserError;
+      }
+    } else {
+      snapshot_config.builder_script_path = std::nullopt;
     }
 
     exit_code = node::SnapshotBuilder::Generate(generated_data.get(),

src/node_snapshotable.cc

@@ -603,7 +603,7 @@ std::vector<char> SnapshotData::ToBlob() const {
   written_total += w.WriteArithmetic<uint32_t>(kMagic);
   w.Debug("Write metadata\n");
   written_total += w.Write<SnapshotMetadata>(metadata);
-
+  w.Debug("Write snapshot blob\n");
   written_total += w.Write<v8::StartupData>(v8_snapshot_blob_data);
   w.Debug("Write isolate_data_indices\n");
   written_total += w.Write<IsolateDataSerializeInfo>(isolate_data_info);