[snapshot] Allow an empty v8_extra_library_files

... and do not artificially add at least one dummy.js file.

Until this CL we used to ensure the existence of at least one 'extra
native' by appending a short dummy.js file if v8_extra_library_files
was empty. This file is uselessly compiled and run at startup.

This CL removes that mechanism along with a minor tweak to handle an
empty natives blob.

Bug: v8:9736
Change-Id: I05bf7c54380e77a9105d5c1a1a76983c692faf60
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1809372
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63868}

src/snapshot/natives-external.cc

@@ -61,9 +61,10 @@ class NativesStore {
     // We expect the libraries in the following format:
     //   int: # of sources.
     //   2N blobs: N pairs of source name + actual source.
-    int library_count = source->GetInt();
-    for (int i = 0; i < library_count; ++i)
+    int library_count = source->GetIntSlow();
+    for (int i = 0; i < library_count; ++i) {
       store->ReadNameAndContentPair(source);
+    }
 
     return store;
   }

src/snapshot/snapshot-source-sink.h

@@ -63,6 +63,24 @@ class SnapshotByteSource final {
     return answer;
   }
 
+  int GetIntSlow() {
+    // Unlike GetInt, this reads only up to the end of the blob, even if less
+    // than 4 bytes are remaining.
+    // TODO(jgruber): Remove once the use in MakeFromScriptsSource is gone.
+    DCHECK(position_ < length_);
+    uint32_t answer = data_[position_];
+    if (position_ + 1 < length_) answer |= data_[position_ + 1] << 8;
+    if (position_ + 2 < length_) answer |= data_[position_ + 2] << 16;
+    if (position_ + 3 < length_) answer |= data_[position_ + 3] << 24;
+    int bytes = (answer & 3) + 1;
+    Advance(bytes);
+    uint32_t mask = 0xffffffffu;
+    mask >>= 32 - (bytes << 3);
+    answer &= mask;
+    answer >>= 2;
+    return answer;
+  }
+
   // Returns length.
   int GetBlob(const byte** data);
 

tools/js2c.py

@@ -202,19 +202,11 @@ def PrepareSources(source_files, native_type, emit_js):
   Returns:
     An instance of Sources.
   """
+  result = Sources()
   filters = BuildFilterChain()
 
   source_files_and_contents = [(f, ReadFile(f)) for f in source_files]
 
-  # Have a single not-quite-empty source file if there are none present;
-  # otherwise you get errors trying to compile an empty C++ array.
-  # It cannot be empty (or whitespace, which gets trimmed to empty), as
-  # the deserialization code assumes each file is nonempty.
-  if not source_files_and_contents:
-    source_files_and_contents = [("dummy.js", "(function() {})")]
-
-  result = Sources()
-
   for (source, contents) in source_files_and_contents:
     try:
       lines = filters(contents)