[wasm] Implement WebAssembly.Module.customSections.

src/wasm/module-decoder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/wasm/module-decoder.h"
 
 #include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/flags.h"
 #include "src/macro-assembler.h"
@@ skipping 1250 matching lines @@
     if (decoder.pc() != table_end) {
       decoder.error("broken asm offset table");
     }
     table.push_back(std::move(func_asm_offsets));
   }
   if (decoder.more()) decoder.error("unexpected additional bytes");
 
   return decoder.toResult(std::move(table));
 }
 
+std::vector<CustomSectionOffset> DecodeCustomSections(const byte* start,
+                                                      const byte* end) {
+  Decoder decoder(start, end);
+  decoder.consume_bytes(4, "wasm magic");
+  decoder.consume_bytes(4, "wasm version");
+
+  std::vector<CustomSectionOffset> result;
+
+  while (decoder.more()) {
+    byte section_code = decoder.consume_u8("section code");
+    uint32_t section_length = decoder.consume_u32v("section length");
+    uint32_t section_start = decoder.pc_offset();
+    if (section_code != 0) {
+      // Skip known sections.
+      decoder.consume_bytes(section_length, "section bytes");
+      continue;
+    }
+    uint32_t name_length = decoder.consume_u32v("name length");
+    uint32_t name_offset = decoder.pc_offset();
+    decoder.consume_bytes(name_length, "section name");
+    uint32_t payload_offset = decoder.pc_offset();
+    uint32_t payload_length = section_length - (payload_offset - section_start);
+    decoder.consume_bytes(payload_length);
+    result.push_back({section_start, name_offset, name_length, payload_offset,
+                      payload_length, section_length});
+  }
+
+  return result;
+}
+
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8