SerializedScriptValue: Add a warning about making wire format changes near branch.

third_party/WebKit/Source/bindings/core/v8/SerializedScriptValue.h

 /*
  * Copyright (C) 2009, 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 62 matching lines @@
   // Version 8: File.lastModified in milliseconds (seconds-based in earlier
   //            versions.)
   // Version 9: Added Map and Set support.
   // [versions skipped]
   // Version 16: Separate versioning between V8 and Blink.
   // Version 17: Remove unnecessary byte swapping.
   //
   // The following versions cannot be used, in order to be able to
   // deserialize version 0 SSVs. The class implementation has details.
   // DO NOT USE: 35, 64, 68, 73, 78, 82, 83, 85, 91, 98, 102, 108, 123.
+  //
+  // WARNING: Increasing this value is a backward-incompatible change.

[jsbell, 2017/04/25 20:08:38]

"backward-incompatible" isn't quite correct - "backwards compatibile" means new
code can read old data, which we always maintain.

I don't know a succinct way to say it; maybe "breaks forwards-compatibility for
previous releases" ?

[jbroman, 2017/04/25 20:13:43]

Ah, you're quite correct. Hmm...I'm not sure your wording is as clear as I'd
like. WDYT of:

  // WARNING: Increasing this value is a change which cannot safely be rolled
  // back without breaking compatibility with data stored on disk. It is
  // strongly recommended that you do not make such changes near a release
  // milestone branch point.
  //
  // Recent changes are routinely reverted in preparation for branch, and this
  // has been the cause of at least one bug in the past.

+  // It is strongly recommended that you do not make backward-incompatible wire
+  // format changes if it is near a release milestone branch point.
+  // Recent changes are routinely reverted in preparation for branch, and this
+  // has been the cause of at least one bug in the past.
   static constexpr uint32_t kWireFormatVersion = 17;
 
   struct SerializeOptions {
     STACK_ALLOCATED();
     Transferables* transferables = nullptr;
     WebBlobInfoArray* blob_info = nullptr;
     bool write_wasm_to_stream = false;
   };
   static PassRefPtr<SerializedScriptValue> Serialize(v8::Isolate*,
                                                      v8::Local<v8::Value>,
@@ skipping 130 matching lines @@
       ExceptionState& exception_state) {
     return SerializedScriptValue::Serialize(
         isolate, value, SerializedScriptValue::SerializeOptions(),
         exception_state);
   }
 };
 
 }  // namespace blink
 
 #endif  // SerializedScriptValue_h