Solution

• Add a comment to FlutterRect that new members cannot be added
• Add a unit test to verify new members aren't added to FlutterRect.

Background

The engine uses FlutterFrameBufferWithDamageCallback to notify the embedder of a damage region using an array of FlutterRect. Adding members to FlutterRect would break the ABI of this callback.

For example:

FlutterDamage damageRegion = ...;
FlutterRect first = damageRegion.damage[0];
FlutterRect second = damageRegion.damage[1]; // BAD! This could crash

The array indexing might crash if the external embedder wasn't recompiled after an engine update. Instead, external embedders must determine the size of FlutterRect using their struct_size member. Something like:

FlutterDamage damageRegion = ...;
FlutterRect first = damageRegion.damage[0];
FlutterRect second = static_cast<FlutterRect*>(
    static_cast<char*>(damageRegion.damage) + 1 * first.struct_size);

This problem would've been avoided if FlutterDamage.damage was an array of pointers to FlutterRect instead of arrays of FlutterRect.