Fix Android memfd buffer registration rollback

[cursor]

Bug and impact

On Android non-split memfd channels, PublisherImpl::CreateOrAttachBuffers advanced ccb_->num_buffers before registering the new buffer FD with the server. If RegisterClientBuffer failed after that CAS, subscribers could observe a new buffer generation that the server could not provide, causing reload failures and eventual abort when a slot referenced the missing buffer.

Root cause

The Android FD broker requires server registration before other clients can map a buffer, but the error path returned without rolling back the shared num_buffers count or clearing the publisher's local unregistered buffer state.

Fix

Roll back ccb_->num_buffers on registration failure when it still points at the failed generation, clear the corresponding BCB sizes, and unmap/drop local buffers beyond the last fully registered generation. Also make SUBSPACE_SHMEM_MODE overridable so the Android memfd path can be tested on Linux, and update memfd includes accordingly.

Validation

• bazelisk test //client:client_test --test_filter=AndroidBufferRegistrationTest.FailedRegistrationRollsBackNumBuffers --cxxopt=-DSUBSPACE_SHMEM_MODE=SUBSPACE_SHMEM_MODE_ANDROID
• bazelisk test //client:client_test --test_filter=ClientTest.Resize1:ClientTest.ResizeCallback
• bazelisk test //client:client_test
• bazelisk test //client:client_test --cxxopt=-DSUBSPACE_SHMEM_MODE=SUBSPACE_SHMEM_MODE_ANDROID

  

[dallison]

Superseded by #82, which contains the same fix re-authored under a CLA-covered account (plus a build/test fix for the Android shmem path on platforms without memfd_create).