backend: use unaligned PRG output for 32-bit bucket coin

[starius]

Summary

This fixes a Debug-build abort in TriplePool::layered_bucket_into_acc by using PRG::random_data_unaligned for the per-layer cyclic-shift coin draw.

The code draws four bytes into a stack uint32_t raw. PRG::random_data requires its destination pointer to be 16-byte aligned, but a uint32_t object is not guaranteed to satisfy that alignment. Debug builds hit emp-tool's alignment assert and abort.

The fix changes this 4-byte draw from random_data to random_data_unaligned, which is the API intended for destinations without 16-byte alignment.

Details

layered_bucket_into_acc derives a public per-layer cyclic-shift coin by hashing the channel digests into a seed block, initializing a local PRG, and drawing a uint32_t used as raw % L.

Before:

uint32_t raw;
{ PRG prg2(&S); prg2.random_data(&raw, sizeof(uint32_t)); }

After:

uint32_t raw;
{ PRG prg2(&S); prg2.random_data_unaligned(&raw, sizeof(uint32_t)); }

This preserves the protocol behavior while removing the invalid alignment assumption. Release builds may not abort because the check is an assert, but the old call still violated the PRG::random_data precondition.

Reproduction

The failure reproduces on the parent commit with a Debug build:

cmake -S . -B build-debug -G Ninja -DCMAKE_BUILD_TYPE=Debug -DCMAKE_PREFIX_PATH=/path/to/emp-prefix
cmake --build build-debug -j$(nproc)
./run ./build-debug/bin/test_context_api

The fixed commit passes the same test under the same build configuration.

Validation

The fixed commit passes these Debug tests:

./run ./build-debug/bin/test_context_api
./run ./build-debug/bin/test_direct_chunks
./run ./build-debug/bin/test_program_replay
./run ./build-debug/bin/test_body_replay_equiv

Crash log from the parent commit

test_context_api: emp-tool/runtime/crypto/prg.h:92: void emp::PRG::random_data(void*, int64_t): Assertion `((uintptr_t)data & (alignof(block) - 1)) == 0 && "random_data requires 16-byte aligned data; use random_data_unaligned"' failed.
test_context_api: emp-tool/runtime/crypto/prg.h:92: void emp::PRG::random_data(void*, int64_t): Assertion `((uintptr_t)data & (alignof(block) - 1)) == 0 && "random_data requires 16-byte aligned data; use random_data_unaligned"' failed.
./run: line 8: Aborted                 "$1" 2 "$PORT" "${@:2}"
./run: line 8: Aborted                 "$1" 1 "$PORT" "${@:2}"

Backtrace from the parent commit

#4  emp::PRG::random_data(void*, long) at emp-tool/runtime/crypto/prg.h:92
#5  TriplePool::layered_bucket_into_acc(...) at emp-ag2pc/backend/triple_pool.h:402
#6  TriplePool::compute_inplace(...) at emp-ag2pc/backend/triple_pool.h:446
#7  emp::AG2PCEngine::run_source(...) at emp-ag2pc/backend/engine.h:175
#8  emp::AG2PCEngine::run_program(...) at emp-ag2pc/backend/engine.h:240
#9  emp::AG2PCSession::flush_(...) at emp-ag2pc/session/ag2pc_session.h:394
#10 emp::AG2PCSession::reveal<emp::UInt_T<emp::AG2PCCtx, 32>>(...) at emp-ag2pc/session/ag2pc_session.h:153
#11 main(...) at test/test_context_api.cpp:29

[wangxiao1254]

Thanks!