Closed in 55f6f7684f (PR #424) — slice (B) WRITE path landed.

PR #424 wired:

• pacquet_store_dir::add_files_from_dir — directory walker that re-CAFSes a post-build package directory. Mirrors pnpm/pnpm@7e3145f9fc:store/cafs/src/addFilesFromDir.ts:16-194 + worker/src/start.ts:312-383. Symlinks read/recurse via the resolved canonical path (TOCTOU-safe), top-level node_modules skipped, cycle-safe via recursion-stack visited set.
• pacquet_store_dir::upload + calculate_diff — upload(store_dir, built_pkg_location, files_index_file, cache_key, writer) walks the directory and queues a WriteMsg::SideEffectsUpload. The actual R/M/W (load existing row → algo == HASH_ALGORITHM check → diff → side_effects[cache_key] = diff) runs inside the writer task's transaction so concurrent uploads to the same row stay commutative.
• StoreIndexWriter driving a WriteMsg enum (Replace vs SideEffectsUpload). The batch loop coalesces per-key updates into one PackageFilesIndex before flushing.
• Config.side_effects_cache_readonly: bool (default false) — mirrors pnpm's sideEffectsCacheReadonly. Helpers Config::side_effects_cache_read() / side_effects_cache_write() consolidate the precedence.
• BuildModules WRITE call site — after run_postinstall_hooks returns Ok and the gate fires, calls pacquet_store_dir::upload(...). Upload errors are swallowed with tracing::warn!, matching upstream's try { upload } catch { logger.warn } at building/during-install/src/index.ts:208-215.
• build_deps_subgraph — the cache-hashing dep graph is bounded to the forward closure of requires_build snapshots, so pure-JS installs skip the walk entirely. Upstream's lockfileToDepGraph builds the full graph because its consumers extend beyond cache hashing; pacquet's graph is consumed only by calc_dep_state today, so the closure-bounded walk produces byte-identical cache keys with strictly less work.
• StoreIndexWriter hoisted to InstallFrozenLockfile::run so the writer spans both the prefetch/download phase (existing Replace queues) and the build phase (new SideEffectsUpload queues). The final batch flushes once BuildModules::run returns.
• Byte-stable msgpack — encode_package_files_index now iterates files, side_effects, and SideEffectsDiff.added in sorted-key order so two pacquet writes of the same logical row produce identical bytes on disk.

Tests ported / added:

• add_files_from_dir::tests (9 cases): top-level files, nested forward-slash paths, exec-bit propagation, top-level-vs-nested node_modules, symlink-out-of-root drop, symlink-in-root follow, directory-cycle termination, missing-root error.
• upload::tests (6 cases): identical / added-only / deleted-only / digest change / mode change / mixed — every calculateDiff branch.
• deps_graph::tests for build_deps_subgraph: empty-roots → empty graph, forward-closure inclusion/exclusion, cycle termination.
• workspace_yaml::tests: sideEffectsCacheReadonly round-trip + 4-state truth table.
• build_modules::tests:
  • write_path_populates_side_effects_row — pre-seed a base row, run a postinstall that creates generated.txt, drain the writer, assert side_effects[cache_key].added carries generated.txt and NOT pristine index.js. Mirrors 'a postinstall script does not modify the original sources added to the store' at pnpm/pnpm@7e3145f9fc:sideEffects.ts:189-223.
  • write_path_disabled_skips_upload — gate off → row's side_effects stays None.
  • upload_error_does_not_interrupt_install — postinstall produces a 0-permission file → walker fails on EACCES → BuildModules swallows with warn! → install completes, base row untouched. Mirrors 'uploading errors do not interrupt installation' at sideEffects.ts:166-187.

Benchmark at merge: cold install on par with main (~2.4s), hot cache ~7% faster than main (690ms vs 738ms), pacquet beats pnpm by 2.5× cold / 3.5× hot.

Both sub-tasks of this issue are now complete:

• ✅ (A) READ path — landed in 97d74398e1 (PR feat(package-manager): is_built gate for the side-effects cache READ path (#421 slice A) #423)
• ✅ (B) WRITE path — landed in 55f6f7684f (PR feat: side-effects cache WRITE path (#421 slice B) #424)

#397 item #10 fully closed.

Written by an agent (Claude Code, claude-opus-4-7).