typewriter/docs/notes/sync-commit-handoff.md

Handoff — the on-device commit must become an O(depth) TreeBuilder walk

Start here if you're picking up :sync performance. Written 2026-07-12 after benching git_bench on a full clone of the real jcalixte/notes repo. Full analysis + numbers: ../tradeoff-curves/sync-commit-staging.md. Latency memory: sync-timing. Why the repo can't shrink: git-sync-images-and-repo-size.md.

TL;DR

The current firmware::git_sync::stage_and_commit (add_all → index.write → write_tree) cannot commit the real repo — it is O(N_tree) and the real repo is 1179 files / 158 dirs / 570 MB pack / 150 MB of images. Measured on device:

• index.write() re-hashes the whole working tree → up to 611 s (10-min freeze).
• The index-free alternative (Index::new + read_tree(HEAD) + write_tree_to) still reads the whole tree cold → 77 s, and drove the esp_map.c mmap cache to 7.4 MB, starving zlib so repo.blob() crashed (zlib (5), 508 KB heap left).

The real repo has almost certainly never completed a :sync on device — only the toy typoena-test (notes.md) has. The repo cannot be shrunk (the 150 MB of images serve another app — see the images note). So the fix is a new commit mechanism, not a tuning knob.

The fix — O(depth) TreeBuilder walk

Rebuild only the edited file's ancestor subtree chain onto HEAD's tree. Never materialise the 1179-entry index; never index.write(); never read_tree the whole tree. Cost is O(depth × dirty_files), flat in repo size, tiny heap, and it carries every unchanged entry (all 260 images, the other 1176 files) forward untouched — which means the device doesn't even need the images in its working tree.

Sketch (git2 0.20 API — all present: Repository::treebuilder(Option<&Tree>), TreeBuilder::{insert,remove,write}, Tree::{get_name,get_path}, TreeEntry::{to_object,filemode}):

/// Return a new tree OID = `base` with `path` set to `new` (Some(blob_oid) to
/// add/replace, None to delete). Reads ~depth subtree objects, writes ~depth trees.
fn splice(repo: &Repository, base: &Tree, path: &[&str], new: Option<Oid>) -> Result<Oid> {
    let (head, rest) = path.split_first().unwrap();
    if rest.is_empty() {
        // leaf level: patch this tree directly
        let mut tb = repo.treebuilder(Some(base))?;
        match new {
            Some(oid) => tb.insert(head, oid, 0o100644)?,   // FileMode::Blob
            None       => { tb.remove(head).ok(); }          // delete
        };
        return Ok(tb.write()?);
    }
    // descend: find (or synthesize) the subtree, recurse, re-insert its new OID
    let sub = base.get_name(head)
        .and_then(|e| e.to_object(repo).ok())
        .and_then(|o| o.peel_to_tree().ok());
    let empty; let sub_ref = match &sub { Some(t) => t, None => { empty = repo.treebuilder(None)?; /* ... */ unreachable!() } };
    let new_sub = splice(repo, sub_ref, rest, new)?;
    let mut tb = repo.treebuilder(Some(base))?;
    // if new_sub is the empty tree and this was a delete, remove the dir entry instead
    tb.insert(head, new_sub, 0o040000)?;                    // FileMode::Tree
    Ok(tb.write()?)
}

Then, for the dirty set, fold each change through splice (thread the running root tree), and commit(Some("HEAD"), …, &final_tree, &[&parent]). Edge cases to handle: new files where an intermediate dir doesn't exist yet (build subtree from None), deletes that empty a directory (remove the dir entry rather than insert an empty tree), and path splitting on / (paths are repo-relative POSIX).

Bench it first (per the established discipline): add a treebuilder splice→tree op to firmware/src/bin/git_bench.rs alongside the existing ones and confirm it's sub-second cold on the real repo, with heap staying healthy (no OOM). Only then wire it into the firmware.

Firmware plumbing (after the bench validates)

1. firmware/src/git_sync.rs

   • Rewrite stage_and_commit (currently ~L271–332) to the splice-walk above. Drop add_all/update_all/index.write/index.write_tree. Keep the commit split — timing log, the tree unchanged → nothing to publish check, and the signature/message code.
   • PublishRequest (~L79) is currently an empty struct — it must carry the dirty set: { changed: Vec<(String /*repo-rel path*/, ...)>, deleted: Vec<String> }. The commit needs the blob content or a way to read it; simplest is to pass the paths and let the git thread repo.blob_path(abs) / read /sd/repo/<path>.
   • reconcile_onto_origin (~L377/L394) uses repo.reset(Mixed) — with an index-free commit there's no index to reset; switch to ResetType::Soft (move HEAD only) or drop the reset and just re-splice onto the new origin tip.
   • The macOS-cruft filter (skip_macos_cruft) is no longer needed — the walk only ever touches paths the editor explicitly hands it, so ._*/.DS_Store can't sneak in. (Keep a note; don't silently lose the Spike-14 lesson.)

2. Dirty-set source — firmware/src/persistence.rs + main.rs

   • Writes funnel through Storage::save_path (~L296) and deletes through Storage::delete_path (~L332), both &self. Accumulate a dirty/deleted set (needs RefCell interior mutability, or move the set up to main.rs).
   • Effect::Publish handler in main.rs (~L222) builds the PublishRequest from that set and clears it on a successful Pushed/UpToDate outcome.

3. esp_map.c cache fix (same pass) — firmware/components/libgit2/esp_map.c

   • Bug: cached buffers are freed only lazily in p_mmap's evict_for, so a window libgit2 has p_munmap'd stays resident until the next map — defeating MWINDOW_MAPPED_LIMIT and starving non-mmap git__malloc (zlib).
   • Fix: on p_munmap when refcount hits 0, evict down to a low-water mark (or free outright); lower ESP_MAP_CACHE_CAP (4 MB → ~1.5–2 MB) and/or SLOTS.
   • ⚠️ Editing this .c needs the fingerprint dance or the change won't rebuild — see the esp-idf-component-rebuild memory: rm -rf firmware/target/xtensa-esp32s3-espidf/release/.fingerprint/esp-idf-sys-* then rebuild.

How to bench / flash

git_bench.rs runs git ops on the 96 KB GIT_STACK thread (the main task stack overflows on these ops — that's why the real service has a dedicated thread). It's Rust-only, so a plain rebuild picks it up (no fingerprint dance unless you also touched esp_map.c).

just flash-gitbench
# = . ~/export-esp.sh && LIBGIT2_SRC=<repo>/firmware/components/libgit2/vendor \
#     LIBGIT2_NO_VENDOR=1 PKG_CONFIG_ALLOW_CROSS=1 \
#     PKG_CONFIG_LIBDIR=<repo>/firmware/pkgconfig \
#     cargo run --release --bin git_bench --features git

Bench on the real repo clone (/sd/repo = full jcalixte/notes), not the toy — the toy pack understates everything by ~2 orders of magnitude.

What's proven vs open

Proven (2026-07-12, real repo): odb.write 142 ms (mmap cache holds); index.write 611 s (whole-tree re-hash via truncate_racily_clean, index.c:822 / index.h:117); index-free read_tree 77 s cold; mmap cache OOM at 7.4 MB → zlib crash. Repository::open 88 ms, odb-open ~6 s cold (maps 1.7 MB .idx).

Open: the TreeBuilder walk is designed but not yet benched or built. Confirm its cold-real-repo latency and heap before wiring. Push (network half, ~6.5 s) is a separate floor, untouched here.