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ai-app/docs/CLIENT_CORE.md
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iris bf3479f5c4 client-core: an unasked page is not an empty one, and two guarded invariants
Review of 73251d6's port of TranscriptSource/joinPages.

`TranscriptSource::page` answered `before == 0` with an empty `Vec`, which
is the same value it answers "this conversation has no more history" with.
That is the state the Kotlin keeps apart: `loadOlderPage` returns false at
`oldestSeq == 0` *without* touching `moreHistory`, and returns false on an
empty page *by latching it*. Collapsing the two moved AGENTS.md's paging
bug one layer down rather than fixing it. `page` returns `OlderPage` now --
`Events(vec![])` is the start of the conversation, `NothingLoaded` is not
an answer about the conversation at all.

`join_pages`' `debug_assert!` on seq ordering across the boundary is not a
true invariant: a peer note carries the seq its turn began at, which can be
older than the page it arrived in, so an ordinary transcript would have
panicked a debug build there. Replaced with the one the function exists to
enforce -- no tool id surviving in both halves.

`fetch_transcript_lines` stores `RawValue`'s exact server bytes, so the
"neither source can produce a newline" comment in `SessionCache::append`
now rests on the server's serializer staying compact rather than on a
local normalization. Checked with a `debug_assert!` in `append` and
`store_page` rather than trusted.

Tests for the failure half, which the port had none of: a 500 mid-page, a
cached line this build cannot read, and the `after` bound in the case that
actually carries one (the existing test asserted only the case with no
bound). `cargo fmt`, `cargo clippy --all-targets`, `cargo test` (112) clean
in client-core; `cargo check -p desktop-app` clean.
2026-09-06 13:00:37 -04:00

214 lines
13 KiB
Markdown

# client-core
`client-core/` is the app's pure logic held once instead of twice, per
RUST.md's recommendation item 1. It is a plain Rust library crate with no UI
framework dependency of any kind, so it can outlive whichever one the app
ends up drawing with (Masonry, iris, or something else -- see RUST.md).
`event-model/` is its sibling: the wire shape both this crate and `server/`
share, extracted from `server/src/session/driver.rs` and
`session/transcript.rs` on 2026-09-04.
Neither crate is wired into anything yet. `server/` re-exports `event-model`
so its own behaviour is unchanged (`./run-tests.sh` covers it); `client-core`
has no caller -- it exists for whichever experiment in RUST.md picks it up
next (a Masonry or iris transcript screen, most likely).
## What's here, and what Kotlin file it replaces
| `client-core/src/…` | Kotlin original | Status |
|------------------------------------------|-------------------------------------------|--------|
| `event-model/src/lib.rs` (shared crate) | `Events.kt` (the enum mirror) | Done |
| `ansi.rs` | `Ansi.kt` | Done, ported test-for-test |
| `highlight/mod.rs`, `languages.rs` | `Highlighter.kt`, `Languages.kt` | Done, ported test-for-test |
| `highlight/markdown.rs` | `MarkdownSyntax.kt` | Done, ported test-for-test |
| `transcript_cache.rs` | `TranscriptCache.kt` | Done, ported test-for-test |
| `sse.rs` | `Sse.kt` (the framing half) | Done, new tests (Kotlin had none of its own beyond integration) |
| `api.rs` | `Api.kt` | Partial -- see below |
| `event_stream.rs` | `EventStream.kt` | Done |
| `transcript_fold.rs` | `TranscriptItems.kt`, `ToolRows.kt` | Done -- see below |
| `config.rs` | `ServerConfig.kt`'s `handleEnrollment` | New, desktop-only so far -- see below |
| `transcript_source.rs` | `TranscriptSource.kt` | Done -- see below |
| *(not ported, and may never be)* | `TranscriptUnits.kt` | Out of scope -- see below |
Every file above whose Kotlin counterpart had a JVM unit test (`AnsiTest`,
`HighlighterTest`, `TranscriptCacheTest`) has had every one of those test
cases ported alongside it, plus new tests for the pieces that had none
(`sse.rs`, `api.rs`, `event_stream.rs`, `transcript_fold.rs`,
`transcript_source.rs` -- the Kotlin `TranscriptSource.kt`/`TranscriptItems.kt`
had no JVM unit tests of their own, so these were written fresh against the
Kotlin source and AGENTS.md's paging incidents as the spec). Test count by
crate as of this writing: **109 in `client-core`**, 0 in `event-model` (its
types carry no logic of their own to test -- `server/`'s own tests exercise
them via `session::transcript`'s round-trip coverage).
## Correspondence notes worth knowing before touching either side
- **`ansi.rs`'s `StyledText`/`Style`/`Rgb`** stand in for Compose's
`AnnotatedString`/`SpanStyle`/`Color`, since this crate has no Compose.
`StyledText` is plain text plus a `Vec<(Range<usize>, Style)>` of
non-overlapping spans. Whatever UI framework ends up consuming this
crate maps `Style` onto its own text-styling type; nothing here should
change to accommodate a particular one.
- **`highlight`'s `Span`/`Kind`** use **char indices, not byte offsets**
(`Vec<char>` internally), mirroring the Kotlin original's `Char`-indexed
strings. `highlight::span_text` turns a `Span` back into text for a
caller working the same way; a caller that wants byte offsets into a
`&str` has to convert.
- **`transcript_cache.rs`'s `SessionCache::guard`** found a real
translation bug while it was being written: an early draft let a
*damaged* chunk (one file unreadable, discard just this session) and a
genuine I/O failure (disk gone, disable the whole cache) both surface as
the same `Err` from one closure, which would have disabled every
session's cache over a single corrupt chunk. Fixed by checking a
thread-local "was this damage" flag before deciding which failure mode
it was -- see the comment on `guard` and the commit message for
`transcript_cache.rs`.
## What `api.rs` covers, and what it does not yet
`ApiClient` wraps a `Transport` trait (network I/O kept out from behind, so
`ApiClient` and `event_stream::follow_session_events` are tested with a
fake transport and no server). `UreqTransport` is the only real
implementation, backed by `ureq` -- see its Cargo.toml comment for why
(blocking, already a project dependency, no extra TLS crate needed since
`ureq::tls::Certificate::from_pem` reads the pinned CA directly).
Covered: session list/read, message send, unqueue, answer, interrupt,
stop, start, rename, cwd, model, permission-mode, notify, command,
compact, delete, and one transcript page.
**Not covered, and each is real work rather than a stub to fill in:**
setups (`/setups*`, machine and provider discovery), the file explorer
(`/setups/{id}/dir|file`), usage (`/usage`), models
(`/models*`, HuggingFace browsing and downloads), attachments
(`/sessions/{id}/attachments`), importing (`/setups/{id}/importable*`),
and the `/notifications` stream. `server/src/routes.rs`'s module doc is
the full table to work from when one of these is next.
## What `transcript_fold.rs` covers, and what it does not yet
`fold_event` covers every `Event` variant server/ can produce today,
including tool-call/question/image attachment and peer-message placement.
`group_tool_runs` groups adjacent calls into `TranscriptRow::Tools`.
`join_pages` (with `heal_split_message` and `adopt_run`, both private) is
now ported too, 2026-09-06 -- the page-boundary healing that merges a tool
call split across two fetched pages, rejoins a message a boundary cut
through, and renames a run of tool calls onto whichever name is already on
screen. Ported with AGENTS.md's "things that have bitten" incidents as the
spec rather than a JVM test file (`TranscriptItems.kt` had none of its
own): `a_clean_boundary_between_two_finished_runs_is_still_healed_into_one_run`
is the regression test for the bug that shipped -- `adopt_run` must run on
*every* join, not only the one where a split call was found, or a boundary
landing cleanly between two already-finished calls (most of them) leaves
one run drawn as two. `a_call_split_across_the_boundary_merges_into_one_row`,
`a_message_split_across_the_boundary_is_rejoined_with_the_newer_halfs_identity`,
and `adopt_run_never_renames_into_a_question_row` cover the other three
edges the Kotlin doc calls out. `join_pages` ends in a `debug_assert!`
that no tool id survives in both halves -- the duplicate row it exists to
prevent, checked rather than assumed. What it deliberately does *not*
assert is seq ordering across the boundary: a peer note carries the seq
its turn began at (`place_peer_note`), which can be older than the page
it arrived in, so the two pages' seqs legitimately interleave there. An
earlier draft asserted it and would have panicked in debug builds on an
ordinary transcript.
**Known gap, and a decision for whoever closes it:** `event_model::Event`
has no `Unknown`/catch-all variant, unlike `Events.kt`'s hand-kept mirror.
A server newer than this build that adds an event type will fail to parse
that line rather than degrading to a placeholder row. Closing this means
deciding how `event_model` itself represents "a shape I don't recognise"
-- a shared-model decision affecting `server/` too, not a `client-core`-only
fix, so it is recorded here rather than silently worked around.
## `config.rs`: `EnrolledServer`
`EnrolledServer` (host, port, bearer token) plus `parse_link`, which reads
the exact `aiapp://enroll?host=H&port=P&token=T` deep link
`wg-app-link`'s `enroll` mints and `ServerConfig.kt`'s `handleEnrollment`
parses on the phone -- so any Rust client enrols from the same text a
phone would scan as a QR, with no second format invented for it (RUST.md's
E4, DECISIONS.md 2026-09-05). Deliberately does not decide where it is
persisted or under what file permissions -- a phone seals its token in the
Android Keystore, `iris/desktop-app/src/config.rs` writes it to
`$XDG_CONFIG_HOME/ai-app-desktop/enrollment.json` at 0600 -- since that is
caller-specific (the code rules' "ask for the least you need"). Its only
caller today is `desktop-app`; a future Android build of this crate would
be a second one, not a reason to move the type.
## What `transcript_source.rs` covers, and what it does not
`TranscriptSource<T: Transport>` is the seam a session screen asks for a
page, ported test-for-test against the Kotlin doc rather than a JVM test
file (there wasn't one): `cached_opening`, `probe`, `fetch_opening`,
`page` and `follow`, each matching its Kotlin namesake's contract --
including `probe`'s three-way outcome (matches / cache purged /
unreachable, told apart so a caller never treats "couldn't ask" as "was
wrong") and `page`'s cache-vs-server split bounded by `covered_up_to`.
Two additions beyond a literal port, both load-bearing:
- **`page(before, ..)` refuses `before == 0` before touching the cache or
the network**, answering `OlderPage::NothingLoaded`. This is AGENTS.md's
`loadOlderPage` incident (`before = 0` is "no event before the first
one," indistinguishable from "reached the start of history" if a caller
ever asks it) moved out of the Kotlin screen and into this layer, so
every future caller gets the guard rather than having to remember it.
**The return type is `OlderPage`, not a `Vec`, and that is the guard.**
The Kotlin's two falses are different answers -- `oldestSeq == 0`
returns without touching `moreHistory`, an empty page latches it false
-- so a port that answered both with an empty list would have moved the
bug rather than fixed it, one layer down and out of sight of the screen
that used to hold the check. `OlderPage::Events(vec![])` means the start
of the conversation; `OlderPage::NothingLoaded` is not an answer about
the conversation at all. Reviewed 2026-09-06.
`paging_before_the_first_event_makes_no_request_at_all` asserts zero
transport calls, not just the variant, since a request that happens to
answer empty is exactly what caused the original bug, and
`a_failing_server_page_is_an_error_rather_than_an_empty_one` plus
`an_unreadable_cached_page_is_a_parse_error_rather_than_an_empty_one`
are the same rule for the two ways a page can fail.
- **`fetch_transcript_lines`** (new in `api.rs`) hands back each line
paired with the exact server bytes it came from, via
`serde_json::value::RawValue` rather than re-serializing a parsed
`Value` -- the cache and a live SSE frame for the same event have to
agree byte-for-byte, which is exactly what the `serde_json`
float-rounding bug (AGENTS.md) was about. The existing
`fetch_transcript_page` is untouched (other callers under `iris/`
depend on its signature); the two share a `transcript_path` helper so
the query string is written in one place.
**Not ported:** `EventStream.kt`'s reconnect-with-backoff loop, and
`TranscriptSource.close`'s ability to cancel a live stream from another
thread. Both are wall-clock/thread-lifetime policy that belongs to
whichever runtime embeds this crate (iris's own timers, a Tokio task, a
Kotlin coroutine scope), not to this pure logic -- `follow` is the same
"write to the cache, then hand the frame to the caller" decorator
`iris/desktop-app/src/app.rs` and `iris/android-app/src/transcript_client.rs`
already hand-wrote around `event_stream::follow_session_events` before this
existed; the cache write moved into one shared place so a third caller
does not repeat it again by hand.
## What is not started at all
- **The markdown *block* model beyond syntax spans** -- `highlight/markdown.rs`
colours a `.md` file or fence for the highlighter, but does not build the
block tree (headings, lists, tables, fences as distinct nodes) that a
renderer walks to lay out prose versus code versus a table.
`CodeFence.kt`'s use of `org.intellij.markdown` for that full CommonMark
AST is Compose rendering plumbing, not something to port as-is; a Rust
UI layer will want its own block parser or a crate for it, decided
alongside the framework choice in RUST.md.
- **`TranscriptUnits.kt`** (see above) -- deliberately out of scope, since
it flattens a row into bounded units for a *specific* lazy-list
framework's composition cost, which is a fact about that framework
rather than about the transcript.
## Verifying
`./run-tests.sh` from the repo root now runs `event-model`, `client-core`
and `server` in that order (each `cargo test`, forwarding arguments the
same way it always has). From `client-core/` directly: `cargo test`
(109 tests), `cargo clippy --all-targets`, `cargo fmt` -- all clean as of
this writing (2026-09-06).