Notes for agents working on Crafter.Debug itself. The README is the user manual. `tooling/AGENTS.consumer.md` is what consumers' agents get.
## Project overview
Crafter.Debug lets an agent inspect and drive an app while it runs. `project.cpp` makes four configurations:
| Configuration | Holds |
|---|---|
| `Crafter.Debug.Core` (static) | What the app and the client share. Partitions:<br>• `:Value`: JSON value with its own parser and printer<br>• `:Reflect`: aggregate members and scoped-enum names, without `<meta>`<br>• `:Codec`: encode, decode, describe<br>• `:Result`: Error, Result, Async<br>• `:Protocol`: envelope, codes, versions, base64<br>• `:BuildInfo`: source id and client path, generated<br>• `:Discovery`: instance files and the lifetime lock |
| `Crafter.Debug.Cli` (static) | The client's logic:<br>• `:Client`: `Cli::Run`, with the transport injected<br>• `:Quic`: `ConnectQuic`, the transport `main` injects |
**The wire:** QUIC on 127.0.0.1, ALPN `crafter-debug/1`, through Crafter.Network. One request per stream the client opens: the request object and a FIN; back come zero or more stream lines (`{"entry": ...}`), then the reply (`{"result"}` or `{"error"}`) and a FIN. Every object is one line of JSON. `implementations/Crafter.Debug-Transport.cpp` opens with the rules on who may hold which lock.
`tooling/` is a separate project, `Crafter.Debug.Build`. Consumers load it with `#pragma crafter dependency ... project "tooling/project.cpp"`. Its `Debug::Use` adds the library under the consumer's flag and writes the consumer's `build/crafter-debug` launcher at load time.
`tooling/AGENTS.consumer.md` is the only notes file consumers get, and it covers both driving the app and coding with it. Crafter.Build merges a dependency's notes only when that dependency is in the build:
- **The tooling** comes in through the pragma, which is unconditional, so its notes are there after every build.
- **The root library** comes in only under the consumer's debug flag. Notes beside the root `project.cpp` would vanish after plain builds and appear twice after debug builds, so the root has none.
`examples/Basic` is a consumer that uses this checkout through a local pragma path.
## Dev environment
crafter-build comes from its Flatpak. Spell it out, since the Bash tool may not expand aliases: `flatpak run --die-with-parent net.catcrafts.CrafterBuild`. The host is Linux x86_64.
Tests run inside the Flatpak sandbox, where:
-`/tmp` is private;
- the toolchain is clang 23 with libc++;
-`/.flatpak-info` exists.
Every `flatpak run` is its own sandbox with its own pid namespace.
**Crafter.Network** comes from forgejo, unpinned, so every load `git pull`s it. `--local` takes `../Crafter.Network`, and `CRAFTER_DEBUG_NETWORK=<path to a project.cpp>` takes any checkout, a worktree say. The environment reaches the build inside the Flatpak.
The transport needs Network's callback API (`OnRecieve`, `SendAsync`, `OnClosed`, `ListenerQUIC` with a bind address and `Port()`) and its fix that lets stopped streams finish before `~ClientQUIC` closes. Until those reach Network's `master`, build with `CRAFTER_DEBUG_NETWORK` pointing at a checkout of the branch that has them.
CRAFTER_DEBUG_NETWORK=../Crafter.Network-quic-callbacks/project.cpp flatpak run --die-with-parent net.catcrafts.CrafterBuild test # Network from a worktree
cd examples/Basic && flatpak run --die-with-parent net.catcrafts.CrafterBuild --debug-api # a consumer's debug build
```
A test's own output appears only when it fails. To see its PASS lines, run the binary from `bin/<Test>-*/` with `flatpak run --die-with-parent --command=<path> net.catcrafts.CrafterBuild`.
## Testing instructions
**Registration:** tests are declared in `project.cpp`, and only when this is the top-level project:
-`ShouldRunClientCommands` and `ShouldServeOverQuic` link the library and the client. The second listens for real and drives the session over loopback with the client's own transport.
- Open `main.cpp` with a comment on the behaviour and the failure it guards against.
- Use a `Check(ok, what)` helper that prints PASS or FAIL, and return nonzero at the end.
**Session tests** use `Session({ .listen = false })` and hand requests to `Session::Handle` through a fake `Detail::Exchange`. Pump from the test's own thread, or from one thread playing the app.
**A listening session** needs `ThreadPool::Start()` with a worker first, `CRAFTER_DEBUG_DIR` set, and `std::_Exit` at the end after `std::fflush(stdout)`: msquic's teardown at exit waits for connections. Run it 30 times in a row before trusting it; one transport race showed in 4 runs of 30.
- Temporary files go under `fs::temp_directory_path() / "crafter-debug-<Test>"`.
- Discovery tests set `CRAFTER_DEBUG_DIR` there.
**Hangs:** anything that can hang gets a watchdog that calls `std::_Exit(1)` before the runner's 60 s.
**Pointers into temporaries:** `Value::Find` returns a pointer into its value. Never keep one from a temporary such as `ResultOf(...).Find(...)` past the full expression; copy the value into a local first. Two tests failed on exactly this.
## Code style
**Nothing enforces it.** No lint rules are registered: a `#pragma crafter dependency` on Crafter.LintRules would be built on every load of every consumer.
**Headers and imports:**
-`// SPDX-License-Identifier: LGPL-3.0-only`, with a space after `//`, then the copyright line, then a blank line. Examples are MIT.
-`import std;`. C headers go only in a global module fragment, or before the imports in a test.
**Formatting and naming:**
- Four-space indentation.
-`PascalCase` for types and functions, `camelCase` for variables, members and constants.
-`std::println` and `std::format`.
- Fixed-width integers.
-`enum class`.
**Comments** say why: the failure mode, the race, the rejected alternative. Long comments are deliberate.
## Design rules
1.**Release builds never link Crafter.Debug.** Consumers gate it through `Debug::Use` and their own define. The library has no no-op mode.
2.**No thread of Crafter.Debug's own, and nothing ever blocks one.** Transport threads only queue work or answer from the session's own state; the Pump thread runs app code; `Detail::Exchange::Reply` and `Stream` never block. The thread pool belongs to the host app. Network's own threads are ListenerQUIC's accept loop and msquic's workers. The one thread Crafter.Debug starts is the timer that ends the process 200 ms after `quit --force` has sent its reply.
3.**App code runs only on the Pump thread:** handlers, getters, setters, and condition evaluation. Never hold the session mutex while calling it, since app code logs, and logging re-enters the session.
4.**Nothing the Flatpak compiles runs on the host.** The launcher is POSIX sh and starts the client with `flatpak run`. On older hosts, the bundled libc++ and the runtime's glibc do not load.
5.**The client is built with the app.**`debug.hello`, the ALPN `crafter-debug/1` and the discovery record's `protocol` and `sourceId` are frozen. Bump `protocolVersion` on any wire change.
6.**Errors name the next step.** Decoding never throws. A request that timed out before `Pump` never runs.
7.**Discovery names instances by random ids and tells liveness by flock, never by pid.** Pids collide across `flatpak run` sandboxes.
- **Discovery files** are 0600 in a 0700 directory.
- **Debuggers:** the session calls `prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY)`, so a debugger the agent starts can attach. That is another reason it never ships in a release build.
## Gotchas
- **`Group` is neither copyable nor movable,** and neither is its owner. Its watches refer to the owner's members.
- **Inside a `Codec<T>` specialization, call the free functions qualified** (`Debug::Encode`): the member `Encode` hides them.
- **`std::format` needs a compile-time format string.** A ternary between two literals does not compile.
- **Tests don't inherit defines.** Add them with `.Define()`.
- **A consumer's `#ifdef`'d `import Crafter.Debug;`** is harmless in its plain builds.
- **`project.cpp` regenerates `build/generated/<core variant>/Crafter.Debug.Core-BuildInfo.cpp`** when the sources or the client's path change. `crafter-build clean` removes it, and the next load writes it again.
- **The launcher is written by `Debug::Use` while crafter-build loads the consumer,** not by the build. A plain build keeps an existing launcher; a fresh plain build writes the stub.
- **A consumer that depends on Crafter.Network itself** must name it exactly as `project.cpp` here does (same URL, no pin, same args). Otherwise the graph holds two Crafter.Network configurations, and crafter-build refuses with "module … built twice".
- **`debug.log`'s `last` of 0 means no backlog**, as `tail -n 0` does. A `TimelineQuery`'s `last` of 0 means all.
- **Answer before you raise a flag the main loop acts on.** The loop may end the Session at once. A reply given before the connections close is delivered; one given after is not. `quit` replies first, then sets `QuitRequested`.
