Set up Cursor Cloud dev environment for 3FS

AGENTS.md

@@ -0,0 +1,70 @@
+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+3FS (Fire-Flyer File System) is a C++/Rust distributed filesystem. Components: `mgmtd`
+(cluster management), `meta` (metadata service, backed by FoundationDB), `storage`
+(CRAQ chain replication), `monitor_collector`, `admin_cli`, and `hf3fs_fuse_main`
+(FUSE client). Build outputs land in `build/bin/`. Standard build/deploy steps live in
+`README.md` and `deploy/README.md`; only the non-obvious, environment-specific caveats
+are captured here.
+
+### Toolchain pinning (important, non-obvious)
+- This VM is Ubuntu 24.04, but the project targets **clang-14 + libstdc++-12** (as in CI,
+  which runs on Ubuntu 22.04). clang-14 auto-selects the newest GCC dir it finds, which
+  here is an **incomplete gcc-14** (`/usr/lib/gcc/x86_64-linux-gnu/14` has `crtbegin.o`
+  but no `libstdc++.so`/headers). So clang-14 must be **pinned to gcc-12** or the build
+  fails with `'algorithm' file not found` / `cannot find -lstdc++`. The pin flags are
+  already baked into the configure command below.
+- `cc`/`c++` alternatives are set to **gcc-12/g++** (matching CI's default). Do not point
+  them back at clang, or the Rust C++ build scripts and linking break.
+- Rust must be **1.85.0** (`rustup default` is already 1.85.0). Do NOT use newer stable
+  (>=1.90): it defaults to `rust-lld`, which fails to link the C++ deps
+  (`unable to find library -lstdc++`).
+
+### Build commands
+Run from the repo root (`/workspace`):
+```bash
+# Rust crates (uses gcc via the cc/c++ alternatives)
+cargo build --release
+
+# C++ (clang-14 pinned to gcc-12 libstdc++; SHUFFLE_METHOD is mandatory)
+GCC12=/usr/lib/gcc/x86_64-linux-gnu/12
+cmake -S . -B build \
+  -DCMAKE_CXX_COMPILER=clang++-14 -DCMAKE_C_COMPILER=clang-14 \
+  -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
+  -DSHUFFLE_METHOD=g++11 \
+  -DCMAKE_CXX_FLAGS="-isystem /usr/include/c++/12 -isystem /usr/include/x86_64-linux-gnu/c++/12 -B$GCC12" \
+  -DCMAKE_EXE_LINKER_FLAGS="-L$GCC12" -DCMAKE_SHARED_LINKER_FLAGS="-L$GCC12" -DCMAKE_MODULE_LINKER_FLAGS="-L$GCC12"
+cmake --build build -j "$(nproc)" -- -k   # -k: keep going (see broken test target below)
+```
+- Known broken target: `tests/common/utils/TestShuffle.cc` fails to compile
+  (`std::basic_ifstream` undefined — the file uses `std::ifstream` without
+  `#include <fstream>`). This is a latent repo issue, unrelated to the toolchain (fails
+  under both libstdc++-12 and -13). Build with `-- -k` so every other target
+  (all service/client binaries + the other test executables) still builds.
+
+### Lint
+From `build/`: `make check-format` (clang-format-14 dry-run) and `make clang-tidy`.
+Rust: `cargo fmt` / `cargo clippy`.
+
+### Tests
+Test binaries are in `build/tests/` (gtest). Run them directly, e.g.
+`./build/tests/test_meta`, `test_mgmtd`, `test_kv`, `test_client`.
+
+### RUNTIME LIMITATION — no RDMA in this VM
+This microVM has **no RDMA kernel subsystem** (no `/sys/class/infiniband`, no loadable
+modules, `ibv_get_device_list` returns ENOSYS). Every service/client binary
+(`mgmtd_main`, `meta_main`, `storage_main`, `hf3fs_fuse_main`, `admin_cli`,
+`simple_example_main`) calls `net::IBManager::start(...)` at startup and treats failure
+as **FATAL**, so none of them can run here. Consequently the full local cluster + FUSE
+harness `tests/fuse/run.sh` cannot run (it also assumes `RDMA://` transport). Soft-RoCE
+(`rdma_rxe`) is not possible because the kernel ships no modules.
+- Functional verification is therefore done via the gtest suites that use `TCP`/`LOCAL`
+  transport and in-process fakes: `test_kv`, `test_mgmtd`, `test_meta`, and `test_client`
+  all pass (the one `test_client` failure, `testRetryUnknownAddrs`, is a TCP-only
+  corner case). Test suites under `tests/storage/`, `tests/migration/`, and
+  `tests/common/net/ib/` call `IBManager::start` and will fail for the same RDMA reason.
+- A local single-node **FoundationDB** server is installed and running
+  (`/etc/foundationdb/fdb.cluster`); `fdbcli --exec "status minimal"` should report the
+  DB available.