# Handoff — the linker error `dune test` prints on every run

Branch: `dev-loop`, worktree `agent-a4f73ab1c6726fdc2`. Scope is the `dev-robust` fixture and the noise it makes;
nothing else in the suite is touched.

## The question

Every `dune test --root .` prints, twice:

```
/usr/bin/ld: cannot open output file /tmp/build_*_dune/flan-devtest-robust.cache/flan-macros-*.so.*: Permission denied
clang: error: linker command failed with exit code 1
```

`HANDOFF-x86-rt.md` section 4 says this is inside the `dev-robust` fixture and deliberate. That has been quoted
forward without ever being checked. The brief names two possibilities: the failure is deliberate and only *looks*
like a broken toolchain, or it is accidental and the fixture has been proving less than it claims.

## The answer: deliberate, and the read is confirmed at the source

`test/test_dev.ml`, the block starting "A build that fails is a refusal, not the end of the session" (~line 2805):

- The fixture gives the daemon a cache directory of its own — `FLAN_CACHE_DIR=<tmp>/flan-devtest-robust.cache`,
  with any inherited `FLAN_CACHE_DIR` filtered out of the environment first.
- Before each step that must fail it runs `Unix.chmod rcache 0o500`, and afterwards `0o700`. A read-only,
  non-writable cache directory is exactly why `ld` cannot open its output there.
- The two failing steps are the `eval-expr` one (C-x C-e) and the `eval` one carrying a `defmacro` (C-c C-c) —
  which is why the message appears **twice** per run, and why the count matches.

So this is possibility 1. The property — a failed build leaves the session standing — is genuinely under test.

## The actual defect, which is narrower

The daemon is spawned with `Unix.stderr` as its stderr, so it inherits the test binary's. `Build.run` shells out
with `Sys.command`, which does not capture anything, so clang's and ld's own words go straight through to the
terminal. Its *stdout* already goes to a log file nobody reads (`rout`), and the comment there says as much; its
stderr was simply never given the same treatment.

The result is four lines of what reads like a broken toolchain on a passing run. A real linker failure in that spot
today would be invisible.

## The plan

1. Give the robustness daemon its own stderr log rather than the suite's, the same way its stdout already has one.
2. Report that log — and say so — only when something in the block actually failed, so a genuine linker error is
   still visible, and a deliberate one is not.
3. Have the fixture announce the deliberate failure in its own voice before it causes it, so a reader of the output
   knows a build is about to be made to fail and why.

Baseline to not regress: `dune test --root .` exit 0, `spike/x86/survey.sh` 101 MATCH / 0 DIFFER / 0 refused,
`spike/x86/cells.sh` 4/4.

## Open questions

- `Build.run` reports only `(exit N)` to the caller; clang's actual words reach the terminal and nothing else. The
  fixture asserts the reply "carries the compiler's message" but only checks for the substring `(exit `, so an
  editor attached to a real `flan dev` sees a bare exit status and no diagnostic. Widening `Build.run` to capture
  and forward stderr would fix that, and is out of this lane's scope — recorded here so it is not lost.