Fix DELETE slug-collision race and widen _discover_jobs error handling

- DELETE now checks under the shared lock whether another job with the
  same slug is queued/running before removing the job entry, so
  deleting a done parent can no longer rmtree a rebuild's active
  work/output directories.
- _discover_jobs now wraps the iterdir/stat/read_warnings calls in the
  same try/except as the JSON parse, so one corrupt or torn sidecar
  file (e.g. from a killed mid-write) no longer crashes the whole app
  at boot.
- Fixed test_delete_tolerates_a_work_dir_that_is_already_gone, which
  had become vacuous since _write_sidecar now eagerly creates the work
  dir; it now removes the dir before asserting DELETE tolerates its
  absence.
- Added a regression test covering the DELETE slug-collision race.

src/songtrace/web.py

@@ -104,17 +104,18 @@ def _discover_jobs(cfg: Config) -> list[Job]:
                 if data.get(key) is not None:
                     data[key] = Path(data[key])
             inp = SongInput(**data)
+            slug = song_slug(inp)
+            out = cfg.output_dir / slug
+            done = out.exists() and any(out.iterdir())
+            mtime = sidecar.stat().st_mtime
+            warnings = read_warnings(sidecar.parent) if done else []
         except (OSError, ValueError, TypeError):
             continue
-        slug = song_slug(inp)
-        out = cfg.output_dir / slug
-        done = out.exists() and any(out.iterdir())
-        mtime = sidecar.stat().st_mtime
         discovered.append(Job(
             id=uuid.uuid4().hex, title=inp.title, artist=inp.artist, inp=inp,
             status="done" if done else "failed",
             output_dir=str(out) if done else None,
-            warnings=read_warnings(sidecar.parent) if done else [],
+            warnings=warnings,
             error=None if done else "interrupted before completion",
             created_at=mtime, finished_at=mtime))
     discovered.sort(key=lambda j: j.created_at)
@@ -358,14 +359,21 @@ def create_app(cfg: Config, runner=run_pipeline) -> FastAPI:
 
     @app.delete("/api/jobs/{job_id}", status_code=204)
     def delete_job(job_id: str):
-        job = _finished_job(job_id)
-        slug = song_slug(job.inp)
-        shutil.rmtree(cfg.work_dir / slug, ignore_errors=True)
-        shutil.rmtree(cfg.output_dir / slug, ignore_errors=True)
         with lock:
+            job = jobs.get(job_id)
+            if job is None:
+                raise HTTPException(404)
+            if job.status in ("queued", "running"):
+                raise HTTPException(409, "job is still in progress")
+            slug = song_slug(job.inp)
+            if any(other.status in ("queued", "running") and song_slug(other.inp) == slug
+                   for other_id, other in jobs.items() if other_id != job_id):
+                raise HTTPException(409, "another job for this song is in progress")
             jobs.pop(job_id, None)
             if job_id in order:
                 order.remove(job_id)
+        shutil.rmtree(cfg.work_dir / slug, ignore_errors=True)
+        shutil.rmtree(cfg.output_dir / slug, ignore_errors=True)
 
     return app
 

tests/test_web.py

@@ -1070,6 +1070,7 @@ def test_delete_removes_work_and_output_and_the_job(tmp_path):
 def test_delete_tolerates_a_work_dir_that_is_already_gone(tmp_path):
     client, cfg = make_client(tmp_path, slug_runner)
     job_id = make_done_job(client)
+    shutil.rmtree(cfg.work_dir / "A - T")
     resp = client.delete(f"/api/jobs/{job_id}")
     assert resp.status_code == 204
 
@@ -1097,6 +1098,29 @@ def test_delete_of_a_running_job_is_409(tmp_path):
     assert client.get(f"/api/jobs/{parent}").status_code == 200
 
 
+def test_delete_of_a_job_with_an_active_rebuild_is_409(tmp_path):
+    """A done job's slug must not be deleted out from under a rebuild in flight."""
+    release = threading.Event()
+    calls = []
+
+    def runner(inp, cfg, log=print, force_from=None, stages=None):
+        calls.append(1)
+        if len(calls) > 1:
+            release.wait(5)
+        return PipelineResult(output_dir=cfg.output_dir / "X")
+
+    client, _ = make_client(tmp_path, runner)
+    parent = make_done_job(client)
+    child = client.post(f"/api/jobs/{parent}/rebuild-full").json()["id"]
+    try:
+        wait_status(client, child, "running")
+        assert client.delete(f"/api/jobs/{parent}").status_code == 409
+    finally:
+        release.set()
+    wait_status(client, child, "done")
+    assert client.get(f"/api/jobs/{parent}").status_code == 200
+
+
 def test_a_plain_job_never_removes_anything(tmp_path):
     client, cfg = make_client(tmp_path, slug_runner)
     make_done_job(client)