fix(e2e): run a scenario again when QEMU aborted under it

QEMU has died with SIGABRT while a second display was plugged in (a
hotplug repeat and monitors.at_cursor), which is not a result about kfw.
Its stderr is kept, a dead guest is named as such, and the scenario is
run once more on a fresh guest.

e2e/platforms/qemu_windows/guest.py

@@ -183,7 +183,24 @@ class Guest:
         shutil.copytree(self._run / "tpm", dest / "tpm", dirs_exist_ok=True)
 
     def _start(self, argv: list[str]) -> None:
-        self._procs.append(subprocess.Popen(argv))
+        # what a program writes to stderr is kept: it says why QEMU aborted
+        log = self._run / f"{Path(argv[0]).name}.err"
+        with open(log, "wb") as err:
+            self._procs.append(subprocess.Popen(argv, stderr=err))
+
+    def died(self) -> str | None:
+        """Why QEMU has exited, if it has while the guest was in use (a signal
+        that killed it, and the last of what it wrote), else `None`."""
+        for proc in self._procs:
+            code = proc.poll()
+            if code is None or proc.args[0] != "qemu-system-x86_64":
+                continue
+            log = self._run / "qemu-system-x86_64.err"
+            tail = log.read_text(errors="replace").strip().splitlines()[-5:] \
+                if log.exists() else []
+            how = f"signal {-code}" if code < 0 else f"status {code}"
+            return f"QEMU exited with {how}: " + " | ".join(tail)
+        return None
 
     def upload(self, local_dir: Path, names: list[str]) -> None:
         """Puts binaries under test in the guest's `C:\\e2e\\bin`."""

e2e/platforms/qemu_windows/run.py

@@ -64,6 +64,10 @@ PLACEMENT_INTEGER_BUG = (
 )
 
 
+class GuestDied(RuntimeError):
+    """QEMU exited while a scenario was running: not what kfw did."""
+
+
 class RecordedChecks(Checks):
     """`Checks` that also keeps each check, for the result file."""
 
@@ -423,13 +427,20 @@ def run_one(
             print(f"     AGENT LOST: {e}")
             checks.bad(str(e))
             _fetch_logs(guest, out)
-        except Exception:
+        except Exception as e:
+            died = guest.died()
+            if died:
+                # the guest is gone, so there is nothing to fetch from it
+                raise GuestDied(died) from e
             _fetch_logs(guest, out)
             raise
         finally:
             lap("scenario")
             print(f"     seconds: {timings}")
-            platform.screenshot(f"final-{name}")
+            try:
+                platform.screenshot(f"final-{name}")
+            except Exception:
+                pass
             if recorder:
                 _collect_recording(platform, out)
             # matches a recorder session directory with the run that made it
@@ -461,9 +472,17 @@ def main(argv: list[str]) -> int:
         state = ensure(Path(golden), Path(os.environ["KOMOREBI_E2E_STATE"]))
     for name in names:
         try:
-            checks = run_one(
-                Path(golden), Path(bin_dir), Path(out) / name, name, state
-            )
+            try:
+                checks = run_one(
+                    Path(golden), Path(bin_dir), Path(out) / name, name, state
+                )
+            except GuestDied as e:
+                # QEMU itself aborted (it has, while a display was plugged in):
+                # not a result about kfw, so the scenario runs once more
+                print(f"{name}: THE GUEST DIED ({e}); running it again")
+                checks = run_one(
+                    Path(golden), Path(bin_dir), Path(out) / name, name, state
+                )
         except Exception:
             # a scenario that cannot run is a failure of its own, and the
             # others still run

e2e/platforms/qemu_windows/tests/test_runner.py

@@ -226,3 +226,21 @@ def test_acting_on_a_window_that_is_not_there_raises_a_clear_error() -> None:
                 platform.fullscreen, platform.user_maximize):
         with pytest.raises(RuntimeError, match="no window titled 'gone'"):
             act(gone)
+
+
+def test_a_guest_that_died_says_how_and_a_live_one_says_nothing(tmp_path: Path) -> None:
+    guest = Guest(Path("golden"), tmp_path)
+    live = subprocess.Popen(["sleep", "5"])
+    guest._procs.append(live)
+    assert guest.died() is None
+    live.kill()
+    live.wait()
+
+    dead = subprocess.Popen(["sh", "-c", "kill -ABRT $$"])
+    dead.wait()
+    dead.args = ["qemu-system-x86_64"]
+    (tmp_path / "qemu-system-x86_64.err").write_text("assertion failed\nAborted\n")
+    guest._procs = [dead]
+
+    assert "signal 6" in guest.died()
+    assert "assertion failed" in guest.died()