mavgpud: a yield stops writing a core and reads as a yield (V-491)

llama-server aborts inside its own static teardown on SIGTERM — the
handler calls exit(), stream_session_manager's destructor throws, and the
process dies "signal: aborted (core dumped)". mavgpud sends that signal on
every eviction, so a routine yield wrote a multi-gigabyte core into
systemd-coredump and logged the same line a real crash would.

LimitCORE=0 in the unit stops the disk cost. A yielding flag, set by stop
and cleared by start, makes the log distinguish the two: only an exit we
did not ask for is still reported as an exit.

Not filed upstream. Searched ggml-org/llama.cpp for
"ggml_uncaught_exception" with SIGTERM and for stream_session_manager and
found nothing matching, so the issue still wants writing — by someone with
an account on that tracker, which is why it is not in this commit.

cmd/mavgpud/runner_test.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// fakeServer writes an executable standing in for llama-server: it ignores
+// SIGTERM the way the real one effectively does — by dying messily rather than
+// cleanly — and reports a non-zero status.
+func fakeServer(t *testing.T, body string) string {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "fake-llama-server")
+	if err := os.WriteFile(path, []byte("#!/bin/sh\n"+body+"\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+// A deliberate stop is a yield, and the log has to say so.
+//
+// llama-server aborts inside its own static teardown on SIGTERM, so the exit
+// status of a routine yield is identical to that of a real crash. Reading the
+// mavgpud log, the two were indistinguishable (Vikunja #491).
+func TestStopMarksTheExitAsAYield(t *testing.T) {
+	r := newRunner(fakeServer(t, "while : ; do sleep 1 ; done"), nil, "")
+	if err := r.start(); err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	r.mu.Lock()
+	if r.yielding {
+		t.Error("a freshly started server is already marked as yielding")
+	}
+	r.mu.Unlock()
+
+	r.stop(2 * time.Second)
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if !r.running() {
+			return
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatal("the child outlived stop")
+}
+
+// Stopping when nothing is running must not arm the flag for the next child.
+// The next exit after that would be a real crash logged as a yield.
+func TestStopWithNoChildDoesNotArmTheFlag(t *testing.T) {
+	r := newRunner("/nonexistent", nil, "")
+	r.stop(10 * time.Millisecond)
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.yielding {
+		t.Error("stop armed the yield flag with no child running")
+	}
+}