mavgpud: test the probe against the sysfs the workstation actually has (V-488)

The fixtures are the live numbers sampled from the box on 02-08-2026, where the
CPT run held 12.8GB of 16 as proc/478104/vram_35881.

The cases that matter are the ones where a mistake is silent: our own
llama-server counting as a contender, an unreadable card reading as free, and
/health hanging or proxying into a closed port instead of answering 503.

cmd/mavgpud/gpu_test.go

@@ -0,0 +1,103 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+)
+
+// fakeKFD builds the sysfs shape the workstation actually has: one directory
+// per ROCm process, each holding a vram_<node> file. Sampled from the live box
+// on 02-08-2026, where the CPT run appeared as proc/478104/vram_35881.
+func fakeKFD(t *testing.T, vramByPID map[int]int64) string {
+	t.Helper()
+	root := t.TempDir()
+	for pid, vram := range vramByPID {
+		dir := filepath.Join(root, strconv.Itoa(pid))
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			t.Fatal(err)
+		}
+		f := filepath.Join(dir, "vram_35881")
+		if err := os.WriteFile(f, []byte(strconv.FormatInt(vram, 10)+"\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return root
+}
+
+func TestForeignExcludesOurChild(t *testing.T) {
+	root := fakeKFD(t, map[int]int64{478104: 12791693312, 999: 4096})
+	p := probe{kfdRoot: root}
+
+	all := p.foreign(0)
+	if len(all) != 2 {
+		t.Fatalf("with no child running, both processes are foreign, got %d", len(all))
+	}
+
+	ours := p.foreign(999)
+	if len(ours) != 1 || ours[0].PID != 478104 {
+		t.Fatalf("our own llama-server must not count as a contender, got %+v", ours)
+	}
+	if ours[0].VRAM != 12791693312 {
+		t.Errorf("per-process VRAM = %d, want the value from vram_35881", ours[0].VRAM)
+	}
+}
+
+// An empty KFD tree is the state that permits a start, so it must read as empty
+// rather than as an error the caller has to interpret.
+func TestForeignEmptyAndMissing(t *testing.T) {
+	if got := (probe{kfdRoot: t.TempDir()}).foreign(0); len(got) != 0 {
+		t.Errorf("empty kfd tree: got %d processes, want 0", len(got))
+	}
+	if got := (probe{kfdRoot: "/nonexistent"}).foreign(0); got != nil {
+		t.Errorf("missing kfd tree: got %+v, want nil", got)
+	}
+}
+
+func TestFreeVRAM(t *testing.T) {
+	dev := t.TempDir()
+	write := func(name, v string) {
+		if err := os.WriteFile(filepath.Join(dev, name), []byte(v), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// The live numbers from the workstation while the CPT run held the card.
+	write("mem_info_vram_total", "17163091968\n")
+	write("mem_info_vram_used", "13396389888\n")
+	p := probe{drmDev: dev}
+	if got, want := p.freeVRAM(), int64(3766702080); got != want {
+		t.Errorf("freeVRAM = %d, want %d", got, want)
+	}
+	if got := (probe{drmDev: "/nonexistent"}).freeVRAM(); got != 0 {
+		t.Errorf("unreadable card reports %d free, want 0 so nothing starts", got)
+	}
+}
+
+// With no model loaded the supervisor must still answer, and it must answer 503
+// rather than hanging or proxying into a closed port. Maven reads this endpoint
+// on a timer forever, including while the workstation is busy.
+func TestHealthAndProxyRefuseWhenNotReady(t *testing.T) {
+	s := &supervisor{run: newRunner("/bin/true", nil, "")}
+	h := s.handler(mustURL(t, "http://127.0.0.1:1"))
+
+	for _, path := range []string{"/health", "/v1/chat/completions"} {
+		w := httptest.NewRecorder()
+		h.ServeHTTP(w, httptest.NewRequest(http.MethodGet, path, nil))
+		if w.Code != http.StatusServiceUnavailable {
+			t.Errorf("%s with no model: got %d, want 503", path, w.Code)
+		}
+	}
+}
+
+func mustURL(t *testing.T, s string) *url.URL {
+	t.Helper()
+	u, err := url.Parse(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return u
+}