fix(depot): retry transient probe responses like curl --retry

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/depot/remote_test.go

@@ -120,6 +120,81 @@ func hostPort(srv *httptest.Server) string {
 	return srv.URL
 }
 
+// stubProbeSleep replaces probeRetrySleep for the test, recording durations.
+func stubProbeSleep(t *testing.T) *[]time.Duration {
+	t.Helper()
+	var slept []time.Duration
+	old := probeRetrySleep
+	probeRetrySleep = func(d time.Duration) { slept = append(slept, d) }
+	t.Cleanup(func() { probeRetrySleep = old })
+	return &slept
+}
+
+func TestProbeRetriesTransientThenPresent(t *testing.T) {
+	slept := stubProbeSleep(t)
+	sha := shaOf("throttled-blob")
+
+	var calls int
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		if calls == 1 {
+			w.WriteHeader(428)
+			return
+		}
+		w.WriteHeader(206)
+	}))
+	defer srv.Close()
+
+	cfg := testCfg(hostPort(srv), "readkey", "writekey")
+	b, err := NewBackend("bunny", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	state, transient, err := b.Probe(context.Background(), sha)
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	if state != Present || transient {
+		t.Fatalf("expected Present/non-transient after retry, got %v/%v", state, transient)
+	}
+	if calls != 2 {
+		t.Fatalf("expected 2 requests (428 then 206), got %d", calls)
+	}
+	if len(*slept) != 1 || (*slept)[0] != time.Second {
+		t.Fatalf("expected one 1s backoff, got %v", *slept)
+	}
+}
+
+func TestProbe404NoRetry(t *testing.T) {
+	slept := stubProbeSleep(t)
+	sha := shaOf("gone-blob")
+
+	f := newFakeBunny()
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+
+	cfg := testCfg(hostPort(srv), "readkey", "writekey")
+	b, err := NewBackend("bunny", "", cfg)
+	if err != nil {
+		t.Fatalf("NewBackend: %v", err)
+	}
+
+	state, transient, err := b.Probe(context.Background(), sha)
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	if state != Absent || transient {
+		t.Fatalf("expected Absent/non-transient, got %v/%v", state, transient)
+	}
+	if got := len(f.requestsSnapshot()); got != 1 {
+		t.Fatalf("expected exactly 1 request for 404, got %d", got)
+	}
+	if len(*slept) != 0 {
+		t.Fatalf("expected zero backoffs for 404, got %v", *slept)
+	}
+}
+
 func TestProbeUsesRangeGetNotHead(t *testing.T) {
 	sha := shaOf("present-blob")
 	f := newFakeBunny()
@@ -157,6 +232,7 @@ func TestProbeUsesRangeGetNotHead(t *testing.T) {
 }
 
 func TestProbeStates(t *testing.T) {
+	stubProbeSleep(t)
 	cases := []struct {
 		status        int
 		wantState     ProbeState