Reconnect to an already-injected DLL (reuse it, survive a tool restart)

Disconnect -> reconnect now reuses the DLL already in the game instead of
injecting again, including across a tool restart or crash: a connected DLL keeps
its per-pid shared section (and worker) alive after the host goes away, so a
fresh host can find it and re-attach to the same section.

- hook_dll_alive(pid) (host/src/inject/dll_probe.cpp): detect a live DLL by
  opening the per-pid section and polling its heartbeat (returns as soon as a
  beat lands; a missing section or stalled worker reads as not-alive). It does
  not check magic -- a graceful disconnect zeroes magic but the DLL keeps
  beating and the worker never re-checks magic post-connect.
- InjectionPanel: the Inject and Connect button branches to reconnect_selected()
  when a live DLL is detected -- IpcServer::start() re-attaches to the SAME
  section the DLL still holds and re-publishes the subsystem state; no
  re-injection. Factored the shared post-connect setup (publish_subsystem_state
  / begin_liveness_tracking). The DLL needed no change -- it just resumes reading
  the re-attached section.
- A false not-alive is benign: the inject path still re-attaches an
  already-injected DLL (LoadLibrary no-ops), so the timeout only needs to clear
  the worker's ~250ms beat period with margin.

Test (mock_game_test test_reconnect): inject -> hooked -> graceful disconnect ->
drop the host handle (simulating a restart while the DLL keeps the section alive)
-> detect via heartbeat -> re-attach to the same section -> hooks re-install
without re-injecting -> and hook_dll_alive goes false once the game is gone.

Roadmap: both current tasks (graceful disconnect, reconnect) done -> removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/mock_game_test.cpp

@@ -24,6 +24,7 @@
 #include <d3d11.h>
 
 #include "capture/shared_texture.hpp"
+#include "inject/dll_probe.hpp"
 #include "coop/audio_ring.hpp"
 #include "coop/log_ring.hpp"
 #include "coop/protocol.hpp"
@@ -1039,6 +1040,89 @@ void test_graceful_disconnect(const char* backend)
 	game.kill();
 }
 
+// Reconnect contract: after a graceful disconnect (and even a simulated tool restart -- the host
+// drops its section handle while the DLL keeps it alive), the host can detect the live DLL via its
+// heartbeat (hook_dll_alive) and re-attach to the SAME per-pid section to resume control, without
+// re-injecting. The DLL, still connected to that section, re-installs its hooks when the reconnected
+// host re-enables them.
+void test_reconnect(const char* backend)
+{
+	std::printf("== reconnect to an already-injected DLL: %s ==\n", backend);
+	std::wstring wbackend;
+	for (const char* p = backend; *p != '\0'; ++p)
+	{
+		wbackend.push_back(static_cast<wchar_t>(*p));
+	}
+	MockGame game = MockGame::launch(wbackend + L" 30");
+	if (!game.ok)
+	{
+		check(false, "launch mock game (reconnect)");
+		return;
+	}
+	Sleep(800);
+
+	const std::uint32_t disabled = 1u << HookSubsys_Audio; // input+focus+video+mkb on; audio off
+	SharedMemory shm_a;
+	SharedBlock* block_a = make_ipc(shm_a, game.pid(), disabled);
+	if (block_a == nullptr || !inject_retry(game.pid()))
+	{
+		check(false, "inject mock game (reconnect)");
+		game.kill();
+		return;
+	}
+
+	bool installed = false;
+	for (int i = 0; i < 100 && game.alive() && !installed; ++i)
+	{
+		Sleep(50);
+		installed = installed_hook_count(block_a) > 0;
+	}
+	check(installed, "first connection installed hooks");
+	check(hook_dll_alive(game.pid()), "hook_dll_alive() detects the live DLL");
+
+	// Graceful disconnect: unhook everything, then simulate the host going away (drop our handle;
+	// the DLL keeps the section alive). This stands in for both an explicit disconnect and a restart.
+	for (std::uint32_t s = 0; s < HookSubsys_Count; ++s)
+	{
+		block_a->control.subsystem_disabled[s].store(1u, std::memory_order_release);
+	}
+	for (int i = 0; i < 100 && installed_hook_count(block_a) != 0; ++i)
+	{
+		Sleep(50);
+	}
+	check(installed_hook_count(block_a) == 0, "graceful disconnect unhooked the game");
+	// Let the DLL settle back to steady heartbeating: the unhook tick runs several bounded drains, so
+	// the worker can briefly not beat right after it. A real reconnect targets an already-dormant DLL,
+	// not one in the microsecond after a mass-unhook.
+	Sleep(500);
+	shm_a.reset(); // host A "exits" -- only the DLL holds the section now
+
+	// The DLL is still alive and holding the section: the restarted host can find it...
+	check(hook_dll_alive(game.pid()), "DLL still detectable after the host dropped its handle");
+
+	// ...and reconnect by re-attaching to the SAME section (no re-inject) and re-enabling subsystems.
+	SharedMemory shm_b;
+	SharedBlock* block_b = make_ipc(shm_b, game.pid(), 0u); // re-attach, all subsystems on
+	if (block_b == nullptr)
+	{
+		check(false, "reconnect: re-attach to the section");
+		game.kill();
+		return;
+	}
+	bool reinstalled = false;
+	for (int i = 0; i < 100 && game.alive() && !reinstalled; ++i)
+	{
+		Sleep(50);
+		reinstalled = installed_hook_count(block_b) > 0;
+	}
+	check(reinstalled, "reconnect re-installed the hooks via the existing DLL (no re-inject)");
+	check(game.alive(), "game alive after reconnect");
+
+	game.kill();
+	Sleep(200);
+	check(!hook_dll_alive(game.pid()), "hook_dll_alive() false once the game (and DLL) is gone");
+}
+
 int main()
 {
 	kill_stray_mock_games(); // clean slate: no leftover game holding coop_hook.dll
@@ -1073,6 +1157,10 @@ int main()
 	// vanilla while the DLL stays injected/dormant (the reconnect-friendly teardown).
 	test_graceful_disconnect("dx11");
 
+	// Reconnect: detect the live DLL and re-attach to the same section (even across a simulated host
+	// restart) to resume control without re-injecting.
+	test_reconnect("dx11");
+
 	// Aggressive hook/unhook storm across every backend: a separate thread thrashes every
 	// subsystem on/off while the game presents, to catch an unsafe install/remove race (the
 	// "spamming Mirror video crashed Brotato" use-after-free). vk uses the early-load path.