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>

README.md

@@ -110,14 +110,6 @@ default** and covers anything the hooked path doesn't.
 
 Completed work lives in **Lessons learned** + the test suite, not here.
 
-### Current tasks
-
-- **Reconnect to an already-injected DLL.** Support disconnect → reconnect reusing the DLL that's
-  already in the game, *including across a tool restart or crash*: the host detects the live DLL
-  (via its advancing IPC heartbeat on the per-pid section), re-attaches to the same shared section,
-  and resumes control without re-injecting. (DLL self-cleanup on host *crash* is explicitly **not**
-  required — relaunch, reconnect, then disconnect gracefully to clean up.)
-
 ### Future work
 
 - **Per-game profiles** — persist each game's subsystem / capture-mode / audio choices and re-apply
@@ -397,6 +389,14 @@ person/account to receive the stream.
    sees the mirrored video, hears the audio, and that their controller drives the
    real game.
 
+**Disconnect / reconnect.** **Disconnect** asks the injected DLL to remove every hook so the game
+behaves exactly as if it was never touched, then drops the channel — but leaves the DLL injected
+(dormant). Clicking **Inject & Connect** on a game that still has a live DLL (left dormant, or
+surviving a tool restart/crash — a connected DLL keeps its shared section alive) **reconnects** to it
+and resumes, without injecting again. Closing the host also unhooks the game on the way out. So a
+clean cycle is: connect → play → disconnect (game back to normal, DLL parked) → reconnect later. The
+DLL is never force-unloaded; it goes away when the game exits.
+
 Useful checks while developing without RPT: tick **Forward synthetic test input**
 in the Injection panel to make the game move on its own (proving forwarding is the
 source), and click away from the game to confirm focus spoofing keeps it running.

host/CMakeLists.txt

@@ -13,6 +13,7 @@ add_executable(coop_host WIN32
 	src/inject/process_list.cpp
 	src/inject/window_list.cpp
 	src/inject/injector.cpp
+	src/inject/dll_probe.cpp
 	src/inject/mkb_forward.cpp
 	src/ipc/ipc_server.cpp
 	src/capture/frame_renderer.cpp

host/src/inject/dll_probe.cpp

@@ -0,0 +1,38 @@
+#include "inject/dll_probe.hpp"
+
+#include <cstdint>
+
+#include <windows.h>
+
+#include "coop/protocol.hpp"
+#include "coop/shared_memory.hpp"
+
+namespace coop
+{
+
+bool hook_dll_alive(unsigned long pid, int timeout_ms)
+{
+	// The per-pid section exists only while someone holds it; a connected DLL keeps it alive across
+	// a host restart. Open it (don't create), then confirm the DLL's worker is actually beating --
+	// a stale section with a dead worker (heartbeat frozen) must read as not-alive so we inject fresh.
+	// Poll rather than sample once: the worker only beats ~every 250 ms, so a single short read can
+	// straddle a gap and miss it; return the instant a beat lands, and give up after the timeout.
+	SharedMemory shm;
+	if (!shm.open(shared_memory_name(pid), sizeof(SharedBlock)))
+	{
+		return false;
+	}
+	auto* block = shm.as<SharedBlock>();
+	const std::uint32_t h0 = block->status.heartbeat.load(std::memory_order_acquire);
+	for (int waited = 0; waited < timeout_ms; waited += 25)
+	{
+		Sleep(25);
+		if (block->status.heartbeat.load(std::memory_order_acquire) != h0)
+		{
+			return true;
+		}
+	}
+	return false;
+}
+
+} // namespace coop

host/src/inject/dll_probe.hpp

@@ -0,0 +1,18 @@
+// Detect whether our hook DLL is already injected and alive in a target process, so the host can
+// reconnect to it (reusing the DLL) instead of injecting again -- including after a tool restart or
+// crash, since a connected DLL keeps the per-pid IPC section alive.
+#pragma once
+
+namespace coop
+{
+
+// True if `pid` already hosts a live coop_hook DLL: the per-pid IPC section exists and its heartbeat
+// advances within `timeout_ms` (the DLL's worker is still beating). Returns as soon as a beat lands,
+// so it's fast when the DLL is healthy; the timeout must clear the worker's beat period (~250 ms), so
+// the default leaves margin. A missing section, or one whose heartbeat has stalled (dead worker),
+// reads as not-alive -> the caller should inject fresh (which still re-attaches an already-injected
+// DLL, so a false negative is benign). The default clears several beat periods for margin; a healthy
+// DLL is detected as soon as the first beat lands, so this returns quickly in the normal case.
+bool hook_dll_alive(unsigned long pid, int timeout_ms = 1000);
+
+} // namespace coop

host/src/injection_panel.cpp

@@ -6,6 +6,7 @@
 
 #include <windows.h>
 
+#include "inject/dll_probe.hpp"
 #include "inject/injector.hpp"
 #include "ui/app_chrome.hpp"
 
@@ -217,6 +218,48 @@ void InjectionPanel::refresh_processes()
 	processes_ = list_processes();
 }
 
+void InjectionPanel::publish_subsystem_state()
+{
+	// Publish the desired subsystem state before the hook's next reconcile tick.
+	server_.set_subsystem_enabled(HookSubsys_Input, want_input_);
+	server_.set_subsystem_enabled(HookSubsys_Focus, want_focus_);
+	server_.set_subsystem_enabled(HookSubsys_Audio, want_audio_);
+	server_.set_subsystem_enabled(HookSubsys_Video, want_video_);
+	server_.set_subsystem_enabled(HookSubsys_Mkb, want_mkb_);
+	server_.set_cursor_clip_allowed(!release_cursor_);
+}
+
+void InjectionPanel::begin_liveness_tracking()
+{
+	injected_ = true;
+	// Track liveness: a SYNCHRONIZE|QUERY handle lets us notice the game exiting,
+	// and seeding the heartbeat clock avoids a spurious "hung" before the first beat.
+	close_target_handle(); // drop any handle from a previous target
+	target_process_ = OpenProcess(SYNCHRONIZE | PROCESS_QUERY_LIMITED_INFORMATION, FALSE, selected_pid_);
+	target_state_ = TargetState::Alive;
+	last_heartbeat_ = 0;
+	last_heartbeat_time_ = ImGui::GetTime();
+	dll_alive_ = true;
+}
+
+void InjectionPanel::reconnect_selected()
+{
+	// Re-attach to a DLL that's already injected and alive (a prior session left it dormant after a
+	// graceful disconnect, or the tool restarted): bring the channel back up on the SAME per-pid
+	// section the DLL still holds and re-publish the desired subsystem state -- no re-injection.
+	if (!server_.start(selected_pid_))
+	{
+		status_ = "Failed to re-attach shared memory.";
+		status_color_ = kRed;
+		return;
+	}
+	publish_subsystem_state();
+	begin_liveness_tracking();
+	status_ = "Reconnected to " + narrow(selected_name_) + " (pid " + std::to_string(selected_pid_) +
+	          ") -- reused the injected DLL.";
+	status_color_ = kGreen;
+}
+
 void InjectionPanel::inject_selected()
 {
 	if (selected_pid_ == 0)
@@ -226,6 +269,15 @@ void InjectionPanel::inject_selected()
 		return;
 	}
 
+	// If our DLL is already injected and alive in this target (left dormant by a graceful disconnect,
+	// or surviving a tool restart -- it keeps the section alive), reconnect to it instead of
+	// injecting a second time.
+	if (hook_dll_alive(selected_pid_))
+	{
+		reconnect_selected();
+		return;
+	}
+
 	// Bring up the shared-memory channel before injecting so the hook finds it
 	// immediately on load.
 	if (!server_.start(selected_pid_))
@@ -235,26 +287,12 @@ void InjectionPanel::inject_selected()
 		return;
 	}
 
-	// Publish the desired subsystem state before the hook's first reconcile tick.
+	publish_subsystem_state();
-	server_.set_subsystem_enabled(HookSubsys_Input, want_input_);
-	server_.set_subsystem_enabled(HookSubsys_Focus, want_focus_);
-	server_.set_subsystem_enabled(HookSubsys_Audio, want_audio_);
-	server_.set_subsystem_enabled(HookSubsys_Video, want_video_);
-	server_.set_subsystem_enabled(HookSubsys_Mkb, want_mkb_);
-	server_.set_cursor_clip_allowed(!release_cursor_);
 
 	const InjectResult result = inject_dll(selected_pid_, hook_dll_path());
 	if (result.status == InjectStatus::Ok)
 	{
-		injected_ = true;
+		begin_liveness_tracking();
-		// Track liveness: a SYNCHRONIZE|QUERY handle lets us notice the game exiting,
-		// and seeding the heartbeat clock avoids a spurious "hung" before the first beat.
-		close_target_handle(); // drop any handle from a previous target
-		target_process_ = OpenProcess(SYNCHRONIZE | PROCESS_QUERY_LIMITED_INFORMATION, FALSE, selected_pid_);
-		target_state_ = TargetState::Alive;
-		last_heartbeat_ = 0;
-		last_heartbeat_time_ = ImGui::GetTime();
-		dll_alive_ = true;
 		status_ = "Injected into " + narrow(selected_name_) + " (pid " + std::to_string(selected_pid_) + ").";
 		status_color_ = kGreen;
 	}

host/src/injection_panel.hpp

@@ -164,7 +164,10 @@ public:
 private:
 	void refresh_targets();      // refresh both the window list and the process list
 	void refresh_processes();
-	void inject_selected();
+	void inject_selected();      // inject fresh, OR reconnect if a live DLL is already in the target
+	void reconnect_selected();   // re-attach to an already-injected, live DLL (no re-inject)
+	void publish_subsystem_state(); // push the desired per-subsystem install state + cursor policy
+	void begin_liveness_tracking(); // mark connected: open the process handle, seed the heartbeat clock
 	// Graceful disconnect: ask the DLL to remove every hook (game returns to vanilla), wait
 	// (bounded) for it to take effect, then drop the channel. The DLL stays injected/dormant for a
 	// later reconnect; we never eject it. Used by the Disconnect button and the destructor.

tests/CMakeLists.txt

@@ -221,7 +221,8 @@ add_test(NAME dx12_present_hook_test COMMAND dx12_present_hook_test)
 # stresses hook/unhook cycles. Reuses the shipping shared-texture reader. Skips without D3D11.
 add_executable(mock_game_test
 	mock_game_test.cpp
-	${CMAKE_SOURCE_DIR}/host/src/capture/shared_texture.cpp)
+	${CMAKE_SOURCE_DIR}/host/src/capture/shared_texture.cpp
+	${CMAKE_SOURCE_DIR}/host/src/inject/dll_probe.cpp)
 target_include_directories(mock_game_test PRIVATE
 	${CMAKE_SOURCE_DIR}/host/src
 	${CMAKE_SOURCE_DIR}/tools/mock_game)

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.