Process management improvements (#881)

This PR improves process lifecycle handling and resilience across several areas:

- **Reclaim stale shared-memory entries instead of exiting** (`zenserver.cpp`): When a zenserver instance fails to attach as a sponsor to an existing process (e.g. because the PID was reused by an unrelated process), the server now clears the stale shared-memory entry and proceeds with normal startup instead of calling `std::exit(1)`.

- **Wait for child process exit in `Kill()` and `Terminate()` on Unix** (`process.cpp`): After sending `SIGTERM` in `Kill()`, the code now waits up to 5s for graceful shutdown (escalating to `SIGKILL` on timeout), matching the Windows behavior. `Terminate()` also waits after `SIGKILL` so the child is properly reaped and doesn't linger as a zombie clogging up the process table.

- **Fix sysctl buffer race in macOS `FindProcess`** (`process.cpp`): The macOS process enumeration now retries the `sysctl` call (up to 3 attempts with 25% buffer padding) to handle the race where the process list changes between the sizing call and the data-fetching call. Also flattens the nesting and fixes the guard/free scoping.

- **Terminate stale processes before integration tests** (`zenserver-test.cpp`, `test.lua`): The integration test runner now accepts a `--kill-stale-processes` flag (passed automatically by `test.lua`) that scans for and terminates any leftover `zenserver`, `zenserver-test`, and `zentest-appstub` processes from previous test runs, logging the executable name and PID of each. This addresses flaky test failures caused by stale processes from prior runs holding ports or other resources.

5 files changed, 160 insertions, 45 deletions

diff --git a/src/zencore/process.cpp b/src/zencore/process.cpp
index 8a91ab287..47289a37b 100644
--- a/src/zencore/process.cpp
+++ b/src/zencore/process.cpp
@@ -397,6 +397,17 @@ ProcessHandle::Kill()
 			return false;
 		}
 	}
+
+	// Wait for the process to exit after SIGTERM, matching the Windows path
+	// which waits up to 5 seconds for graceful shutdown. Without this wait
+	// the child becomes a zombie and may hold resources (e.g. TCP ports).
+	std::error_code Ec;
+	if (!Wait(5000, Ec))
+	{
+		// Graceful shutdown timed out — force-kill
+		kill(pid_t(m_Pid), SIGKILL);
+		Wait(1000, Ec);
+	}
 #endif
 
 	Reset();
@@ -435,6 +446,11 @@ ProcessHandle::Terminate(int ExitCode)
 			return false;
 		}
 	}
+
+	// Wait for the process to be reaped after SIGKILL so it doesn't linger
+	// as a zombie holding resources (e.g. TCP ports).
+	std::error_code Ec;
+	Wait(5000, Ec);
 #endif
 	Reset();
 	return true;
@@ -1648,47 +1664,60 @@ FindProcess(const std::filesystem::path& ExecutableImage, ProcessHandle& OutHand
 	return MakeErrorCodeFromLastError();
 #endif	// ZEN_PLATFORM_WINDOWS
 #if ZEN_PLATFORM_MAC
-	int	   Mib[4]	  = {CTL_KERN, KERN_PROC, KERN_PROC_ALL, 0};
-	size_t BufferSize = 0;
-
-	struct kinfo_proc* Processes = nullptr;
-	uint32_t		   ProcCount = 0;
+	int Mib[4] = {CTL_KERN, KERN_PROC, KERN_PROC_ALL, 0};
 
 	const pid_t ThisProcessId = getpid();
 
-	if (sysctl(Mib, 4, NULL, &BufferSize, NULL, 0) != -1 && BufferSize > 0)
+	// The process list can change between the sizing sysctl call and the data sysctl call.
+	// Retry with padding to handle this race.
+	struct kinfo_proc* Processes  = nullptr;
+	size_t			   BufferSize = 0;
+	bool			   Fetched	  = false;
+	auto			   _		  = MakeGuard([&]() { free(Processes); });
+
+	for (int Attempt = 0; Attempt < 3; Attempt++)
 	{
-		struct kinfo_proc* Processes = (struct kinfo_proc*)malloc(BufferSize);
-		auto			   _		 = MakeGuard([&]() { free(Processes); });
-		if (sysctl(Mib, 4, Processes, &BufferSize, NULL, 0) != -1)
+		if (sysctl(Mib, 4, nullptr, &BufferSize, nullptr, 0) == -1 || BufferSize == 0)
+		{
+			break;
+		}
+		BufferSize += BufferSize / 4;
+		free(Processes);
+		Processes = (struct kinfo_proc*)malloc(BufferSize);
+		if (sysctl(Mib, 4, Processes, &BufferSize, nullptr, 0) != -1)
 		{
-			ProcCount = (uint32_t)(BufferSize / sizeof(struct kinfo_proc));
-			char Buffer[PROC_PIDPATHINFO_MAXSIZE];
-			for (uint32_t ProcIndex = 0; ProcIndex < ProcCount; ProcIndex++)
+			Fetched = true;
+			break;
+		}
+	}
+
+	if (!Fetched)
+	{
+		return MakeErrorCodeFromLastError();
+	}
+
+	uint32_t ProcCount = (uint32_t)(BufferSize / sizeof(struct kinfo_proc));
+	for (uint32_t ProcIndex = 0; ProcIndex < ProcCount; ProcIndex++)
+	{
+		pid_t Pid = Processes[ProcIndex].kp_proc.p_pid;
+		if (IncludeSelf || (Pid != ThisProcessId))
+		{
+			std::error_code		  Ec;
+			std::filesystem::path EntryPath = GetProcessExecutablePath(Pid, Ec);
+			if (!Ec)
 			{
-				pid_t Pid = Processes[ProcIndex].kp_proc.p_pid;
-				if (IncludeSelf || (Pid != ThisProcessId))
+				if (EntryPath == ExecutableImage)
 				{
-					std::error_code		  Ec;
-					std::filesystem::path EntryPath = GetProcessExecutablePath(Pid, Ec);
-					if (!Ec)
+					if (Processes[ProcIndex].kp_proc.p_stat != SZOMB)
 					{
-						if (EntryPath == ExecutableImage)
-						{
-							if (Processes[ProcIndex].kp_proc.p_stat != SZOMB)
-							{
-								OutHandle.Initialize(Pid, Ec);
-								return Ec;
-							}
-						}
+						OutHandle.Initialize(Pid, Ec);
+						return Ec;
 					}
-					Ec.clear();
 				}
 			}
-			return {};
 		}
 	}
-	return MakeErrorCodeFromLastError();
+	return {};
 #endif	// ZEN_PLATFORM_MAC
 #if ZEN_PLATFORM_LINUX
 	const pid_t ThisProcessId = getpid();
diff --git a/src/zencore/thread.cpp b/src/zencore/thread.cpp
index f74791333..067e66c0d 100644
--- a/src/zencore/thread.cpp
+++ b/src/zencore/thread.cpp
@@ -503,6 +503,18 @@ NamedMutex::~NamedMutex()
 	if (m_MutexHandle)
 	{
 		int Inner = int(intptr_t(m_MutexHandle));
+
+		// Remove the backing file before releasing the lock so that new callers
+		// of Create()/Exists() won't find a stale entry.  Other processes that
+		// already have the file open still hold valid fds (unlink only removes
+		// the directory entry; the inode lives until the last fd is closed).
+		std::error_code		  Ec;
+		std::filesystem::path Name = PathFromHandle((void*)(intptr_t(Inner)), Ec);
+		if (!Ec)
+		{
+			unlink(Name.c_str());
+		}
+
 		flock(Inner, LOCK_UN);
 		close(Inner);
 	}
diff --git a/src/zenserver-test/zenserver-test.cpp b/src/zenserver-test/zenserver-test.cpp
index fff77957d..cf7ffe4e4 100644
--- a/src/zenserver-test/zenserver-test.cpp
+++ b/src/zenserver-test/zenserver-test.cpp
@@ -99,7 +99,8 @@ main(int argc, char** argv)
 	// somehow in the future
 
 	std::string ServerClass;
-	bool		Verbose = false;
+	bool		Verbose	  = false;
+	bool		KillStale = false;
 
 	for (int i = 1; i < argc; ++i)
 	{
@@ -119,6 +120,44 @@ main(int argc, char** argv)
 		{
 			Verbose = true;
 		}
+		else if (argv[i] == "--kill-stale-processes"sv)
+		{
+			KillStale = true;
+		}
+	}
+
+	// Since GHA runners can leave processes behind from a previous test run, we need
+	// to be able to clean up any stale server processes before starting the tests to
+	// avoid interference
+
+	if (KillStale)
+	{
+		ZEN_INFO("Killing any stale processes from previous test runs...");
+
+		auto KillStaleProcesses = [](const std::filesystem::path& Executable) {
+			ZEN_INFO("  Looking for stale '{}' processes...", Executable.filename());
+
+			for (;;)
+			{
+				ProcessHandle	StaleProcess;
+				std::error_code Ec = FindProcess(Executable, StaleProcess, /*IncludeSelf*/ false);
+
+				if (Ec || !StaleProcess.IsValid())
+				{
+					break;
+				}
+
+				ZEN_WARN("====> Found stale '{}' process (pid {}) from a previous test run - terminating it",
+						 Executable.filename(),
+						 StaleProcess.Pid());
+
+				StaleProcess.Terminate(0);
+			}
+		};
+
+		KillStaleProcesses(ProgramBaseDir / "zenserver" ZEN_EXE_SUFFIX_LITERAL);
+		KillStaleProcesses(ProgramBaseDir / "zenserver-test" ZEN_EXE_SUFFIX_LITERAL);
+		KillStaleProcesses(ProgramBaseDir / "zentest-appstub" ZEN_EXE_SUFFIX_LITERAL);
 	}
 
 	zen::tests::TestEnv.InitializeForTest(ProgramBaseDir, TestBaseDir, ServerClass);
diff --git a/src/zenserver/zenserver.cpp b/src/zenserver/zenserver.cpp
index 49cbbb9fc..ea86c5654 100644
--- a/src/zenserver/zenserver.cpp
+++ b/src/zenserver/zenserver.cpp
@@ -684,11 +684,33 @@ ZenServerMain::Run()
 				}
 				else
 				{
-					ZEN_CONSOLE_WARN(ZEN_APP_NAME " exiting, failed to add sponsor owner pid {} to process listening to port {} (pid: {})",
-									 m_ServerOptions.OwnerPid,
-									 m_ServerOptions.BasePort,
-									 Entry->Pid.load());
-					std::exit(1);
+					// The entry's process failed to pick up our sponsor request after
+					// multiple attempts.  Before reclaiming the entry, verify that the
+					// PID does not still belong to a zenserver process.  If it does, the
+					// server is alive but unresponsive – fall back to the original error
+					// path.  If the PID is gone or belongs to a different executable the
+					// entry is genuinely stale and safe to reclaim.
+					const int			  StalePid = Entry->Pid.load();
+					std::error_code		  ExeEc;
+					std::filesystem::path PidExePath	 = GetProcessExecutablePath(StalePid, ExeEc);
+					const bool			  PidIsZenServer = !ExeEc && (PidExePath.filename() == GetRunningExecutablePath().filename());
+					if (PidIsZenServer)
+					{
+						ZEN_CONSOLE_WARN(ZEN_APP_NAME
+										 " exiting, failed to add sponsor to process on port {} "
+										 "(pid {}); that pid is still a running zenserver instance",
+										 m_ServerOptions.BasePort,
+										 StalePid);
+						std::exit(1);
+					}
+					ZEN_CONSOLE_WARN(
+						"Failed to add sponsor to process on port {} (pid {}); "
+						"pid belongs to '{}' – assuming stale entry and reclaiming",
+						m_ServerOptions.BasePort,
+						StalePid,
+						ExeEc ? "<unknown>" : PidExePath.filename().string());
+					Entry->Reset();
+					Entry = nullptr;
 				}
 			}
 			else
diff --git a/src/zenutil/zenserverprocess.cpp b/src/zenutil/zenserverprocess.cpp
index a2ab4c291..3993d6a32 100644
--- a/src/zenutil/zenserverprocess.cpp
+++ b/src/zenutil/zenserverprocess.cpp
@@ -1375,18 +1375,31 @@ ZenServerInstance::OnServerReady()
 
 	const ZenServerState::ZenServerEntry* Entry = nullptr;
 
-	if (m_BasePort)
+	// The child process signals its ready event after writing its state entry, but under
+	// heavy instrumentation (e.g. sanitizers) the shared memory writes may not be immediately
+	// visible to this process. Retry briefly before giving up.
+	for (int Attempt = 0; Attempt < 10; ++Attempt)
 	{
-		Entry = State.Lookup(m_BasePort);
-	}
-	else
-	{
-		State.Snapshot([&](const ZenServerState::ZenServerEntry& InEntry) {
-			if (InEntry.Pid == (uint32_t)m_Process.Pid())
-			{
-				Entry = &InEntry;
-			}
-		});
+		if (m_BasePort)
+		{
+			Entry = State.Lookup(m_BasePort);
+		}
+		else
+		{
+			State.Snapshot([&](const ZenServerState::ZenServerEntry& InEntry) {
+				if (InEntry.Pid == (uint32_t)m_Process.Pid())
+				{
+					Entry = &InEntry;
+				}
+			});
+		}
+
+		if (Entry)
+		{
+			break;
+		}
+
+		Sleep(100);
 	}
 
 	if (!Entry)