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.

1 files changed, 27 insertions, 5 deletions

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