Core/Process: Add Exists function

include/Nazara/Core/Process.hpp

@@ -29,6 +29,7 @@ namespace Nz
 			Process& operator=(const Process&) = delete;
 			Process& operator=(Process&&) = delete;
 
+			static Result<bool, std::string> Exists(Pid pid);
 			static Pid GetCurrentPid();
 			static Result<Pid, std::string> SpawnDetached(const std::filesystem::path& program, std::span<const std::string> arguments = {}, const std::filesystem::path& workingDirectory = {});
 	};

src/Nazara/Core/Posix/ProcessImpl.cpp

@@ -15,6 +15,17 @@
 
 namespace Nz::PlatformImpl
 {
+	Result<bool, std::string> CheckProcessExistence(Pid pid)
+	{
+		if (::kill(pid, 0) == 0)
+			return Ok(true);
+
+		if (errno == ESRCH)
+			return OK(false);
+
+		return Err(Error::GetLastSystemError());
+	}
+
 	Pid GetCurrentProcessId()
 	{
 		return ::getpid();
@@ -34,6 +45,8 @@ namespace Nz::PlatformImpl
 
 		// Double fork (see https://0xjet.github.io/3OHA/2022/04/11/post.html)
 		// We will create a child and a grand-child process, using a pipe to retrieve the grand-child pid
+		// TODO: Maybe use vfork for the child process too?
+		// TODO: Use posix_spawn if possible instead
 		pid_t childPid = ::fork();
 		if (childPid == -1)
 			return Err("failed to create child: " + Error::GetLastSystemError());
@@ -76,7 +89,7 @@ namespace Nz::PlatformImpl
 				::execve(program.c_str(), argv.data(), envs);
 
 				// If we get here, execve failed
-				// Remember we share the memory of our parent (vfork) so we need to exit using _exit() to avoid calling the parent exit handler
+				// Remember we share the memory of our parent (vfork) so we need to exit using _exit() to avoid calling the parent exit handlers
 
 				PidOrErr err;
 				err.pid = -1;

src/Nazara/Core/Posix/ProcessImpl.hpp

@@ -12,6 +12,7 @@
 
 namespace Nz::PlatformImpl
 {
+	Result<bool, std::string> CheckProcessExistence(Pid pid);
 	Pid GetCurrentProcessId();
 	Result<Pid, std::string> SpawnDetachedProcess(const std::filesystem::path& program, std::span<const std::string> arguments = {}, const std::filesystem::path& workingDirectory = {});
 }

src/Nazara/Core/Process.cpp

@@ -16,6 +16,11 @@
 
 namespace Nz
 {
+	Result<bool, std::string> Process::Exists(Pid pid)
+	{
+		return PlatformImpl::CheckProcessExistence(pid);
+	}
+
 	Pid Process::GetCurrentPid()
 	{
 		return PlatformImpl::GetCurrentProcessId();

src/Nazara/Core/Win32/ProcessImpl.cpp

@@ -6,6 +6,7 @@
 #include <Nazara/Core/Error.hpp>
 #include <Nazara/Core/StringExt.hpp>
 #include <Nazara/Core/Win32/Win32Utils.hpp>
+#include <NazaraUtils/CallOnExit.hpp>
 #include <Windows.h>
 #include <Nazara/Core/Debug.hpp>
 
@@ -85,6 +86,27 @@ namespace Nz::PlatformImpl
 		return commandLine;
 	}
 
+	Result<bool, std::string> CheckProcessExistence(Pid pid)
+	{
+		HANDLE processHandle = ::OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, FALSE, pid);
+		if (!processHandle)
+		{
+			DWORD lastErr = ::GetLastError();
+			if (lastErr == ERROR_INVALID_PARAMETER)
+				return Ok(false);
+
+			return Err("failed to open process handle: " + Error::GetLastSystemError());
+		}
+		CallOnExit releaseHandle([processHandle] { ::CloseHandle(processHandle); });
+
+		// Opening a handle doesn't mean the process hasn't exited (zombie processes), check if it has an exit code
+		DWORD exitCode;
+		if (!::GetExitCodeProcess(processHandle, &exitCode))
+			return Err("failed to get exit code: " + Error::GetLastSystemError());
+
+		return Ok(exitCode == STILL_ACTIVE);
+	}
+
 	Pid GetCurrentProcessId()
 	{
 		return ::GetCurrentProcessId();

src/Nazara/Core/Win32/ProcessImpl.hpp

@@ -12,6 +12,7 @@
 
 namespace Nz::PlatformImpl
 {
+	Result<bool, std::string> CheckProcessExistence(Pid pid);
 	Pid GetCurrentProcessId();
 	Result<Pid, std::string> SpawnDetachedProcess(const std::filesystem::path& program, std::span<const std::string> arguments = {}, const std::filesystem::path& workingDirectory = {});
 }