Avoid signal-unsafe operations in signal handler

TSan reported a signal-unsafe call to `ostream::put` in the POSIX signal
handler. This patch replaces the use of `dstream`, which generated those
calls, with calls to `write` and `strlen`, which are both async-signal
safe in POSIX.

This patch also changes the type of the global kill flag from `bool` to
`volatile std::sig_atomic_t`, to be certain there is no undefined
behavior. TSan did not mention that issue; I am not sure whether that is
something TSan is able to detect.

src/main.cpp

@@ -46,6 +46,8 @@ extern "C" {
 #endif
 }
 
+#include <csignal>
+
 #if !defined(__cpp_rtti) || !defined(__cpp_exceptions)
 #error Luanti cannot be built without exceptions or RTTI
 #endif
@@ -1138,7 +1140,7 @@ static bool run_dedicated_server(const GameParams &game_params, const Settings &
 			return false;
 		}
 		ChatInterface iface;
-		bool &kill = *porting::signal_handler_killstatus();
+		volatile std::sig_atomic_t &kill = *porting::signal_handler_killstatus();
 
 		try {
 			// Create server
@@ -1181,7 +1183,7 @@ static bool run_dedicated_server(const GameParams &game_params, const Settings &
 			server.start();
 
 			// Run server
-			bool &kill = *porting::signal_handler_killstatus();
+			volatile std::sig_atomic_t &kill = *porting::signal_handler_killstatus();
 			dedicated_server_loop(server, kill);
 
 		} catch (const ModError &e) {
@@ -1226,7 +1228,7 @@ static bool migrate_map_database(const GameParams &game_params, const Settings &
 
 	u32 count = 0;
 	u64 last_update_time = 0;
-	bool &kill = *porting::signal_handler_killstatus();
+	volatile std::sig_atomic_t &kill = *porting::signal_handler_killstatus();
 
 	std::vector<v3s16> blocks;
 	old_db->listAllLoadableBlocks(blocks);
@@ -1280,7 +1282,7 @@ static bool recompress_map_database(const GameParams &game_params, const Setting
 
 	u32 count = 0;
 	u64 last_update_time = 0;
-	bool &kill = *porting::signal_handler_killstatus();
+	volatile std::sig_atomic_t &kill = *porting::signal_handler_killstatus();
 	const u8 serialize_as_ver = SER_FMT_VER_HIGHEST_WRITE;
 	const s16 map_compression_level = rangelim(g_settings->getS16("map_compression_level_disk"), -1, 9);
 

src/porting.cpp

@@ -60,6 +60,7 @@
 #include "util/string.h"
 #include "util/tracy_wrapper.h"
 #include <vector>
+#include <csignal>
 #include <cstdarg>
 #include <cstdio>
 #include <signal.h>
@@ -81,24 +82,27 @@ namespace porting
 	Signal handler (grabs Ctrl-C on POSIX systems)
 */
 
-static bool g_killed = false;
+volatile static std::sig_atomic_t g_killed = false;
 
-bool *signal_handler_killstatus()
+volatile std::sig_atomic_t *signal_handler_killstatus()
 {
 	return &g_killed;
 }
 
 #if !defined(_WIN32) // POSIX
+#define STDOUT 1
 
 static void signal_handler(int sig)
 {
 	if (!g_killed) {
 		if (sig == SIGINT) {
-			dstream << "INFO: signal_handler(): "
-				<< "Ctrl-C pressed, shutting down." << std::endl;
+			const char *dbg_text{"INFO: signal_handler(): "
+				"Ctrl-C pressed, shutting down.\n"};
+			write(STDOUT, dbg_text, strlen(dbg_text) + 1);
 		} else if (sig == SIGTERM) {
-			dstream << "INFO: signal_handler(): "
-				<< "got SIGTERM, shutting down." << std::endl;
+			const char *dbg_text{"INFO: signal_handler(): "
+				"got SIGTERM, shutting down.\n"};
+			write(STDOUT, dbg_text, strlen(dbg_text) + 1);
 		}
 
 		// Comment out for less clutter when testing scripts

src/porting.h

@@ -13,6 +13,7 @@
 #endif
 
 // Be mindful of what you include here!
+#include <csignal>
 #include <string>
 #include "config.h"
 #include "irrlichttypes.h" // u64
@@ -77,7 +78,7 @@ namespace porting
 void signal_handler_init();
 // Returns a pointer to a bool.
 // When the bool is true, program should quit.
-[[nodiscard]] bool *signal_handler_killstatus();
+[[nodiscard]] volatile std::sig_atomic_t *signal_handler_killstatus();
 
 /*
 	Path of static data directory.

src/server.cpp

@@ -65,6 +65,8 @@
 #include "gettext.h"
 #include "util/tracy_wrapper.h"
 
+#include <csignal>
+
 class ClientNotFoundException : public BaseException
 {
 public:
@@ -4105,7 +4107,7 @@ std::unique_ptr<PlayerSAO> Server::emergePlayer(const char *name, session_t peer
 	return playersao;
 }
 
-void dedicated_server_loop(Server &server, bool &kill)
+void dedicated_server_loop(Server &server, volatile std::sig_atomic_t &kill)
 {
 	verbosestream<<"dedicated_server_loop()"<<std::endl;
 

src/server.h

@@ -25,6 +25,7 @@
 #include "translation.h"
 #include "script/common/c_types.h" // LuaError
 #include <atomic>
+#include <csignal>
 #include <string>
 #include <list>
 #include <map>
@@ -794,4 +795,4 @@ private:
 
 	Shuts down when kill is set to true.
 */
-void dedicated_server_loop(Server &server, bool &kill);
+void dedicated_server_loop(Server &server, volatile std::sig_atomic_t &kill);

src/terminal_chat_console.h

@@ -9,6 +9,8 @@
 #include "util/container.h"
 #include "log.h"
 #include "log_internal.h"
+
+#include <csignal>
 #include <set>
 #include <sstream>
 
@@ -45,7 +47,7 @@ public:
 
 	void setup(
 		ChatInterface *iface,
-		bool *kill_requested,
+		volatile std::sig_atomic_t *kill_requested,
 		const std::string &nick)
 	{
 		m_nick = nick;
@@ -96,9 +98,9 @@ private:
 	int m_rows;
 	bool m_can_draw_text;
 
-	bool *m_kill_requested = nullptr;
-	ChatBackend m_chat_backend;
-	ChatInterface *m_chat_interface;
+	volatile std::sig_atomic_t *m_kill_requested = nullptr;
+	ChatBackend                 m_chat_backend;
+	ChatInterface              *m_chat_interface;
 
 	TermLogOutput m_log_output;