Add a project setting to enable stdout flushing in release builds

This can be used in server builds for journalctl compatibility.

core/io/logger.cpp

@@ -30,6 +30,7 @@
 
 #include "logger.h"
 
+#include "core/config/project_settings.h"
 #include "core/os/dir_access.h"
 #include "core/os/os.h"
 #include "core/string/print_string.h"
@@ -201,15 +202,14 @@ void RotatedFileLogger::logv(const char *p_format, va_list p_list, bool p_err) {
 		}
 		va_end(list_copy);
 		file->store_buffer((uint8_t *)buf, len);
+
 		if (len >= static_buf_size) {
 			Memory::free_static(buf);
 		}
-#ifdef DEBUG_ENABLED
+
-		const bool need_flush = true;
+		if (p_err || GLOBAL_GET("application/run/flush_stdout_on_print")) {
-#else
+			// Don't always flush when printing stdout to avoid performance
-		bool need_flush = p_err;
+			// issues when `print()` is spammed in release builds.
-#endif
-		if (need_flush) {
 			file->flush();
 		}
 	}
@@ -228,9 +228,11 @@ void StdLogger::logv(const char *p_format, va_list p_list, bool p_err) {
 		vfprintf(stderr, p_format, p_list);
 	} else {
 		vprintf(p_format, p_list);
-#ifdef DEBUG_ENABLED
+		if (GLOBAL_GET("application/run/flush_stdout_on_print")) {
+			// Don't always flush when printing stdout to avoid performance
+			// issues when `print()` is spammed in release builds.
 			fflush(stdout);
-#endif
+		}
 	}
 }
 

doc/classes/ProjectSettings.xml

@@ -269,6 +269,14 @@
 		<member name="application/run/disable_stdout" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], disables printing to standard output in an exported build.
 		</member>
+		<member name="application/run/flush_stdout_on_print" type="bool" setter="" getter="" default="false">
+			If [code]true[/code], flushes the standard output stream every time a line is printed. This affects both terminal logging and file logging.
+			When running a project, this setting must be enabled if you want logs to be collected by service managers such as systemd/journalctl. This setting is disabled by default on release builds, since flushing on every printed line will negatively affect performance if lots of lines are printed in a rapid succession. Also, if this setting is enabled, logged files will still be written successfully if the application crashes or is otherwise killed by the user (without being closed "normally").
+			[b]Note:[/b] Regardless of this setting, the standard error stream ([code]stderr[/code]) is always flushed when a line is printed to it.
+		</member>
+		<member name="application/run/flush_stdout_on_print.debug" type="bool" setter="" getter="" default="true">
+			Debug build override for [member application/run/flush_stdout_on_print], as performance is less important during debugging.
+		</member>
 		<member name="application/run/frame_delay_msec" type="int" setter="" getter="" default="0">
 			Forces a delay between frames in the main loop (in milliseconds). This may be useful if you plan to disable vertical synchronization.
 		</member>

main/main.cpp

@@ -1121,6 +1121,11 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 	}
 #endif
 
+	// Only flush stdout in debug builds by default, as spamming `print()` will
+	// decrease performance if this is enabled.
+	GLOBAL_DEF("application/run/flush_stdout_on_print", false);
+	GLOBAL_DEF("application/run/flush_stdout_on_print.debug", true);
+
 	GLOBAL_DEF("logging/file_logging/enable_file_logging", false);
 	// Only file logging by default on desktop platforms as logs can't be
 	// accessed easily on mobile/Web platforms (if at all).