Add --log-file command line argument to write output log to a file

This works even if file logging is disabled in the project settings,
or for the editor/project manager.

`--log-file`'s value can be an absolute path or relative to the project
directory (similar to existing arguments like `--write-movie`).

doc/classes/ProjectSettings.xml

@@ -433,16 +433,18 @@
 			If canvas item redraw debugging is active, this will be the time the flash will last each time they redraw.
 		</member>
 		<member name="debug/file_logging/enable_file_logging" type="bool" setter="" getter="" default="false">
-			If [code]true[/code], logs all output to files.
+			If [code]true[/code], logs all output and error messages to files. See also [member debug/file_logging/log_path], [member debug/file_logging/max_log_files], and [member application/run/flush_stdout_on_print].
 		</member>
 		<member name="debug/file_logging/enable_file_logging.pc" type="bool" setter="" getter="" default="true">
 			Desktop override for [member debug/file_logging/enable_file_logging], as log files are not readily accessible on mobile/Web platforms.
 		</member>
 		<member name="debug/file_logging/log_path" type="String" setter="" getter="" default="&quot;user://logs/godot.log&quot;">
 			Path at which to store log files for the project. Using a path under [code]user://[/code] is recommended.
+			This can be specified manually on the command line using the [code]--log-file &lt;file&gt;[/code] [url=$DOCS_URL/tutorials/editor/command_line_tutorial.html]command line argument[/url]. If this command line argument is specified, log rotation is automatically disabled (see [member debug/file_logging/max_log_files]).
 		</member>
 		<member name="debug/file_logging/max_log_files" type="int" setter="" getter="" default="5">
-			Specifies the maximum number of log files allowed (used for rotation).
+			Specifies the maximum number of log files allowed (used for rotation). Set to [code]1[/code] to disable log file rotation.
+			If the [code]--log-file &lt;file&gt;[/code] [url=$DOCS_URL/tutorials/editor/command_line_tutorial.html]command line argument[/url] is used, log rotation is always disabled.
 		</member>
 		<member name="debug/gdscript/warnings/assert_always_false" type="int" setter="" getter="" default="1">
 			When set to [code]warn[/code] or [code]error[/code], produces a warning or an error respectively when an [code]assert[/code] call always evaluates to false.

main/main.cpp

@@ -172,6 +172,7 @@ static bool editor = false;
 static bool project_manager = false;
 static bool cmdline_tool = false;
 static String locale;
+static String log_file;
 static bool show_help = false;
 static uint64_t quit_after = 0;
 static OS::ProcessID editor_pid = 0;
@@ -450,7 +451,9 @@ void Main::print_help(const char *p_binary) {
 	OS::get_singleton()->print("  --text-driver <driver>            Text driver (Fonts, BiDi, shaping).\n");
 	OS::get_singleton()->print("  --tablet-driver <driver>          Pen tablet input driver.\n");
 	OS::get_singleton()->print("  --headless                        Enable headless mode (--display-driver headless --audio-driver Dummy). Useful for servers and with --script.\n");
-	OS::get_singleton()->print("  --write-movie <file>              Writes a video to the specified path (usually with .avi or .png extension).\n");
+	OS::get_singleton()->print("  --log-file <file>                 Write output/error log to the specified path instead of the default location defined by the project.\n");
+	OS::get_singleton()->print("                                    <file> path should be absolute or relative to the project directory.\n");
+	OS::get_singleton()->print("  --write-movie <file>              Write a video to the specified path (usually with .avi or .png extension).\n");
 	OS::get_singleton()->print("                                    --fixed-fps is forced when enabled, but it can be used to change movie FPS.\n");
 	OS::get_singleton()->print("                                    --disable-vsync can speed up movie writing but makes interaction more difficult.\n");
 	OS::get_singleton()->print("                                    --quit-after can be used to specify the number of frames to write.\n");
@@ -1165,6 +1168,15 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 			audio_driver = NULL_AUDIO_DRIVER;
 			display_driver = NULL_DISPLAY_DRIVER;
 
+		} else if (I->get() == "--log-file") { // write to log file
+
+			if (I->next()) {
+				log_file = I->next()->get();
+				N = I->next()->next();
+			} else {
+				OS::get_singleton()->print("Missing log file path argument, aborting.\n");
+				goto error;
+			}
 		} else if (I->get() == "--profiling") { // enable profiling
 
 			use_debug_profiler = true;
@@ -1689,12 +1701,24 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 	GLOBAL_DEF("debug/file_logging/log_path", "user://logs/godot.log");
 	GLOBAL_DEF(PropertyInfo(Variant::INT, "debug/file_logging/max_log_files", PROPERTY_HINT_RANGE, "0,20,1,or_greater"), 5);
 
-	if (!project_manager && !editor && FileAccess::get_create_func(FileAccess::ACCESS_USERDATA) &&
+	// If `--log-file` is used to override the log path, allow creating logs for the project manager or editor
-			GLOBAL_GET("debug/file_logging/enable_file_logging")) {
+	// and even if file logging is disabled in the Project Settings.
+	// `--log-file` can be used with any path (including absolute paths outside the project folder),
+	// so check for filesystem access if it's used.
+	if (FileAccess::get_create_func(!log_file.is_empty() ? FileAccess::ACCESS_FILESYSTEM : FileAccess::ACCESS_USERDATA) &&
+			(!log_file.is_empty() || (!project_manager && !editor && GLOBAL_GET("debug/file_logging/enable_file_logging")))) {
 		// Don't create logs for the project manager as they would be written to
 		// the current working directory, which is inconvenient.
-		String base_path = GLOBAL_GET("debug/file_logging/log_path");
+		String base_path;
-		int max_files = GLOBAL_GET("debug/file_logging/max_log_files");
+		int max_files;
+		if (!log_file.is_empty()) {
+			base_path = log_file;
+			// Ensure log file name respects the specified override by disabling log rotation.
+			max_files = 1;
+		} else {
+			base_path = GLOBAL_GET("debug/file_logging/log_path");
+			max_files = GLOBAL_GET("debug/file_logging/max_log_files");
+		}
 		OS::get_singleton()->add_logger(memnew(RotatedFileLogger(base_path, max_files)));
 	}
 

misc/dist/shell/_godot.zsh-completion

@@ -51,7 +51,8 @@ _arguments \
   '--text-driver[set the text driver]:text driver name' \
   '--tablet-driver[set the pen tablet input driver]:tablet driver name' \
   '--headless[enable headless mode (--display-driver headless --audio-driver Dummy), useful for servers and with --script]' \
-  '--write-movie[writes a video to the specified path (usually with .avi or .png extension)]:path to output video file' \
+  '--log-file[write output/error log to the specified path instead of the default location defined by the project]:path to output log file' \
+  '--write-movie[write a video to the specified path (usually with .avi or .png extension)]:path to output video file' \
   '(-f --fullscreen)'{-f,--fullscreen}'[request fullscreen mode]' \
   '(-m --maximized)'{-m,--maximized}'[request a maximized window]' \
   '(-w --windowed)'{-w,--windowed}'[request windowed mode]' \

misc/dist/shell/godot.bash-completion

@@ -54,6 +54,7 @@ _complete_godot_options() {
 --text-driver
 --tablet-driver
 --headless
+--log-file
 --write-movie
 --fullscreen
 --maximized

misc/dist/shell/godot.fish

@@ -67,7 +67,8 @@ complete -c godot -l gpu-index -d "Use a specific GPU (run with --verbose to get
 complete -c godot -l text-driver -d "Set the text driver" -x
 complete -c godot -l tablet-driver -d "Set the pen tablet input driver" -x
 complete -c godot -l headless -d "Enable headless mode (--display-driver headless --audio-driver Dummy). Useful for servers and with --script"
-complete -c godot -l write-movie -d "Writes a video to the specified path (usually with .avi or .png extension). --fixed-fps is forced when enabled" -x
+complete -c godot -l log-file -d "Write output/error log to the specified path instead of the default location defined by the project" -x
+complete -c godot -l write-movie -d "Write a video to the specified path (usually with .avi or .png extension). --fixed-fps is forced when enabled" -x
 
 # Display options:
 complete -c godot -s f -l fullscreen -d "Request fullscreen mode"