Estimate buffer size in advance for render_frame

The size of the surface used to draw the indicator depends on
the extents of the text being drawn on and under the indicator.

This commit refactors the `render_frame` function so that the surface
size is computed before drawing; before, `render_frame` drew onto a
buffer, estimated the size the buffer should have had, and recursively
called itself to try again with the estimated size, if necessary. This
was done because Cairo's methods to estimate font and text size
require that a cairo_t context object and an associated cairo_surface
already have been set up. Since the surface size depends on the text
size, the natural way to use Cairo would have a circular dependency.

In order to compute sizes _before_ the buffer is created, this commit
adds a 1x1 surface and a matching `test_cairo` context which is set to
the same font and drawing parameters as the buffer that will be
created. Font/text extent measurements should give the same results as
for the final buffer.

include/swaylock.h

@@ -86,6 +86,8 @@ struct swaylock_state {
 	struct swaylock_args args;
 	struct swaylock_password password;
 	struct swaylock_xkb xkb;
+	cairo_surface_t *test_surface;
+	cairo_t *test_cairo; // used to estimate font/text sizes
 	enum auth_state auth_state;
 	int failed_attempts;
 	bool run_display;

main.c

@@ -1291,6 +1291,9 @@ int main(int argc, char **argv) {
 		return 1;
 	}
 
+	state.test_surface = cairo_image_surface_create(CAIRO_FORMAT_RGB24, 1, 1);
+	state.test_cairo = cairo_create(state.test_surface);
+
 	struct swaylock_surface *surface;
 	wl_list_for_each(surface, &state.surfaces, link) {
 		create_surface(surface);
@@ -1325,5 +1328,7 @@ int main(int argc, char **argv) {
 	}
 
 	free(state.args.font);
+	cairo_destroy(state.test_cairo);
+	cairo_surface_destroy(state.test_surface);
 	return 0;
 }

render.c

@@ -68,88 +68,14 @@ void render_frame_background(struct swaylock_surface *surface) {
 	wl_surface_commit(surface->surface);
 }
 
-void render_frame(struct swaylock_surface *surface) {
+static void configure_font_drawing(cairo_t *cairo, struct swaylock_state *state,
-	struct swaylock_state *state = surface->state;
+		enum wl_output_subpixel subpixel, int arc_radius) {
-
-	int arc_radius = state->args.radius * surface->scale;
-	int arc_thickness = state->args.thickness * surface->scale;
-	int buffer_diameter = (arc_radius + arc_thickness) * 2;
-
-	int buffer_width = surface->indicator_width;
-	int buffer_height = surface->indicator_height;
-	int new_width = buffer_diameter;
-	int new_height = buffer_diameter;
-
-	int subsurf_xpos;
-	int subsurf_ypos;
-
-	// Center the indicator unless overridden by the user
-	if (state->args.override_indicator_x_position) {
-		subsurf_xpos = state->args.indicator_x_position -
-			buffer_width / (2 * surface->scale) + 2 / surface->scale;
-	} else {
-		subsurf_xpos = surface->width / 2 -
-			buffer_width / (2 * surface->scale) + 2 / surface->scale;
-	}
-
-	if (state->args.override_indicator_y_position) {
-		subsurf_ypos = state->args.indicator_y_position -
-			(state->args.radius + state->args.thickness);
-	} else {
-		subsurf_ypos = surface->height / 2 -
-			(state->args.radius + state->args.thickness);
-	}
-
-	wl_subsurface_set_position(surface->subsurface, subsurf_xpos, subsurf_ypos);
-
-	struct pool_buffer *buffer = get_next_buffer(state->shm,
-			surface->indicator_buffers, buffer_width, buffer_height);
-	if (buffer == NULL) {
-		return;
-	}
-
-	cairo_t *cairo = buffer->cairo;
-	cairo_set_antialias(cairo, CAIRO_ANTIALIAS_BEST);
 	cairo_font_options_t *fo = cairo_font_options_create();
 	cairo_font_options_set_hint_style(fo, CAIRO_HINT_STYLE_FULL);
 	cairo_font_options_set_antialias(fo, CAIRO_ANTIALIAS_SUBPIXEL);
-	cairo_font_options_set_subpixel_order(fo, to_cairo_subpixel_order(surface->subpixel));
+	cairo_font_options_set_subpixel_order(fo, to_cairo_subpixel_order(subpixel));
+
 	cairo_set_font_options(cairo, fo);
-	cairo_font_options_destroy(fo);
-	cairo_identity_matrix(cairo);
-
-	// Clear
-	cairo_save(cairo);
-	cairo_set_source_rgba(cairo, 0, 0, 0, 0);
-	cairo_set_operator(cairo, CAIRO_OPERATOR_SOURCE);
-	cairo_paint(cairo);
-	cairo_restore(cairo);
-
-	float type_indicator_border_thickness =
-		TYPE_INDICATOR_BORDER_THICKNESS * surface->scale;
-
-	if (state->args.show_indicator && (state->auth_state != AUTH_STATE_IDLE ||
-			state->args.indicator_idle_visible)) {
-		// Fill inner circle
-		cairo_set_line_width(cairo, 0);
-		cairo_arc(cairo, buffer_width / 2, buffer_diameter / 2,
-				arc_radius - arc_thickness / 2, 0, 2 * M_PI);
-		set_color_for_state(cairo, state, &state->args.colors.inside);
-		cairo_fill_preserve(cairo);
-		cairo_stroke(cairo);
-
-		// Draw ring
-		cairo_set_line_width(cairo, arc_thickness);
-		cairo_arc(cairo, buffer_width / 2, buffer_diameter / 2, arc_radius,
-				0, 2 * M_PI);
-		set_color_for_state(cairo, state, &state->args.colors.ring);
-		cairo_stroke(cairo);
-
-		// Draw a message
-		char *text = NULL;
-		const char *layout_text = NULL;
-		char attempts[4]; // like i3lock: count no more than 999
-		set_color_for_state(cairo, state, &state->args.colors.text);
 	cairo_select_font_face(cairo, state->args.font,
 		CAIRO_FONT_SLANT_NORMAL, CAIRO_FONT_WEIGHT_NORMAL);
 	if (state->args.font_size > 0) {
@@ -157,6 +83,20 @@ void render_frame(struct swaylock_surface *surface) {
 	} else {
 		cairo_set_font_size(cairo, arc_radius / 3.0f);
 	}
+	cairo_font_options_destroy(fo);
+}
+
+void render_frame(struct swaylock_surface *surface) {
+	struct swaylock_state *state = surface->state;
+
+	// First, compute the text that will be drawn, if any, since this
+	// determines the size/positioning of the surface
+
+	char attempts[4]; // like i3lock: count no more than 999
+	char *text = NULL;
+	const char *layout_text = NULL;
+
+	if (state->args.show_indicator) {
 		switch (state->auth_state) {
 		case AUTH_STATE_VALIDATING:
 			text = "Verifying";
@@ -201,6 +141,104 @@ void render_frame(struct swaylock_surface *surface) {
 		default:
 			break;
 		}
+	}
+
+	// Compute the size of the buffer needed
+	int arc_radius = state->args.radius * surface->scale;
+	int arc_thickness = state->args.thickness * surface->scale;
+	int buffer_diameter = (arc_radius + arc_thickness) * 2;
+	int buffer_width = buffer_diameter;
+	int buffer_height = buffer_diameter;
+
+	if (text || layout_text) {
+		cairo_set_antialias(state->test_cairo, CAIRO_ANTIALIAS_BEST);
+		configure_font_drawing(state->test_cairo, state, surface->subpixel, arc_radius);
+
+		if (text) {
+			cairo_text_extents_t extents;
+			cairo_text_extents(state->test_cairo, text, &extents);
+			if (buffer_width < extents.width) {
+				buffer_width = extents.width;
+			}
+		}
+		if (layout_text) {
+			cairo_text_extents_t extents;
+			cairo_font_extents_t fe;
+			double box_padding = 4.0 * surface->scale;
+			cairo_text_extents(state->test_cairo, layout_text, &extents);
+			cairo_font_extents(state->test_cairo, &fe);
+			buffer_height += fe.height + 2 * box_padding;
+			if (buffer_width < extents.width + 2 * box_padding) {
+				buffer_width = extents.width + 2 * box_padding;
+			}
+		}
+	}
+	// Ensure buffer size is multiple of buffer scale - required by protocol
+	buffer_height += surface->scale - (buffer_height % surface->scale);
+	buffer_width += surface->scale - (buffer_width % surface->scale);
+
+	int subsurf_xpos;
+	int subsurf_ypos;
+
+	// Center the indicator unless overridden by the user
+	if (state->args.override_indicator_x_position) {
+		subsurf_xpos = state->args.indicator_x_position -
+			buffer_width / (2 * surface->scale) + 2 / surface->scale;
+	} else {
+		subsurf_xpos = surface->width / 2 -
+			buffer_width / (2 * surface->scale) + 2 / surface->scale;
+	}
+
+	if (state->args.override_indicator_y_position) {
+		subsurf_ypos = state->args.indicator_y_position -
+			(state->args.radius + state->args.thickness);
+	} else {
+		subsurf_ypos = surface->height / 2 -
+			(state->args.radius + state->args.thickness);
+	}
+
+	struct pool_buffer *buffer = get_next_buffer(state->shm,
+			surface->indicator_buffers, buffer_width, buffer_height);
+	if (buffer == NULL) {
+		return;
+	}
+
+	// Render the buffer
+	cairo_t *cairo = buffer->cairo;
+	cairo_set_antialias(cairo, CAIRO_ANTIALIAS_BEST);
+
+	cairo_identity_matrix(cairo);
+
+	// Clear
+	cairo_save(cairo);
+	cairo_set_source_rgba(cairo, 0, 0, 0, 0);
+	cairo_set_operator(cairo, CAIRO_OPERATOR_SOURCE);
+	cairo_paint(cairo);
+	cairo_restore(cairo);
+
+	float type_indicator_border_thickness =
+		TYPE_INDICATOR_BORDER_THICKNESS * surface->scale;
+
+	if (state->args.show_indicator && (state->auth_state != AUTH_STATE_IDLE ||
+			state->args.indicator_idle_visible)) {
+		// Fill inner circle
+		cairo_set_line_width(cairo, 0);
+		cairo_arc(cairo, buffer_width / 2, buffer_diameter / 2,
+				arc_radius - arc_thickness / 2, 0, 2 * M_PI);
+		set_color_for_state(cairo, state, &state->args.colors.inside);
+		cairo_fill_preserve(cairo);
+		cairo_stroke(cairo);
+
+		// Draw ring
+		cairo_set_line_width(cairo, arc_thickness);
+		cairo_arc(cairo, buffer_width / 2, buffer_diameter / 2, arc_radius,
+				0, 2 * M_PI);
+		set_color_for_state(cairo, state, &state->args.colors.ring);
+		cairo_stroke(cairo);
+
+		// Draw a message
+		configure_font_drawing(cairo, state, surface->subpixel, arc_radius);
+		set_color_for_state(cairo, state, &state->args.colors.text);
 
 		if (text) {
 			cairo_text_extents_t extents;
@@ -217,10 +255,6 @@ void render_frame(struct swaylock_surface *surface) {
 			cairo_show_text(cairo, text);
 			cairo_close_path(cairo);
 			cairo_new_sub_path(cairo);
-
-			if (new_width < extents.width) {
-				new_width = extents.width;
-			}
 		}
 
 		// Typing indicator: Highlight random part on keypress
@@ -300,25 +334,11 @@ void render_frame(struct swaylock_surface *surface) {
 			cairo_set_source_u32(cairo, state->args.colors.layout_text);
 			cairo_show_text(cairo, layout_text);
 			cairo_new_sub_path(cairo);
-
-			new_height += fe.height + 2 * box_padding;
-			if (new_width < extents.width + 2 * box_padding) {
-				new_width = extents.width + 2 * box_padding;
-			}
 		}
 	}
 
-	// Ensure buffer size is multiple of buffer scale - required by protocol
+	// Send Wayland requests
-	new_height += surface->scale - (new_height % surface->scale);
+	wl_subsurface_set_position(surface->subsurface, subsurf_xpos, subsurf_ypos);
-	new_width += surface->scale - (new_width % surface->scale);
-
-	if (buffer_width != new_width || buffer_height != new_height) {
-		destroy_buffer(buffer);
-		surface->indicator_width = new_width;
-		surface->indicator_height = new_height;
-		render_frame(surface);
-		return;
-	}
 
 	wl_surface_set_buffer_scale(surface->child, surface->scale);
 	wl_surface_attach(surface->child, buffer->buffer, 0, 0);