Godot 4 simple canvas item shader to create a circle mask.

bordered_circle_mask.shader

shader_type canvas_item;

uniform vec4 border_color: source_color = vec4(1.0);
uniform float border_width: hint_range(0, 10) = 0;

void fragment() {
	float alpha = step(length(UV - vec2(0.5, 0.5)), 0.5);
	if (alpha > 0.0) {
		float border_width_uv = border_width * max(TEXTURE_PIXEL_SIZE.x, TEXTURE_PIXEL_SIZE.y);
		if (step(0.5 - border_width_uv, length(UV - vec2(0.5, 0.5))) == 1.0) {
			COLOR = border_color;
		}
	} else {
		COLOR.a = 0.0;
	}
}

circle_mask.shader

shader_type canvas_item;

void fragment() {
	COLOR.a *= step(length(UV - vec2(0.5, 0.5)), 0.5);
}