docs: add notice about known problems with SDL2 and V's garbage collector (#745) (#754)

Author: larpon

README.md

@@ -56,6 +56,22 @@ against newer versions of the SDL2 library.
 
 Also note that SDL2 **is not** compatible with SDL `v1.x`.
 
+## Notes on garbage collection and memory issues
+
+Currently, with some setups, SDL2 is known to trigger crashes when used in conjunction
+with V's default garbage collector. In these cases running apps importing `sdl` with
+`v run` you may experience runtime crashes and output similar to this:
+
+```
+main__main: RUNTIME ERROR: invalid memory access
+```
+
+We are tracking the issue here: https://github.com/vlang/sdl/issues/744
+
+The crashes can be avoided by passing `-d sdl_memory_no_gc` when compiling V applications
+that contains `import sdl` and managing SDL2's memory manually with calls to the various
+`destroy` and `sdl.free/1` functions.
+
 ## Support
 
 `sdl` is currently supported on:

tests/crash_with_gc.vv

@@ -0,0 +1,61 @@
+// This file serves as a MRE (minimal reproducible example) of a runtime crash triggered by compiling and running
+// this file with just `v run ~/.vmodules/sdl/tests/crash_with_gc.vv`. On some setups, the problem seems to be
+// memory corruption happening between actions in SDL2's memory allocations and V's default garbage collector.
+// Especially when a lot of heap allocations occur.
+//
+// The example does not crash if compiled with `-d sdl_memory_no_gc`.
+// See also: https://github.com/vlang/sdl/issues/744
+module main
+
+import sdl
+
+struct Data1 {
+mut:
+	a int
+}
+
+fn main() {
+	mut data1 := []&Data1{cap: 200}
+	for i in 0 .. 200 {
+		data1 << &Data1{
+			a: i
+		}
+	}
+
+	sdl.init(sdl.init_video)
+	window := sdl.create_window('Hello SDL2'.str, 300, 300, 500, 300, 0)
+	renderer := sdl.create_renderer(window, -1, u32(sdl.RendererFlags.accelerated) | u32(sdl.RendererFlags.presentvsync))
+
+	mut should_close := false
+	mut ticks := 0
+	for {
+		ticks++
+		evt := sdl.Event{}
+		for 0 < sdl.poll_event(&evt) {
+			match evt.@type {
+				.quit { should_close = true }
+				else {}
+			}
+		}
+
+		data1[0].a = ticks
+		data1.delete(10)
+		data1 << &Data1{
+			a: ticks
+		}
+
+		println('ticks: ${ticks}')
+		if should_close || ticks == 1000 {
+			break
+		}
+
+		sdl.set_render_draw_color(renderer, 255, 55, 55, 255)
+		sdl.render_clear(renderer)
+		sdl.render_present(renderer)
+	}
+	println('Exiting. If this was compiled without `-d sdl_memory_no_gc`, an invalid memory access error should occur')
+
+	sdl.destroy_renderer(renderer)
+	sdl.destroy_window(window)
+	sdl.quit()
+}