Use a single foreign object guardian.

Rather than each foreign resource type creating its own guardian, let's
just use one instead so that there is only a single guardian to talk to
each frame.

* sly/guardian.scm: New file.
* Makefile.am (SOURCES): Add it.
* sly/game.scm (run-game-loop): Start guardian worker.
* sly/render/mesh.scm (mesh-guardian): Delete.
(free-mesh): New procedure.
(make-mesh): Guard newly created meshes.
* sly/render/shader.scm (shader-stage-guardian): Delete.
(free-shader-stage): New procedure.
(make-shader-stage): Guard newly created shader stages.
(shader-guardian): Delete.
(free-shader): New procedure.
(make-shader): Guard newly created shaders.
* sly/render/texture.scm (texture-guardian): Delete.
(free-texture): New procedure.
(make-texture): Guard newly created textures.

6 files changed, 109 insertions, 38 deletions

diff --git a/Makefile.am b/Makefile.am
index f63e3da..fe5348a 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -29,6 +29,7 @@ SOURCES = 					\
   sly/event.scm					\
   sly/fps.scm					\
   sly/game.scm					\
+  sly/guardian.scm				\
   sly/input/keyboard.scm			\
   sly/input/mouse.scm				\
   sly/live-reload.scm				\
diff --git a/sly/game.scm b/sly/game.scm
index 38d230e..66f0893 100644
--- a/sly/game.scm
+++ b/sly/game.scm
@@ -31,6 +31,7 @@
   #:use-module (gl)
   #:use-module (sly agenda)
   #:use-module (sly event)
+  #:use-module (sly guardian)
   #:use-module (sly math)
   #:use-module (sly signal)
   #:use-module (sly math vector)
@@ -184,6 +185,7 @@ milliseconds of the last iteration of the game loop."
            (stop-game-loop)))
        ;; Let's play!
        (run-hook game-start-hook)
+       (run-guardian)
        (game-loop (sdl-ticks) 0))
      (lambda (cont callback)
        (when (procedure? callback)
diff --git a/sly/guardian.scm b/sly/guardian.scm
new file mode 100644
index 0000000..6bdfcd0
--- /dev/null
+++ b/sly/guardian.scm
@@ -0,0 +1,69 @@
+;;; Sly
+;;; Copyright © 2016 David Thompson <davet@gnu.org>
+;;;
+;;; Sly is free software: you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Sly is distributed in the hope that it will be useful, but WITHOUT
+;;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+;;; or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+;;; License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Garbage collector for foreign objects.
+;;
+;;; Code:
+
+(define-module (sly guardian)
+  #:use-module (ice-9 match)
+  #:use-module (sly agenda)
+  #:export (register-finalizer
+            guard
+            run-guardian))
+
+(define-syntax-rule (push! variable value)
+  (set! variable (cons value variable)))
+
+(define %guardian (make-guardian))
+
+(define %finalizers '())
+
+(define (register-finalizer predicate finalizer)
+  "Register FINALIZER, a procedure that frees a foreign resource, to
+be used for objects that satisfy PREDICATE."
+  (push! %finalizers (cons predicate finalizer)))
+
+(define (guard obj)
+  "Protect OBJ from garbage collection until its finalizer has been
+applied.  OBJ is returned unmodified."
+  (%guardian obj)
+  obj)
+
+(define (lookup-finalizer obj)
+  "Return the finalization procedure for OBJ, or #f if none is found."
+  (let loop ((finalizers %finalizers))
+    (match finalizers
+      (() #f)
+      (((predicate . finalizer) . rest)
+       (if (predicate obj)
+           finalizer
+           (loop rest))))))
+
+(define (run-guardian)
+  "Start the guardian worker coroutine on the current agenda."
+  (schedule-each
+   (lambda ()
+     (let loop ((obj (%guardian)))
+       (when obj
+         (let ((finalizer (lookup-finalizer obj)))
+           (if finalizer
+               (finalizer obj)
+               (error "no finalizer found for object" obj)))
+         (loop (%guardian)))))))
diff --git a/sly/render/mesh.scm b/sly/render/mesh.scm
index acf9a0f..f1d6e2c 100644
--- a/sly/render/mesh.scm
+++ b/sly/render/mesh.scm
@@ -34,6 +34,7 @@
   #:use-module (gl low-level)
   #:use-module (gl enums)
   #:use-module (sly utils)
+  #:use-module (sly guardian)
   #:use-module (sly wrappers gl)
   #:use-module (sly math vector)
   #:use-module (sly render color)
@@ -264,17 +265,18 @@ allows vertex buffers to be updated with new vertex data."
   (length mesh-length)
   (vertex-buffers mesh-vertex-buffers))
 
-(define-guardian mesh-guardian
-  (lambda (mesh)
-    ;; Delete vertex array and vertex buffers.
-    (glDeleteVertexArrays 1 (u32vector (mesh-id mesh)))
-    (let ((buffers (mesh-vertex-buffers mesh)))
-      (glDeleteBuffers (length buffers)
-                       (list->u32vector
-                        (map (match-lambda
-                              ((_ . vbo)
-                               (vertex-buffer-id vbo)))
-                             buffers))))))
+(define (free-mesh mesh)
+  ;; Delete vertex array and vertex buffers.
+  (glDeleteVertexArrays 1 (u32vector (mesh-id mesh)))
+  (let ((buffers (mesh-vertex-buffers mesh)))
+    (glDeleteBuffers (length buffers)
+                     (list->u32vector
+                      (map (match-lambda
+                             ((_ . vbo)
+                              (vertex-buffer-id vbo)))
+                           buffers)))))
+
+(register-finalizer mesh? free-mesh)
 
 (define null-mesh (%make-mesh 0 0 '()))
 
@@ -309,8 +311,7 @@ allows vertex buffers to be updated with new vertex data."
       (vertex-attrib-pointer vertex-position-location position-buffer)
       (vertex-attrib-pointer vertex-texture-location texture-buffer)
       (apply-vertex-buffer index-buffer))
-    (mesh-guardian mesh)
-    mesh))
+    (guard mesh)))
 
 (define (build-mesh indices positions textures)
   (let ((index-buffer (vector->vertex-buffer indices #t))
diff --git a/sly/render/shader.scm b/sly/render/shader.scm
index 556d626..3367b57 100644
--- a/sly/render/shader.scm
+++ b/sly/render/shader.scm
@@ -26,6 +26,7 @@
   #:use-module (gl)
   #:use-module (gl low-level)
   #:use-module (sly utils)
+  #:use-module (sly guardian)
   #:use-module (sly math transform)
   #:use-module (sly math vector)
   #:use-module (sly render color)
@@ -88,10 +89,11 @@
   (eq? (shader-stage-type stage) 'fragment))
 
 ;; Reap GL shaders when their wrapper objects are GC'd.
-(define-guardian shader-stage-guardian
-  (lambda (stage)
-    (false-if-exception
-     (glDeleteShader (shader-stage-id stage)))))
+(define (free-shader-stage stage)
+  (false-if-exception
+   (glDeleteShader (shader-stage-id stage))))
+
+(register-finalizer shader-stage? free-shader-stage)
 
 (define-status %shader-stage-compiled? glGetShaderiv compile-status)
 
@@ -135,10 +137,9 @@ or 'fragment') and compile the GLSL program contained in the string
 SOURCE."
   (let* ((id (glCreateShader (gl-shader-type type)))
          (stage (%make-shader-stage type id)))
-    (shader-stage-guardian stage)
     (set-shader-stage-source stage source)
     (compile-shader-stage stage)
-    stage))
+    (guard stage)))
 
 (define (make-vertex-shader-stage source)
   "Create a new GLSL vertex shader stage and compile the GLSL program
@@ -205,10 +206,11 @@ stored in the file FILE-NAME."
         (uniform-location uniform)
         (error "Uniform not found: " name))))
 
-(define-guardian shader-guardian
-  (lambda (shader)
-    (false-if-exception
-     (glDeleteProgram (shader-id shader)))))
+(define (free-shader shader)
+  (false-if-exception
+   (glDeleteProgram (shader-id shader))))
+
+(register-finalizer shader? free-shader)
 
 (define-status shader-linked? glGetProgramiv link-status)
 (define-logger display-linking-error glGetProgramiv glGetProgramInfoLog)
@@ -252,10 +254,8 @@ VERTEX-STAGE and FRAGMENT-STAGE."
         (for-each (lambda (stage)
                     (glDetachShader id (shader-stage-id stage)))
                   stages)
-        (let* ((uniforms (map build-uniform uniforms))
-               (shader (%make-shader id uniforms)))
-          (shader-guardian shader)
-          shader))
+        (let* ((uniforms (map build-uniform uniforms)))
+          (guard (%make-shader id uniforms))))
       throw
       (lambda _
         ;; Make sure to delete program in case linking fails.
diff --git a/sly/render/texture.scm b/sly/render/texture.scm
index bb7f88c..2e77b0f 100644
--- a/sly/render/texture.scm
+++ b/sly/render/texture.scm
@@ -31,8 +31,9 @@
   #:use-module (gl contrib packed-struct)
   #:use-module (sdl2 image)
   #:use-module (sdl2 surface)
-  #:use-module (sly render color)
+  #:use-module (sly guardian)
   #:use-module (sly utils)
+  #:use-module (sly render color)
   #:use-module (sly math vector)
   #:use-module (sly wrappers gl)
   #:export (make-texture
@@ -90,9 +91,7 @@ a texture object (if this texture only represents a region of another
 texture) or #f. WIDTH and HEIGHT are the texture dimensions in
 pixels. S1, T1, S2, and T2 are the OpenGL texture coordinates
 representing the area of the texture that will be rendered."
-  (let ((texture (%make-texture id parent width height s1 t1 s2 t2)))
-    (texture-guardian texture)
-    texture))
+  (guard (%make-texture id parent width height s1 t1 s2 t2)))
 
 (define (make-texture-region texture x y width height)
   "Creates new texture region object. TEXTURE is the region's parent
@@ -109,13 +108,12 @@ that will be rendered, in pixels."
                   (/ (+ x width) w)
                   (/ (+ y height) h))))
 
-;; Use a guardian and an after GC hook that ensures that OpenGL
-;; textures are deleted when texture objects are GC'd.
-(define-guardian texture-guardian
-  (lambda (texture)
-    ;; Do not reap texture regions
-    (unless (texture-region? texture)
-      (gl-delete-texture (texture-id texture)))))
+(define (free-texture texture)
+  ;; Do not reap texture regions.
+  (unless (texture-region? texture)
+    (gl-delete-texture (texture-id texture))))
+
+(register-finalizer texture? free-texture)
 
 (define* (bytevector->texture pixels width height min-filter mag-filter
                               #:optional (format (pixel-format rgba)))