til: introduce til_module_render_limited() variant

With the introduction of discoverable on-stream contexts, with
the intention of modules finding them at runtime for nested
rendering, relying exclusively on limiting n_cpus @
til_module_create_context() is no longer adequate.

When the nested rendering makes use of a context created
elsewhere, it can't make any assumptions about what its n_cpus
is, nor should it be attempting to change that value for its use.

So this variant just adds a max_cpus parameter for setting an
upper bound to whatever is found in the context's n_cpus.

The impetus for this is to fix the ref builtin, which you can
currently trick into deadlock by performing a nested threaded
render within a threaded render.  What the ref module should be
doing is propagating its own context's n_cpus down as the upper
bound, via this new render variant.

2 files changed, 24 insertions, 7 deletions

diff --git a/src/til.c b/src/til.c
index d374759..b4a903c 100644
--- a/src/til.c
+++ b/src/til.c
@@ -386,7 +386,7 @@ char * til_get_module_names(unsigned flags_excluded, const char **exclusions)
 }
 
 
-static void module_render_fragment(til_module_context_t *context, til_stream_t *stream, til_threads_t *threads, unsigned ticks, til_fb_fragment_t **fragment_ptr)
+static void module_render_fragment(til_module_context_t *context, til_stream_t *stream, til_threads_t *threads, unsigned n_cpus, unsigned ticks, til_fb_fragment_t **fragment_ptr)
 {
 	const til_module_t	*module;
 	int			touched = 0;
@@ -395,6 +395,7 @@ static void module_render_fragment(til_module_context_t *context, til_stream_t *
 	assert(context->module);
 	assert(threads);
 	assert(fragment_ptr && *fragment_ptr);
+	assert(n_cpus);
 
 	module = context->module;
 
@@ -409,7 +410,7 @@ static void module_render_fragment(til_module_context_t *context, til_stream_t *
 		assert(frame_plan.fragmenter);
 		assert(module->render_fragment);
 
-		if (context->n_cpus > 1) {
+		if (n_cpus > 1) {
 			til_threads_frame_submit(threads, fragment_ptr, &frame_plan, module->render_fragment, context, stream, ticks);
 			til_threads_wait_idle(threads);
 		} else {
@@ -437,14 +438,11 @@ static void module_render_fragment(til_module_context_t *context, til_stream_t *
 }
 
 
-/* This is a public interface to the threaded module rendering intended for use by
- * modules that wish to get the output of other modules for their own use.
- */
-void til_module_render(til_module_context_t *context, til_stream_t *stream, unsigned ticks, til_fb_fragment_t **fragment_ptr)
+static void _til_module_render(til_module_context_t *context, til_stream_t *stream, unsigned n_cpus, unsigned ticks, til_fb_fragment_t **fragment_ptr)
 {
 	unsigned	start = til_ticks_now();
 
-	module_render_fragment(context, stream, til_threads, ticks, fragment_ptr);
+	module_render_fragment(context, stream, til_threads, n_cpus, ticks, fragment_ptr);
 
 	context->last_render_duration = til_ticks_now() - start;
 	if (context->last_render_duration > context->max_render_duration)
@@ -454,6 +452,24 @@ void til_module_render(til_module_context_t *context, til_stream_t *stream, unsi
 }
 
 
+/* This is a public interface to the threaded module rendering intended for use by
+ * modules that wish to get the output of other modules for their own use.
+ */
+void til_module_render(til_module_context_t *context, til_stream_t *stream, unsigned ticks, til_fb_fragment_t **fragment_ptr)
+{
+	return _til_module_render(context, stream, context->n_cpus, ticks, fragment_ptr);
+}
+
+
+/* Identical to til_module_render() except with a parameterized upper bound for context->c_cpus,
+ * this is primarily intended for modules performing nested rendering
+ */
+void til_module_render_limited(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned max_cpus, til_fb_fragment_t **fragment_ptr)
+{
+	return _til_module_render(context, stream, MIN(context->n_cpus, max_cpus), ticks, fragment_ptr);
+}
+
+
 /* if n_cpus == 0, it will be automatically set to n_threads.
  * to explicitly set n_cpus, just pass the value.  This is primarily intended for
  * the purpose of explicitly constraining rendering parallelization to less than n_threads,
diff --git a/src/til.h b/src/til.h
index eea8009..ec078db 100644
--- a/src/til.h
+++ b/src/til.h
@@ -45,6 +45,7 @@ const til_module_t * til_lookup_module(const char *name);
 void til_get_modules(const til_module_t ***res_modules, size_t *res_n_modules);
 char * til_get_module_names(unsigned flags_excluded, const char **exclusions);
 void til_module_render(til_module_context_t *context, til_stream_t *stream, unsigned ticks, til_fb_fragment_t **fragment_ptr);
+void til_module_render_limited(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned max_cpus, til_fb_fragment_t **fragment_ptr);
 int til_module_create_contexts(const til_module_t *module, til_stream_t *stream, unsigned seed, unsigned ticks, unsigned n_cpus, til_setup_t *setup, size_t n_contexts, til_module_context_t **res_contexts);
 int til_module_create_context(const til_module_t *module, til_stream_t *stream, unsigned seed, unsigned ticks, unsigned n_cpus, til_setup_t *setup, til_module_context_t **res_context);
 til_module_context_t * til_module_destroy_context(til_module_context_t *context, til_stream_t *stream);