From 99f471d738e0c2a0dcb61a559be3be512659d113 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Sat, 4 Jan 2020 19:49:26 +0100
Subject: [PATCH] render: trigger terminal refreshes in an FDM hook

In some cases, we end up calling render_refresh() multiple times in
the same FDM iteration. This means will render the first update
immediately, and then set the 'pending' flag, causing the updated
content to be rendered in the next frame.

This can cause flicker, or flashes, since we're presenting one or more
intermediate frames until the final content is shown.

Not to mention that it is inefficient to render multiple frames like
this.

Fix by:

* render_refresh() only sets a flag in the terminal

* install an FDM hook; this hook loops all terminals and executes what
  render_refresh() _used_ to do (that is, render immediately if we're
  not waiting for a frame callback, otherwise set 'pending' flag). for
  all terminals that have the 'refresh_needed' flag set.
---
 main.c     |  6 ++++++
 render.c   | 62 ++++++++++++++++++++++++++++++++++++++++++++++++------
 render.h   |  7 ++++--
 terminal.h |  1 +
 4 files changed, 68 insertions(+), 8 deletions(-)

diff --git a/main.c b/main.c
index ec44b8b3..e418b584 100644
--- a/main.c
+++ b/main.c
@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "fdm.h"
+#include "render.h"
 #include "server.h"
 #include "shm.h"
 #include "terminal.h"
@@ -218,6 +219,7 @@ main(int argc, char *const *argv)
 
     struct fdm *fdm = NULL;
     struct wayland *wayl = NULL;
+    struct renderer *renderer = NULL;
     struct terminal *term = NULL;
     struct server *server = NULL;
     struct shutdown_context shutdown_ctx = {.term = &term, .exit_code = EXIT_FAILURE};
@@ -238,6 +240,9 @@ main(int argc, char *const *argv)
     if ((wayl = wayl_init(&conf, fdm)) == NULL)
         goto out;
 
+    if ((renderer = render_init(fdm, wayl)) == NULL)
+        goto out;
+
     if (!as_server && (term = term_init(
                            &conf, fdm, wayl, conf.term, "foot", cwd, argc, argv,
                            &term_shutdown_cb, &shutdown_ctx)) == NULL) {
@@ -276,6 +281,7 @@ out:
     term_destroy(term);
 
     shm_fini();
+    render_destroy(renderer);
     wayl_destroy(wayl);
     fdm_destroy(fdm);
 
diff --git a/render.c b/render.c
index d97da212..2da90187 100644
--- a/render.c
+++ b/render.c
@@ -22,6 +22,11 @@
 #define min(x, y) ((x) < (y) ? (x) : (y))
 #define max(x, y) ((x) > (y) ? (x) : (y))
 
+struct renderer {
+    struct fdm *fdm;
+    struct wayland *wayl;
+};
+
 static struct {
     size_t total;
     size_t zero;  /* commits presented in less than one frame interval */
@@ -29,6 +34,56 @@ static struct {
     size_t two;   /* commits presented in two or more frame intervals */
 } presentation_statistics = {0};
 
+static void
+fdm_hook_refresh_pending_terminals(struct fdm *fdm, void *data)
+{
+    struct renderer *renderer = data;
+    tll_foreach(renderer->wayl->terms, it) {
+        struct terminal *term = it->item;
+
+        if (!term->render.refresh_needed)
+            continue;
+
+        assert(term->window->is_configured);
+        term->render.refresh_needed = false;
+
+        if (term->window->frame_callback == NULL) {
+            LOG_INFO("rendering immediately");
+            grid_render(term);
+        } else {
+            LOG_INFO("setting pending");
+            term->render.pending = true;
+        }
+    }
+}
+
+struct renderer *
+render_init(struct fdm *fdm, struct wayland *wayl)
+{
+    struct renderer *renderer = calloc(1, sizeof(*renderer));
+    *renderer = (struct renderer) {
+        .fdm = fdm,
+        .wayl = wayl,
+    };
+
+    if (!fdm_hook_add(fdm, &fdm_hook_refresh_pending_terminals, renderer)) {
+        LOG_ERR("failed to register FDM hook");
+        free(renderer);
+        return NULL;
+    }
+
+    return renderer;
+}
+
+void
+render_destroy(struct renderer *renderer)
+{
+    if (renderer == NULL)
+        return;
+
+    fdm_hook_del(renderer->fdm, &fdm_hook_refresh_pending_terminals);
+}
+
 static void __attribute__((destructor))
 log_presentation_statistics(void)
 {
@@ -1079,10 +1134,5 @@ render_set_title(struct terminal *term, const char *_title)
 void
 render_refresh(struct terminal *term)
 {
-    assert(term->window->is_configured);
-
-    if (term->window->frame_callback == NULL)
-        grid_render(term);
-    else
-        term->render.pending = true;
+    term->render.refresh_needed = true;
 }
diff --git a/render.h b/render.h
index ad6b6578..74d9b456 100644
--- a/render.h
+++ b/render.h
@@ -1,9 +1,12 @@
 #pragma once
 
 #include "terminal.h"
+#include "fdm.h"
+#include "wayland.h"
 
-struct font *attrs_to_font(
-    const struct terminal *term, const struct attributes *attrs);
+struct renderer;
+struct renderer *render_init(struct fdm *fdm, struct wayland *wayl);
+void render_destroy(struct renderer *renderer);
 
 void grid_render(struct terminal *term);
 void render_resize(struct terminal *term, int width, int height);
diff --git a/terminal.h b/terminal.h
index 9d28ff41..fe33ff98 100644
--- a/terminal.h
+++ b/terminal.h
@@ -283,6 +283,7 @@ struct terminal {
     bool visual_focus;
 
     struct {
+        bool refresh_needed;
         int scrollback_lines;
 
         struct {