ui: Refactor so that busy state won't be the default

Even though assuming nvim is busy most times is simpler, it has a problem: A lot
of unnecessary busy_start/busy_stop notifications are sent to the UI. That's
because in the majority of scenarios almost no time is spent between
`event_poll` calls.

This restores the normal behavior which is to call busy_start only when nvim is
going to perform some task that can take a significant amount of time. Also
improve the usage of buffering in the TUI when changing the cursor state.

src/nvim/ex_getln.c

@@ -1364,6 +1364,7 @@ cmdline_changed:
       if (ccline.cmdlen == 0)
         i = 0;
       else {
+        ui_busy_start();
         ui_flush();
         ++emsg_off;            /* So it doesn't beep if bad expr */
         /* Set the time limit to half a second. */
@@ -1381,6 +1382,7 @@ cmdline_changed:
         } else if (char_avail())
           /* cancelled searching because a char was typed */
           incsearch_postponed = TRUE;
+        ui_busy_stop();
       }
       if (i != 0)
         highlight_match = TRUE;                 /* highlight position */

src/nvim/getchar.c

@@ -2379,6 +2379,11 @@ inchar (
   int retesc = FALSE;               /* return ESC with gotint */
   int script_char;
 
+  if (wait_time == -1L || wait_time > 100L) {
+    // flush output before waiting
+    ui_flush();
+  }
+
   /*
    * Don't reset these when at the hit-return prompt, otherwise an endless
    * recursive loop may result (write error in swapfile, hit-return, timeout

src/nvim/misc1.c

@@ -2364,6 +2364,7 @@ int get_keystroke(void)
 
   mapped_ctrl_c = FALSE;        /* mappings are not used here */
   for (;; ) {
+    // flush output before waiting
     ui_flush();
     /* Leave some room for check_termcode() to insert a key code into (max
      * 5 chars plus NUL).  And fix_input_buffer() can triple the number of

src/nvim/os/event.h

@@ -8,15 +8,9 @@
 #include "nvim/os/job_defs.h"
 #include "nvim/os/time.h"
 
-void ui_busy_start(void);
-void ui_busy_stop(void);
-
 // Poll for events until a condition or timeout
 #define event_poll_until(timeout, condition)                                 \
   do {                                                                       \
-    if (timeout < 0 || timeout > 100) {                                      \
-      ui_busy_stop();                                                        \
-    }                                                                        \
     int remaining = timeout;                                                 \
     uint64_t before = (remaining > 0) ? os_hrtime() : 0;                     \
     while (!(condition)) {                                                   \
@@ -32,9 +26,6 @@ void ui_busy_stop(void);
         }                                                                    \
       }                                                                      \
     }                                                                        \
-    if (timeout < 0 || timeout > 100) {                                      \
-      ui_busy_start();                                                       \
-    }                                                                        \
   } while (0)
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS

src/nvim/os/shell.c

@@ -241,6 +241,7 @@ static int shell(const char *cmd,
   // invoke busy_start here so event_poll_until wont change the busy state for
   // the UI
   ui_busy_start();
+  ui_flush();
   status = job_wait(job, -1);
   ui_busy_stop();
 

src/nvim/tui/tui.c

@@ -16,6 +16,11 @@
 #include "nvim/os/event.h"
 #include "nvim/tui/tui.h"
 
+// Space reserved in the output buffer to restore the cursor to normal when
+// flushing. No existing terminal will require 32 bytes to do that.
+#define CNORM_COMMAND_MAX_SIZE 32
+#define OUTBUF_SIZE 0xffff
+
 typedef struct term_input TermInput;
 
 #include "term_input.inl"
@@ -31,8 +36,8 @@ typedef struct {
 
 typedef struct {
   unibi_var_t params[9];
-  char buf[0xffff];
-  size_t bufpos;
+  char buf[OUTBUF_SIZE];
+  size_t bufpos, bufsize;
   TermInput *input;
   uv_loop_t *write_loop;
   unibi_term *ut;
@@ -81,13 +86,13 @@ typedef struct {
 void tui_start(void)
 {
   TUIData *data = xcalloc(1, sizeof(TUIData));
-  data->busy = true;
   UI *ui = xcalloc(1, sizeof(UI));
   ui->data = data;
   data->attrs = data->print_attrs = EMPTY_ATTRS;
   data->fg = data->bg = -1;
   data->can_use_terminal_scroll = true;
   data->bufpos = 0;
+  data->bufsize = sizeof(data->buf) - CNORM_COMMAND_MAX_SIZE;
   data->unibi_ext.enable_mouse = -1;
   data->unibi_ext.disable_mouse = -1;
   data->unibi_ext.enable_bracketed_paste = -1;
@@ -108,7 +113,6 @@ void tui_start(void)
     data->ut = unibi_dummy();
   }
   fix_terminfo(data);
-  unibi_out(ui, unibi_cursor_invisible);
   // Enter alternate screen and clear
   unibi_out(ui, unibi_enter_ca_mode);
   unibi_out(ui, unibi_clear_screen);
@@ -172,6 +176,7 @@ static void tui_stop(UI *ui)
   tui_normal_mode(ui);
   tui_mouse_off(ui);
   unibi_out(ui, unibi_exit_attribute_mode);
+  // cursor should be set to normal before exiting alternate screen
   unibi_out(ui, unibi_cursor_normal);
   unibi_out(ui, unibi_exit_ca_mode);
   // Disable bracketed paste
@@ -661,7 +666,7 @@ static void out(void *ctx, const char *str, size_t len)
 {
   UI *ui = ctx;
   TUIData *data = ui->data;
-  size_t available = sizeof(data->buf) - data->bufpos;
+  size_t available = data->bufsize - data->bufpos;
 
   if (len > available) {
     flush_buf(ui);
@@ -786,29 +791,26 @@ end:
 static void flush_buf(UI *ui)
 {
   uv_write_t req;
-  uv_buf_t buf[2];
-  unsigned int buf_count = 1;
-
+  uv_buf_t buf;
   TUIData *data = ui->data;
 
-  buf[0].base = data->buf;
-  buf[0].len = data->bufpos;
-
-  char normal_buf[64];
   if (!data->busy) {
-    // Cannot use unibi_out(ui, unibi_cursor_normal), in case there is not
-    // enough remaining space in data->buf.
-    const char *str = unibi_get_str(data->ut, unibi_cursor_normal);
-    buf[1].base = normal_buf;
-    buf[1].len = unibi_run(str, data->params, normal_buf, sizeof(normal_buf));
-    buf_count++;
+    // not busy and the cursor is invisible(see below). Append a "cursor
+    // normal" command to the end of the buffer.
+    data->bufsize += CNORM_COMMAND_MAX_SIZE;
+    unibi_out(ui, unibi_cursor_normal);
+    data->bufsize -= CNORM_COMMAND_MAX_SIZE;
   }
 
-  uv_write(&req, (uv_stream_t *)&data->output_handle, buf, buf_count, NULL);
+  buf.base = data->buf;
+  buf.len = data->bufpos;
+  uv_write(&req, (uv_stream_t *)&data->output_handle, &buf, 1, NULL);
   uv_run(data->write_loop, UV_RUN_DEFAULT);
   data->bufpos = 0;
 
   if (!data->busy) {
+    // not busy and cursor is visible(see above), append a "cursor invisible"
+    // command to the beginning of the buffer for the next flush
     unibi_out(ui, unibi_cursor_invisible);
   }
 }

src/nvim/ui.c

@@ -45,7 +45,7 @@ static struct {
 } sr;
 static int current_attr_code = 0;
 static bool pending_cursor_update = false;
-static int busy = 1;
+static int busy = 0;
 static int height, width;
 
 // This set of macros allow us to use UI_CALL to invoke any function on
@@ -155,7 +155,6 @@ void ui_busy_start(void)
 {
   if (!(busy++)) {
     UI_CALL(busy_start);
-    ui_flush();
   }
 }
 
@@ -163,11 +162,9 @@ void ui_busy_stop(void)
 {
   if (!(--busy)) {
     UI_CALL(busy_stop);
-    ui_flush();
   }
 }
 
-
 void ui_mouse_on(void)
 {
   UI_CALL(mouse_on);

test/functional/ui/screen.lua

@@ -166,7 +166,7 @@ function Screen.new(width, height)
     _cursor = {
       row = 1, col = 1
     },
-    _busy = true
+    _busy = false
   }, Screen)
   self:_handle_resize(width, height)
   return self