fix(terminal): don't poll for output during scrollback refresh (#38365)

Problem:
If buffer update callbacks poll for uv events during terminal scrollback
refresh, new output from PTY process may lead to incorrect scrollback.

Solution:
Don't poll for output to the same terminal as the one being refreshed.

src/nvim/api/vim.c

@@ -1153,6 +1153,7 @@ Integer nvim_open_term(Buffer buffer, Dict(open_term) *opts, Error *err)
     // displaying the buffer
     .width = (uint16_t)MAX(curwin->w_view_width - win_col_off(curwin), 0),
     .height = (uint16_t)curwin->w_view_height,
+    .read_pause_cb = term_read_pause,
     .write_cb = term_write,
     .resize_cb = term_resize,
     .resume_cb = term_resume,
@@ -1185,6 +1186,11 @@ Integer nvim_open_term(Buffer buffer, Dict(open_term) *opts, Error *err)
   return (Integer)chan->id;
 }
 
+static void term_read_pause(bool pause, void *data)
+{
+  // Not currently needed as sending to channel isn't allowed during buffer updates.
+}
+
 static void term_write(const char *buf, size_t size, void *data)
 {
   Channel *chan = data;

src/nvim/channel.c

@@ -847,6 +847,7 @@ void channel_terminal_alloc(buf_T *buf, Channel *chan)
     .data = chan,
     .width = chan->stream.pty.width,
     .height = chan->stream.pty.height,
+    .read_pause_cb = term_read_pause,
     .write_cb = term_write,
     .resize_cb = term_resize,
     .resume_cb = term_resume,
@@ -858,6 +859,19 @@ void channel_terminal_alloc(buf_T *buf, Channel *chan)
   chan->term = terminal_alloc(buf, topts);
 }
 
+static void term_read_pause(bool pause, void *data)
+{
+  Channel *chan = data;
+  if (chan->stream.proc.out.s.closed) {
+    return;
+  }
+  if (pause) {
+    rstream_stop_inner(&chan->stream.proc.out);
+  } else {
+    rstream_start_inner(&chan->stream.proc.out);
+  }
+}
+
 static void term_write(const char *buf, size_t size, void *data)
 {
   Channel *chan = data;

src/nvim/terminal.c

@@ -2530,6 +2530,10 @@ static void adjust_scrollback(Terminal *term, buf_T *buf)
 // Refresh the scrollback of an invalidated terminal.
 static void refresh_scrollback(Terminal *term, buf_T *buf)
 {
+  // Buffer update callbacks may poll for uv events.
+  // Avoid polling for output to the same terminal as the one being refreshed.
+  term->opts.read_pause_cb(true, term->opts.data);
+
   linenr_T deleted = (linenr_T)(term->sb_deleted - term->old_sb_deleted);
   deleted = MIN(deleted, buf->b_ml.ml_line_count);
   mark_adjust_buf(buf, 1, deleted, MAXLNUM, -deleted, true, kMarkAdjustTerm, kExtmarkUndo);
@@ -2569,6 +2573,8 @@ static void refresh_scrollback(Terminal *term, buf_T *buf)
   }
 
   adjust_scrollback(term, buf);
+
+  term->opts.read_pause_cb(false, term->opts.data);
 }
 
 // Refresh the screen (visible part of the buffer when the terminal is

src/nvim/terminal.h

@@ -7,6 +7,7 @@
 #include "nvim/api/private/defs.h"  // IWYU pragma: keep
 #include "nvim/types_defs.h"  // IWYU pragma: keep
 
+typedef void (*terminal_read_pause_cb)(bool pause, void *data);
 typedef void (*terminal_write_cb)(const char *buffer, size_t size, void *data);
 typedef void (*terminal_resize_cb)(uint16_t width, uint16_t height, void *data);
 typedef void (*terminal_resume_cb)(void *data);
@@ -15,6 +16,7 @@ typedef void (*terminal_close_cb)(void *data);
 typedef struct {
   void *data;  // PTY process channel
   uint16_t width, height;
+  terminal_read_pause_cb read_pause_cb;
   terminal_write_cb write_cb;
   terminal_resize_cb resize_cb;
   terminal_resume_cb resume_cb;