mksession: restore same :term buf in split windows

Problem: When session-restore creates a terminal buffer with command
         like `:edit term://.//16450:/bin/bash`, the buffer gets
         a different name (depends on PID). Thus the later call to
         `bufexists('term://.//16450:/bin/bash)` will return false.
Solution: Force the buffer name with :file. This as least ensures
         the same buffer will show in multiple windows correctly, as
         expected when saving the session.  But it still has problems:
           1. the PID in the buffer name is bogus
           2. redundant :terminal buffers still hang around

fix #5250

src/nvim/ex_docmd.c

@@ -9346,6 +9346,10 @@ makeopens(
     /*
      * Restore the view of the window (options, file, cursor, etc.).
      */
+    if (put_line(fd, "let s:buffer_names = []") == FAIL) {
+      return FAIL;
+    }
+
     for (wp = tab_firstwin; wp != NULL; wp = wp->w_next) {
       if (!ses_do_win(wp))
         continue;
@@ -9357,6 +9361,16 @@ makeopens(
       next_arg_idx = wp->w_arg_idx;
     }
 
+    if (put_line(fd, "for [s:name, s:tbuf] in s:buffer_names") == FAIL
+        || put_line(fd, "  if buflisted(s:tbuf)") == FAIL
+        || put_line(fd, "    execute 'buffer' fnameescape(s:tbuf)") == FAIL
+        || put_line(fd, "    execute 'file' fnameescape(s:name)") == FAIL
+        || put_line(fd, "  endif") == FAIL
+        || put_line(fd, "endfor") == FAIL
+        || put_line(fd, "unlet! s:buffer_names s:tbuf s:name") == FAIL) {
+      return FAIL;
+    }
+
     /* The argument index in the first tab page is zero, need to set it in
      * each window.  For further tab pages it's the window where we do
      * "tabedit". */
@@ -9662,6 +9676,19 @@ put_view(
           || put_eol(fd) == FAIL) {
         return FAIL;
       }
+
+      if (fputs("call add(s:buffer_names, [bufname('%'),'", fd) < 0
+          || ses_fname(fd, wp->w_buffer, flagp, false) == FAIL
+          || fputs("'])", fd) < 0
+          || put_eol(fd) == FAIL) {
+        return FAIL;
+      }
+
+      if (fputs("file ", fd) < 0
+          || ses_fname(fd, wp->w_buffer, flagp, false) == FAIL
+          || put_eol(fd) == FAIL) {
+        return FAIL;
+      }
     } else {
       // No file in this buffer, just make it empty.
       if (put_line(fd, "enew") == FAIL) {

test/functional/ex_cmds/mksession_spec.lua

@@ -91,4 +91,23 @@ describe(':mksession', function()
     matches('^term://'..pesc(expected_cwd)..'//%d+:', funcs.expand('%'))
     command('qall!')
   end)
+
+	it('restores multiple windows with same terminal instances', function()
+		-- Create a view with two buffers referencing the same terminal instance
+		command('terminal')
+		command('split')
+		command('mksession ' .. session_file)
+
+		clear()
+
+		command('source ' .. session_file)
+		-- Getting the name of the buffer shown to compare with the other window
+		local eval = helpers.eval
+
+		command('exe 1 . "wincmd w"')
+		local expected_pid = eval('b:terminal_job_pid')
+
+		command('exe 2 . "wincmd w"')
+		eq(expected_pid, eval('b:terminal_job_pid'))
+	end)
 end)