patch 9.1.0751: Error callback for term_start() not used

Problem:  Error callback for term_start() not used
          (darkseid-is)
Solution: attach pipe to stderr if an error callback exists

fixes: #15665
closes: #15729

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/os_unix.c

@@ -5763,8 +5763,11 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options, int is_terminal)
 	    goto failed;
 	}
     }
+    // only create a pipe for the error fd, when either a callback has been setup
+    // or pty is not used (e.g. terminal uses pty by default)
     else if (!use_out_for_err && !use_null_for_err
-				      && pty_master_fd < 0 && pipe(fd_err) < 0)
+		&& (pty_master_fd < 0 || (options->jo_set & JO_ERR_CALLBACK))
+			    && pipe(fd_err) < 0)
 	goto failed;
 
     if (!use_null_for_in || !use_null_for_out || !use_null_for_err)

src/testdir/test_channel.vim

@@ -2765,4 +2765,42 @@ func Test_channel_lsp_mode()
   call RunServer('test_channel_lsp.py', 'LspTests', [])
 endfunc
 
+func Test_error_callback_terminal()
+  CheckUnix
+  CheckFeature terminal
+  let g:out = ''
+  let g:error = ''
+
+  func! s:Out(channel, msg)
+      let g:out .= string(a:msg)
+  endfunc
+
+  func! s:Err(channel, msg)
+      let g:error .= string(a:msg)
+  endfunc
+
+  let buf = term_start(['sh'], #{term_finish: 'close', out_cb: 's:Out', err_cb: 's:Err', err_io: 'pipe'})
+  let job = term_getjob(buf)
+  let dict = job_info(job).channel->ch_info()
+
+  call assert_true(dict.id != 0)
+  call assert_equal('open', dict.status)
+  call assert_equal('open', dict.out_status)
+  call assert_equal('RAW', dict.out_mode)
+  call assert_equal('buffer', dict.out_io)
+  call assert_equal('open', dict.err_status)
+  call assert_equal('RAW', dict.err_mode)
+  call assert_equal('pipe', dict.err_io)
+  call term_sendkeys(buf, "XXXX\<cr>")
+  call term_wait(buf)
+  call term_sendkeys(buf, "exit\<cr>")
+  call term_wait(buf)
+  call assert_match('XXX.*exit', g:out)
+  call assert_match('sh:.*XXXX:.*not found', g:error)
+
+  delfunc s:Out
+  delfunc s:Err
+  unlet! g:out g:error
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    751,
 /**/
     750,
 /**/