Ticket MidnightCommander#4625: Fix slow start with zsh on macOS, part 2/2

Avoid a deadlock arising by zsh draining its output channel (i.e.
waiting for mc to read them), while mc is waiting over a different
channel for zsh's response to some magic keypresses to test the
persistent command line feature.

Fix this by making mc behave as it's expected from a terminal master:
still consuming incoming data while performing that other task.

The bug did not affect Linux because there draining the channel, at
least over local pseudoterminals, is a no-op, it does not actually
wait for the master side to read that data.

Signed-off-by: Egmont Koblinger <[email protected]>

Author: egmontkob

src/subshell/common.c

@@ -614,7 +614,7 @@ read_command_line_buffer (gboolean test_mode)
     FD_ZERO (&read_set);
     FD_SET (command_buffer_pipe[READ], &read_set);
 
-    const int maxfdp = command_buffer_pipe[READ];
+    const int maxfdp = MAX (command_buffer_pipe[READ], mc_global.tty.subshell_pty);
 
     /* First, flush the command buffer pipe. This pipe shouldn't be written
      * to under normal circumstances, but if it somehow does get written
@@ -643,11 +643,15 @@ read_command_line_buffer (gboolean test_mode)
                sizeof (ESC_STR SHELL_BUFFER_KEYBINDING) - 1);
 
     subshell_prompt_timer.tv_sec = 1;
-    FD_ZERO (&read_set);
-    FD_SET (command_buffer_pipe[READ], &read_set);
 
-    while ((rc = select (maxfdp + 1, &read_set, NULL, NULL, &subshell_prompt_timer)) != 1)
+    while (TRUE)
     {
+        FD_ZERO (&read_set);
+        FD_SET (command_buffer_pipe[READ], &read_set);
+        FD_SET (mc_global.tty.subshell_pty, &read_set);
+
+        rc = select (maxfdp + 1, &read_set, NULL, NULL, &subshell_prompt_timer);
+
         if (rc == -1)
         {
             if (errno == EINTR)
@@ -658,6 +662,17 @@ read_command_line_buffer (gboolean test_mode)
 
         if (rc == 0)
             return FALSE;
+
+        if (FD_ISSET (mc_global.tty.subshell_pty, &read_set))
+        {
+            /* Keep reading the pty to avoid possible deadlock with the shell. Throw away the data.
+             */
+            char buf[BUF_SMALL];
+            read_nonblock (mc_global.tty.subshell_pty, buf, sizeof (buf));
+        }
+
+        if (FD_ISSET (command_buffer_pipe[READ], &read_set))
+            break;
     }
 
     bytes =
@@ -676,11 +691,15 @@ read_command_line_buffer (gboolean test_mode)
 
     subshell_prompt_timer.tv_sec = 1;
     subshell_prompt_timer.tv_usec = 0;
-    FD_ZERO (&read_set);
-    FD_SET (command_buffer_pipe[READ], &read_set);
 
-    while ((rc = select (maxfdp + 1, &read_set, NULL, NULL, &subshell_prompt_timer)) != 1)
+    while (TRUE)
     {
+        FD_ZERO (&read_set);
+        FD_SET (command_buffer_pipe[READ], &read_set);
+        FD_SET (mc_global.tty.subshell_pty, &read_set);
+
+        rc = select (maxfdp + 1, &read_set, NULL, NULL, &subshell_prompt_timer);
+
         if (rc == -1)
         {
             if (errno == EINTR)
@@ -691,6 +710,17 @@ read_command_line_buffer (gboolean test_mode)
 
         if (rc == 0)
             return FALSE;
+
+        if (FD_ISSET (mc_global.tty.subshell_pty, &read_set))
+        {
+            /* Keep reading the pty to avoid possible deadlock with the shell. Throw away the data.
+             */
+            char buf[BUF_SMALL];
+            read_nonblock (mc_global.tty.subshell_pty, buf, sizeof (buf));
+        }
+
+        if (FD_ISSET (command_buffer_pipe[READ], &read_set))
+            break;
     }
 
     bytes =