subshell: reorder pty/pipe file descriptor acquisition

Swap the code allocating the pty and the block getting
the pipe for cwd communication, to hopefully get an fd
lower than 10 for the latter, to ameliorate a few from
the problems mentioned in #4634 as long as there isn’t
more global code allocating file descriptors added.

Specifically, this moves the shell’s side of the fd in
my local tests on Debian trixie from 10 to 8.

I’ve merely swapped two blocks without changing any of
the actual code. Swapping WRITE with READ in line 158ff
would gain us another one in this specific instance as
it would then use the lower number of the pair.

#4634 (comment)
mentions a few ideas I have for really actually fixing
this in the long term (and ensuring the issue will not
reappear) which need more invasive refactoring though.

Signed-off-by: Thorsten Glaser <tglaser@b1-systems.de>

Author: mirabilos

src/subshell/common.c

@@ -1556,37 +1556,6 @@ init_subshell (void)
         if (mc_global.shell->type == SHELL_NONE)
             return;
 
-        // Open a pty for talking to the subshell
-
-        // FIXME: We may need to open a fresh pty each time on SVR4
-
-#ifdef HAVE_OPENPTY
-        if (openpty (&mc_global.tty.subshell_pty, &subshell_pty_slave, NULL, NULL, NULL))
-        {
-            fprintf (stderr, "Cannot open master and slave sides of pty: %s\n",
-                     unix_error_string (errno));
-            mc_global.tty.use_subshell = FALSE;
-            return;
-        }
-#else
-        mc_global.tty.subshell_pty = pty_open_master (pty_name);
-        if (mc_global.tty.subshell_pty == -1)
-        {
-            fprintf (stderr, "Cannot open master side of pty: %s\r\n", unix_error_string (errno));
-            mc_global.tty.use_subshell = FALSE;
-            return;
-        }
-
-        subshell_pty_slave = pty_open_slave (pty_name);
-        if (subshell_pty_slave == -1)
-        {
-            fprintf (stderr, "Cannot open slave side of pty %s: %s\r\n", pty_name,
-                     unix_error_string (errno));
-            mc_global.tty.use_subshell = FALSE;
-            return;
-        }
-#endif
-
         // Create a pipe for receiving the subshell's CWD
 
         if (mc_global.shell->type == SHELL_TCSH)
@@ -1629,6 +1598,41 @@ init_subshell (void)
             mc_global.tty.use_subshell = FALSE;
             return;
         }
+
+        // Open a pty for talking to the subshell; do this after
+        // acquiring the pipes, so the latter have a higher chance
+        // of getting a number lower than 10 so the "pwd >&%d" in
+        // the mc-injected precmd does not cause a syntax error, a
+        // ten-second sleep at startup, and other problems.
+
+        // FIXME: We may need to open a fresh pty each time on SVR4
+
+#ifdef HAVE_OPENPTY
+        if (openpty (&mc_global.tty.subshell_pty, &subshell_pty_slave, NULL, NULL, NULL))
+        {
+            fprintf (stderr, "Cannot open master and slave sides of pty: %s\n",
+                     unix_error_string (errno));
+            mc_global.tty.use_subshell = FALSE;
+            return;
+        }
+#else
+        mc_global.tty.subshell_pty = pty_open_master (pty_name);
+        if (mc_global.tty.subshell_pty == -1)
+        {
+            fprintf (stderr, "Cannot open master side of pty: %s\r\n", unix_error_string (errno));
+            mc_global.tty.use_subshell = FALSE;
+            return;
+        }
+
+        subshell_pty_slave = pty_open_slave (pty_name);
+        if (subshell_pty_slave == -1)
+        {
+            fprintf (stderr, "Cannot open slave side of pty %s: %s\r\n", pty_name,
+                     unix_error_string (errno));
+            mc_global.tty.use_subshell = FALSE;
+            return;
+        }
+#endif
     }
 
     // Fork the subshell