Try to not prioritise reading from lower file descriptors

* src/process.c (wait_reading_process_output): When looping through
fds, continue from where we left off.
(syms_of_process): Vprocess_prioritize_lower_fds: New variable
(bug#48118).

diff --git a/etc/NEWS b/etc/NEWS
index 1541b74a3be8639c066f71140a8568a711623dd2..7f9bf5c2423d2cfe19042e38229e4d62d2ebcdf1 100644 (file)
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2705,6 +2705,13 @@ the Emacs Lisp reference manual for background.
 \f
 * Lisp Changes in Emacs 28.1
 
+---
+** New variable 'process-prioritize-lower-fds'.
+When looping through file descriptors to handle subprocess output, try
+to continue from where the previous loop left off instead of always
+beginning from file descriptor zero.  Set this variable to t to get
+the old behaviour.
+
 +++
 ** New function 'sxhash-equal-including-properties'.
 This is identical to 'sxhash-equal' but accounting also for string

diff --git a/src/process.c b/src/process.c
index 47a2a6f1a314f200fb01fa05aaec2228ceba09dd..2f32eadd50d594bbf49dbdf293833e9e1be175a7 100644 (file)
--- a/src/process.c
+++ b/src/process.c
@@ -5134,6 +5134,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
                             Lisp_Object wait_for_cell,
                             struct Lisp_Process *wait_proc, int just_wait_proc)
 {
+  static int last_read_channel = -1;
   int channel, nfds;
   fd_set Available;
   fd_set Writeok;
@@ -5188,6 +5189,8 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
   while (1)
     {
       bool process_skipped = false;
+      bool wrapped;
+      int channel_start;
 
       /* If calling from keyboard input, do not quit
         since we want to return C-g as an input character.
@@ -5722,8 +5725,21 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
             d->func (channel, d->data);
        }
 
-      for (channel = 0; channel <= max_desc; channel++)
+      /* Do round robin if `process-pritoritize-lower-fds' is nil. */
+      channel_start
+       = process_prioritize_lower_fds ? 0 : last_read_channel + 1;
+
+      for (channel = channel_start, wrapped = false;
+          !wrapped || (channel < channel_start && channel <= max_desc);
+          channel++)
        {
+         if (channel > max_desc)
+           {
+             wrapped = true;
+             channel = -1;
+             continue;
+           }
+
          if (FD_ISSET (channel, &Available)
              && ((fd_callback_info[channel].flags & (KEYBOARD_FD | PROCESS_FD))
                  == PROCESS_FD))
@@ -5761,6 +5777,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
                     don't try to read from any other processes
                     before doing the select again.  */
                  FD_ZERO (&Available);
+                 last_read_channel = channel;
 
                  if (do_display)
                    redisplay_preserve_echo_area (12);
@@ -8477,6 +8494,13 @@ non-nil value means that the delay is not reset on write.
 The variable takes effect when `start-process' is called.  */);
   Vprocess_adaptive_read_buffering = Qt;
 
+  DEFVAR_BOOL ("process-prioritize-lower-fds", process_prioritize_lower_fds,
+              doc: /* If nil, try to not prioritize reading from any specific process.
+Emacs loops through file descriptors to receive data from subprocesses.  After
+accepting output from the first file descriptor with available data, restart the
+loop from the file descriptor 0 if this option is non-nil.  */);
+  process_prioritize_lower_fds = 0;
+
   DEFVAR_LISP ("interrupt-process-functions", Vinterrupt_process_functions,
               doc: /* List of functions to be called for `interrupt-process'.
 The arguments of the functions are the same as for `interrupt-process'.