[PATCH] use non-blocking stdin for interactive mode

[Eric Wong <normalperson@yhbt.net>]

Tarim <sox@mediaplaygrounds.co.uk> wrote:
>   I'm now thinking that this is actually a bug in the latest Ubuntu kernel
> 3.13.0-65.105-generic where select's FD_ISSET returns true even when
> there is no data to read!

This is a kernel bug from a performance standpoint, but the select
manpage explicitly documents spurious wakeups as a possibility which
userspace must be prepared for.

The following patch should fix the problem:

------------------8<------------------
Subject: [PATCH] use non-blocking stdin for interactive mode

When accepting keyboard input, it is possible for select() to
return a false-positive with spurious wakeups from inside the
update_status callback.

Using the FIONREAD ioctl in place of select is also a possibility,
but may be less portable.
---
  Downloadable patch:
  http://80x24.org/spew/1443909787-27821-1-git-send-email-e%4080x24.org/raw

  In case upstream prefers a pull request:

  The following changes since commit 7e74b254b2a7c963be0bfce751fc5911fe681c12:

    Remove hepler script. It's mostly unmaintained, I don't know if anyone but me ever used it. In any case, those who want a custom Debian package should be capable of updating the debian/changelog entry on their own. (2015-02-26 22:48:40 -0500)

  are available in the git repository at:

    git://bogomips.org/sox nb-stdin

  for you to fetch changes up to 62a370a2a01cf1dcc0d5f9b9f49d1515706346cd:

    use non-blocking stdin for interactive mode (2015-10-03 22:04:50 +0000)

  ----------------------------------------------------------------
  Eric Wong (1):
        use non-blocking stdin for interactive mode

 src/sox.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/sox.c b/src/sox.c
index bab0f45..fdb7616 100644
--- a/src/sox.c
+++ b/src/sox.c
@@ -1789,6 +1789,18 @@ static int process(void)
     tcsetattr(fileno(stdin), TCSANOW, &modified_termios);
   }
 #endif
+#if defined(F_GETFL) && defined(F_SETFL) && defined(O_NONBLOCK)
+  if (interactive) {
+    int fd = fileno(stdin);
+    int flags = fcntl(fd, F_GETFL);
+    if (flags == -1) {
+      lsx_warn("error getting flags on stdin descriptor: %s", strerror(errno));
+    } else if (!(flags & O_NONBLOCK)) {
+      if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1)
+        lsx_warn("error setting non-blocking on stdin: %s", strerror(errno));
+    }
+  }
+#endif
 
   signal(SIGTERM, sigint); /* Stop gracefully, as soon as we possibly can. */
   signal(SIGINT , sigint); /* Either skip current input or behave as SIGTERM. */
-- 
EW

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