build: Never use poll() on Darwin family (macOS, etc.) or Interix

Doing a runtime check in configure.ac (AC_RUN_IFELSE) has several
disadvantages:

* It doesn't work when cross-compiling. For example, if we build macOS
  binaries on a Linux system, we'd assume that poll() works, but in
  fact it won't.

* It checks the build system capabilities, but that is not necessarily
  appropriate if (for example) a macOS 10.10 user builds binaries that
  could be used by macOS 10.12 or macOS 10.9 users.

* It checks for one specific failure mode, but macOS seems to have a
  history of various implementation issues in poll().

* If we want it to work in CMake, we have to duplicate it in the CMake
  build system.

None of these is a showstopper on its own, but the combination of all
of them makes the current approach to avoiding the broken poll() on
macOS look unreliable. libcurl, a widely-portable library making
extensive use of sockets, specifically doesn't use poll() on Darwin
(macOS, iOS, etc.) or on Interix; let's follow their example here.

See also https://bugzilla.gnome.org/show_bug.cgi?id=302672 and
https://daniel.haxx.se/blog/2016/10/11/poll-on-mac-10-12-is-broken/
for some relevant history.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: https://gitlab.freedesktop.org/dbus/dbus/issues/232

1 files changed, 26 insertions, 0 deletions

diff --git a/dbus/dbus-sysdeps.h b/dbus/dbus-sysdeps.h
index 8642dda5..a5987ae5 100644
--- a/dbus/dbus-sysdeps.h
+++ b/dbus/dbus-sysdeps.h
@@ -48,6 +48,32 @@
 #include <string.h>
 #include <stdarg.h>
 
+#if !defined(BROKEN_POLL) && (defined(__APPLE__) || defined(__INTERIX))
+  /* Following libcurl's example, we blacklist poll() on Darwin
+   * (macOS, iOS, etc.) and Interix due to a history of implementation
+   * issues.
+   * https://github.com/curl/curl/blob/master/m4/curl-functions.m4
+   *
+   * On unspecified older macOS versions, poll() failed if given a
+   * device node to poll.
+   *
+   * On macOS < 10.9, poll() with nfds=0 failed instead of waiting for
+   * the timeout and then succeeding.
+   *
+   * On macOS >= 10.12, poll() with nfds=0 succeeded immediately
+   * instead of waiting for the timeout, resulting in busy-looping.
+   *
+   * On Interix, poll() apparently only works for files in /proc.
+   *
+   * The "legacy" build flavour in our CI machinery defines BROKEN_POLL
+   * on whatever platform is in use (normally Linux) to force use of the
+   * same select()-based poll() emulation that we use for macOS, Interix,
+   * and any platform that lacks a real poll(), so that we can test it
+   * more regularly.
+   */
+# define BROKEN_POLL
+#endif
+
 /* AIX sys/poll.h does #define events reqevents, and other
  * wonderousness, so must include sys/poll before declaring
  * DBusPollFD