Hi all,

I think there is consensus for:
* Using a sysctl (simpler than a device node),
* Setting this sysctl on all architectures,
* Calling the sysctl kern.suspend_blocked,
* Consulting the sysctl from the ACPI code (for now) and possibly from
other platform-specific forms of sleeping (in the indefinite future).

Points without consensus:
* jkim thinks we should prevent suspend when we're dropping to single-user
mode; I'm not sure I see the point, but I don't think there's any harm in
doing that too.
* Ian Smith would like to have suspend blocked for the last 5 minutes before
shutdown(8) signals init to shut the system down.  I don't think anyone else
has expressed a desire for this, and some people have raised concerns about
blocking suspend for too long in case a system is running out of battery; so
I'm inclined to leave this out at this point.  (It would be easy enough to
add the sysctl-frobbing to shutdown(8) if desired later.)

With the above in mind, I've written (but not yet actually compiled or tested,
so beware of typos) a patch which I think makes sense.  If nobody is violently
opposed to this I'll make sure I got the details right and then commit this in
a few days.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
Index: sbin/init/init.c
===================================================================
--- sbin/init/init.c	(revision 288249)
+++ sbin/init/init.c	(working copy)
@@ -1481,6 +1481,16 @@
 }
 
 /*
+ * Block (if block == 1) or unblock (if block == 0) suspend.
+ */
+static void
+block_suspend(int block)
+{
+
+	sysctlbyname("kern.suspend_blocked", NULL, NULL, &block, sizeof(block));
+}
+
+/*
  * Bring the system down to single user.
  */
 static state_func_t
@@ -1487,7 +1497,16 @@
 death(void)
 {
 	session_t *sp;
+	int block, blocked;
+	size_t len;
 
+	/* Temporarily block suspend. */
+	len = sizeof(blocked);
+	block = 1;
+	if (sysctlbyname("kern.suspend_blocked", &blocked, &len,
+	    &block, sizeof(block)))
+		blocked = 0;
+
 	/*
 	 * Also revoke the TTY here.  Because runshutdown() may reopen
 	 * the TTY whose getty we're killing here, there is no guarantee
@@ -1503,6 +1522,11 @@
 	/* Try to run the rc.shutdown script within a period of time */
 	runshutdown();
 
+	/* Unblock suspend if we blocked it. */
+	if (!blocked)
+		sysctlbyname(kern.suspend_blocked", NULL, NULL,
+		    &blocked, sizeof(blocked));
+
 	return (state_func_t) death_single;
 }
 
Index: sys/dev/acpica/acpi.c
===================================================================
--- sys/dev/acpica/acpi.c	(revision 288249)
+++ sys/dev/acpica/acpi.c	(working copy)
@@ -2574,8 +2574,11 @@
     if (!acpi_sleep_states[state])
 	return (EOPNOTSUPP);
 
-    /* If a suspend request is already in progress, just return. */
-    if (sc->acpi_next_sstate != 0) {
+    /*
+     * If a reboot/shutdown/suspend request is already in progress or
+     * suspend is blocked due to an upcoming shutdown, just return.
+     */
+    if (rebooting || sc->acpi_next_sstate != 0 || suspend_blocked) {
 	return (0);
     }
 
Index: sys/kern/kern_shutdown.c
===================================================================
--- sys/kern/kern_shutdown.c	(revision 288249)
+++ sys/kern/kern_shutdown.c	(working copy)
@@ -137,6 +137,10 @@
 SYSCTL_INT(_kern_shutdown, OID_AUTO, show_busybufs, CTLFLAG_RW,
 	&show_busybufs, 0, "");
 
+int suspend_blocked = 0;
+SYSCTL_INT(_kern, OID_AUTO, suspend_blocked, CTLFLAG_RW,
+	&suspend_blocked, 0, "Block suspend due to a pending shutdown");
+
 /*
  * Variable panicstr contains argument to first call to panic; used as flag
  * to indicate that the kernel has already called panic.
Index: sys/sys/systm.h
===================================================================
--- sys/sys/systm.h	(revision 288249)
+++ sys/sys/systm.h	(working copy)
@@ -46,6 +46,7 @@
 #include <sys/stdint.h>		/* for people using printf mainly */
 
 extern int cold;		/* nonzero if we are doing a cold boot */
+extern int suspend_blocked;	/* block suspend due to pending shutdown */
 extern int rebooting;		/* kern_reboot() has been called. */
 extern const char *panicstr;	/* panic message */
 extern char version[];		/* system version */
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