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Add a special exit stack to our thread library.

	Because OpenBSD wants a valid stack pointer in any code
	that enters the kernel, unmapping our stack as part of
	exiting the process is rather unreliable.

	This change allocates a stack page that we can switch to
	when a thread is deallocating itself, which keeps the
	kernel happy.

--- a/6/insns.def

+++ b/6/insns.def

@@ -287,7 +287,7 @@

 /* branch instructions */

 Insn(Icall,

-    "\tcall %v\n",

+    "\tcall %v@PLT\n",

     "\tCALL %V\n",

     Use(.l={1}, .r={

             Rrdi, Rrsi, Rrdx, Rrcx, Rr8, Rr9,

--- a/lib/sys/sys+openbsd:6.1-x64.myr

+++ b/lib/sys/sys+openbsd:6.1-x64.myr

@@ -272,6 +272,7 @@

 	const Mfixed	: mopt = 0x10

 	const Mfile	: mopt = 0x0

 	const Manon	: mopt = 0x1000

+	const Mstack	: mopt = 0x4000

 	const Mnoreplace	: mopt = 0x0800

 	/* file types */

--- a/lib/thread/exit+openbsd-x64.s

+++ b/lib/thread/exit+openbsd-x64.s

@@ -10,6 +10,13 @@

 	andq	$~0xfff,%rdi	/* align it */

 	addq	$0x1000,%rdi

+	/* 

+	  Because OpenBSD wants a valid stack whenever

+	  we enter the kernel, we need to toss a preallocated

+	  stack pointer into %rsp.

+	 */

+	movq	thread$exitstk,%rsp

+

 	/* munmap(base, size) */

 	movq	$73,%rax	/* munmap */

 	movq	-8(%rdi),%rsi	/* size */

--- a/lib/thread/spawn+openbsd.myr

+++ b/lib/thread/spawn+openbsd.myr

@@ -5,10 +5,16 @@

 	type tid = uint64

 	const spawn : (fn : (-> void) -> std.result(tid, byte[:]))

+	pkglocal var exitstk : byte#

 ;;

 const Stacksz = 8*std.MiB

 extern const exit : (-> void)

+

+var exitstk

+const __init__ = {

+	exitstk = getstk(16)

+}

 const spawn = {fn;

 	-> spawnstk(fn, Stacksz)

--- a/lib/thread/test/spawn.myr

+++ b/lib/thread/test/spawn.myr

@@ -21,5 +21,6 @@

 	;;

 	std.assert(capture == 333, "capture wasn't written to correctly\n")

+	std.usleep(100_000)

 }