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Fix fork on osx

    fork() has a funky ABI, and you need to check %edx to know
    if you're the parent or child, and clear the PID in userspace.

--- a/libstd/sys-osx.myr

+++ b/libstd/sys-osx.myr

@@ -531,14 +531,16 @@

 	const sysctl	: (mib : int[:], old : byte[:]#, new : byte[:] -> int)

 ;;

+extern const __osx_fork : (->int64)

+

 /* process control */

 const exit	= {status;		syscall(Sysexit, status castto(int64))}

 const getpid	= {;			-> syscall(Sysgetpid, 1)}

 const kill	= {pid, sig;		-> syscall(Syskill, pid, sig)}

-const fork	= {;			-> syscall(Sysfork)}

+const fork	= {;			-> __osx_fork()}

 const wait4	= {pid, loc, opt, usage;	-> syscall(Syswait4, pid, loc, opt, usage)}

 const waitpid	= {pid, loc, opt;

-	-> wait4(pid, loc, opt, 0 castto(byte#)) 

+	-> wait4(pid, loc, opt, 0 castto(rusage#)) 

 }

 const execv	= {cmd, args

--- a/libstd/syscall-osx.s

+++ b/libstd/syscall-osx.s

@@ -23,3 +23,30 @@

 success:

 	popq %rbp

 	ret

+

+/*

+ * OSX is dumb about fork, and needs an assembly wrapper.

+ * The fork() syscall, when called directly, returns the pid in both

+ * processes, which means that both parent and child think they're

+ * the parent.

+ *

+ * checking this involves peeking in %edx, so we need to do this in asm.

+ */

+.globl _std$__osx_fork

+_std$__osx_fork:

+	pushq %rbp

+

+	movq $0x2000002,%rax

+	syscall

+

+	jae forksuccess

+	negq %rax

+

+forksuccess:

+	testl %edx,%edx

+	jz isparent

+	xorq %rax,%rax

+isparent:

+	popq %rbp

+	ret

+