runtime: os.Executable runtime support for Darwin

Change-Id: Ie21df37016c90cd0479c23ec4845f8195dd90fda
Reviewed-on: https://go-review.googlesource.com/16518Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_darwin.go

@@ -551,3 +551,21 @@ func sigaddset(mask *sigset, i int) {
 func sigdelset(mask *sigset, i int) {
 	*mask &^= 1 << (uint32(i) - 1)
 }
+
+//go:linkname executablePath os.executablePath
+var executablePath string
+
+func sysargs(argc int32, argv **byte) {
+	// skip over argv, envv and the first string will be the path
+	n := argc + 1
+	for argv_index(argv, n) != nil {
+		n++
+	}
+	executablePath = gostringnocopy(argv_index(argv, n+1))
+
+	// strip "executable_path=" prefix if available, it's added after OS X 10.11.
+	const prefix = "executable_path="
+	if len(executablePath) > len(prefix) && executablePath[:len(prefix)] == prefix {
+		executablePath = executablePath[len(prefix):]
+	}
+}

src/runtime/vdso_none.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build !linux
+// +build !darwin
 
 package runtime