os: do not use procfs for os.Executable in dragonfly

  procfs(5) is not always mounted in DragonFly BSD, for example during
  the binary package build with synth. os.Executable() consumers
  will then fail, we've spotted this when trying to build tinygo:

    [...]

    copying source files
    ./build/tinygo build-builtins -target=armv6m-none-eabi [...]
    panic: could not get executable path: readlink /proc/curproc/file:
    no such file or directory

    [...]

  Use KERN_PROC_PATHNAME as FreeBSD does.

Change-Id: Ic65bea02cd0309fb24dec8ba8d2b151d1acde67b
GitHub-Last-Rev: 083120a43b3158bb45d7e1a66fa32e3335a2d407
GitHub-Pull-Request: golang/go#36826
Reviewed-on: https://go-review.googlesource.com/c/go/+/216622
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/src/os/executable_sysctl.go b/src/os/executable_sysctl.go
new file mode 100644
index 0000000000..f9a4b18f60
--- /dev/null
+++ b/src/os/executable_sysctl.go
@@ -0,0 +1,35 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build freebsd dragonfly
+
+package os
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func executable() (string, error) {
+	mib := [4]int32{_CTL_KERN, _KERN_PROC, _KERN_PROC_PATHNAME, -1}
+
+	n := uintptr(0)
+	// get length
+	_, _, err := syscall.Syscall6(syscall.SYS___SYSCTL, uintptr(unsafe.Pointer(&mib[0])), 4, 0, uintptr(unsafe.Pointer(&n)), 0, 0)
+	if err != 0 {
+		return "", err
+	}
+	if n == 0 { // shouldn't happen
+		return "", nil
+	}
+	buf := make([]byte, n)
+	_, _, err = syscall.Syscall6(syscall.SYS___SYSCTL, uintptr(unsafe.Pointer(&mib[0])), 4, uintptr(unsafe.Pointer(&buf[0])), uintptr(unsafe.Pointer(&n)), 0, 0)
+	if err != 0 {
+		return "", err
+	}
+	if n == 0 { // shouldn't happen
+		return "", nil
+	}
+	return string(buf[:n-1]), nil
+}