Create proper mounter for OpenBSD

OpenBSD's mount(2) works differently since it uses separate structs to
present options for different filesystems, unlike in FreeBSD. It also
means that support for other filesystems need to be added explicitly.

I'm not sure which ones are required for now so I just write support
for only FFS, which is the go-to filesystem used by OpenBSD systems.

Signed-off-by: Muhammad Kaisar Arkhan <hi@yukiisbo.red>

mount/mounter_bsd.go → mount/mounter_freebsd.go

@@ -1,4 +1,4 @@
-// +build freebsd,cgo openbsd,cgo
+// +build freebsd,cgo
 
 package mount
 

mount/mounter_openbsd.go

@@ -0,0 +1,77 @@
+// +build openbsd,cgo
+
+/*
+   Due to how OpenBSD mount(2) works, filesystem types need to be
+   supported explicitly since it uses separate structs to pass
+   filesystem-specific arguments.
+
+   For now only UFS/FFS is supported as it's the default fs
+   on OpenBSD systems.
+
+   See: https://man.openbsd.org/mount.2
+*/
+
+package mount
+
+/*
+#include <sys/types.h>
+#include <sys/mount.h>
+*/
+import "C"
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+func createExportInfo(readOnly bool) C.struct_export_args {
+	exportFlags := C.int(0)
+	if readOnly {
+		exportFlags = C.MNT_EXRDONLY
+	}
+	out := C.struct_export_args{
+		ex_root:  0,
+		ex_flags: exportFlags,
+	}
+	return out
+}
+
+func createUfsArgs(device string, readOnly bool) unsafe.Pointer {
+	out := &C.struct_ufs_args{
+		fspec:       C.CString(device),
+		export_info: createExportInfo(readOnly),
+	}
+	return unsafe.Pointer(out)
+}
+
+func mount(device, target, mType string, flag uintptr, data string) error {
+	readOnly := flag&RDONLY != 0
+
+	var fsArgs unsafe.Pointer
+
+	switch mType {
+	case "ffs":
+		fsArgs = createUfsArgs(device, readOnly)
+	default:
+		return &mountError{
+			op:     "mount",
+			source: device,
+			target: target,
+			flags:  flag,
+			err:    fmt.Errorf("unsupported file system type: %s", mType),
+		}
+	}
+
+	if errno := C.mount(C.CString(mType), C.CString(target), C.int(flag), fsArgs); errno != 0 {
+		return &mountError{
+			op:     "mount",
+			source: device,
+			target: target,
+			flags:  flag,
+			err:    syscall.Errno(errno),
+		}
+	}
+
+	return nil
+}