pkg/fileutil: helper function to get major, minor numbers of a device…

… file.

pkg/fileutil/fileutil_linux.go

@@ -17,6 +17,7 @@
 package fileutil
 
 import (
+	"fmt"
 	"os"
 	"syscall"
 	"unsafe"
@@ -105,3 +106,39 @@ func Lsetxattr(path string, attr string, data []byte, flags int) error {
 	}
 	return nil
 }
+
+// GetDeviceInfo returns the type, major, and minor numbers of a device.
+// Kind is 'b' or 'c' for block and character devices, respectively.
+// This does not follow symlinks.
+func GetDeviceInfo(path string) (kind rune, major uint64, minor uint64, err error) {
+	d, err := os.Lstat(path)
+	if err != nil {
+		return
+	}
+	mode := d.Mode()
+
+	if mode&os.ModeDevice == 0 {
+		err = fmt.Errorf("not a device: %s", path)
+		return
+	}
+	stat_t, ok := d.Sys().(*syscall.Stat_t)
+	if !ok {
+		err = fmt.Errorf("cannot determine device number")
+		return
+	}
+	return getDeviceInfo(mode, stat_t.Rdev)
+}
+
+// Parse the device info out of the mode bits. Separate for testability.
+func getDeviceInfo(mode os.FileMode, rdev uint64) (kind rune, major uint64, minor uint64, err error) {
+	kind = 'b'
+
+	if mode&os.ModeCharDevice != 0 {
+		kind = 'c'
+	}
+
+	major = (rdev >> 8) & 0xfff
+	minor = (rdev & 0xff) | ((rdev >> 12) & 0xfff00)
+
+	return
+}

pkg/fileutil/fileutil_test.go

@@ -183,3 +183,81 @@ func TestFileIsExecutable(t *testing.T) {
 		}
 	}
 }
+
+func TestDeviceInfo(t *testing.T) {
+	// First, test the main call
+	{
+		kind, major, minor, err := GetDeviceInfo("/dev/null")
+		if kind != 'c' || major != 1 || minor != 3 || err != nil {
+			t.Errorf("GetDeviceInfo(/dev/null) wrong result")
+		}
+	}
+
+	{
+		_, _, _, err := GetDeviceInfo("/usr")
+		if err == nil {
+			t.Errorf("GetDeviceInfo(/usr) should return err")
+		}
+	}
+
+	// Then test the logic more specifically
+	for i, tt := range []struct {
+		mode  os.FileMode
+		rdev  uint64
+		kind  rune
+		major uint64
+		minor uint64
+	}{
+		// /dev/null
+		{
+			69206454,
+			259,
+			'c',
+			1,
+			3,
+		},
+		// /dev/sda1
+		{
+			67109296,
+			2049,
+			'b',
+			8,
+			1,
+		},
+		// /dev/pts/3
+		{
+			69206416,
+			34819,
+			'c',
+			136,
+			3,
+		},
+		// /dev/dm-0
+		{
+			67109296,
+			64768,
+			'b',
+			253,
+			0,
+		},
+	} {
+		kind, major, minor, err := getDeviceInfo(tt.mode, tt.rdev)
+		if err != nil {
+			t.Errorf("getDeviceInfo %d not as expected, got err %s", i, err)
+		}
+		// don't care about result when err
+		if err != nil {
+			continue
+		}
+
+		if tt.kind != kind {
+			t.Errorf("getDeviceInfo %d kind expected %v got %v", i, tt.kind, kind)
+		}
+		if tt.major != major {
+			t.Errorf("getDeviceInfo %d major expected %v got %v", i, tt.major, major)
+		}
+		if tt.minor != minor {
+			t.Errorf("getDeviceInfo %d minor expected %v got %v", i, tt.minor, minor)
+		}
+	}
+}