filesystem: add device number utilities

Add a utility type and functions for handling device numbers.

2 files changed, 80 insertions, 0 deletions

diff --git a/filesystem/path.go b/filesystem/path.go
index cfc3dc0..a99b743 100644
--- a/filesystem/path.go
+++ b/filesystem/path.go
@@ -20,6 +20,7 @@
 package filesystem
 
 import (
+	"fmt"
 	"log"
 	"os"
 	"path/filepath"
@@ -99,3 +100,28 @@ func isRegularFile(path string) bool {
 	info, err := loggedStat(path)
 	return err == nil && info.Mode().IsRegular()
 }
+
+// DeviceNumber represents a combined major:minor device number.
+type DeviceNumber uint64
+
+func (num DeviceNumber) String() string {
+	return fmt.Sprintf("%d:%d", unix.Major(uint64(num)), unix.Minor(uint64(num)))
+}
+
+func newDeviceNumberFromString(str string) (DeviceNumber, error) {
+	var major, minor uint32
+	if count, _ := fmt.Sscanf(str, "%d:%d", &major, &minor); count != 2 {
+		return 0, errors.Errorf("invalid device number string %q", str)
+	}
+	return DeviceNumber(unix.Mkdev(major, minor)), nil
+}
+
+// getDeviceNumber returns the device number of the device node at the given
+// path.  If there is a symlink at the path, it is dereferenced.
+func getDeviceNumber(path string) (DeviceNumber, error) {
+	var stat unix.Stat_t
+	if err := unix.Stat(path, &stat); err != nil {
+		return 0, err
+	}
+	return DeviceNumber(stat.Rdev), nil
+}
diff --git a/filesystem/path_test.go b/filesystem/path_test.go
new file mode 100644
index 0000000..eef5ce3
--- /dev/null
+++ b/filesystem/path_test.go
@@ -0,0 +1,54 @@
+/*
+ * path_test.go - Tests for path utilities.
+ *
+ * Copyright 2019 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package filesystem
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestDeviceNumber(t *testing.T) {
+	num, err := getDeviceNumber("/NONEXISTENT")
+	if num != 0 || err == nil {
+		t.Error("Should have failed to get device number of nonexistent file")
+	}
+	// /dev/null is always device 1:3 on Linux.
+	num, err = getDeviceNumber("/dev/null")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if str := num.String(); str != "1:3" {
+		t.Errorf("Wrong device number string: %q", str)
+	}
+	if str := fmt.Sprintf("%v", num); str != "1:3" {
+		t.Errorf("Wrong device number string: %q", str)
+	}
+	var num2 DeviceNumber
+	num2, err = newDeviceNumberFromString("1:3")
+	if err != nil {
+		t.Error("Failed to parse device number")
+	}
+	if num2 != num {
+		t.Errorf("Wrong device number: %d", num2)
+	}
+	num2, err = newDeviceNumberFromString("foo")
+	if num2 != 0 || err == nil {
+		t.Error("Should have failed to parse invalid device number")
+	}
+}