builder: Add a set_rdev() convenience

src/builder.rs

@@ -535,11 +535,7 @@ fn append_special(
     prepare_header_path(dst, &mut header, path)?;
 
     header.set_entry_type(entry_type);
-    let dev_id = stat.rdev();
-    let dev_major = ((dev_id >> 32) & 0xffff_f000) | ((dev_id >> 8) & 0x0000_0fff);
-    let dev_minor = ((dev_id >> 12) & 0xffff_ff00) | ((dev_id) & 0x0000_00ff);
-    header.set_device_major(dev_major as u32)?;
-    header.set_device_minor(dev_minor as u32)?;
+    header.set_rdev(stat.rdev())?;
 
     header.set_cksum();
     dst.write_all(header.as_bytes())?;

src/header.rs

@@ -645,6 +645,17 @@ impl Header {
         }
     }
 
+    /// Encodes the device number into the major and minor fields of this header.
+    ///
+    /// This function will return an error if this header format cannot encode a
+    /// a device number.
+    pub fn set_rdev(&mut self, rdev: libc::dev_t) -> io::Result<()> {
+        let dev_major = ((rdev >> 32) & 0xffff_f000) | ((rdev >> 8) & 0x0000_0fff);
+        self.set_device_major(dev_major as u32)?;
+        let dev_minor = ((rdev >> 12) & 0xffff_ff00) | ((rdev) & 0x0000_00ff);
+        self.set_device_minor(dev_minor as u32)
+    }
+
     /// Returns the device minor number, if present.
     ///
     /// This field may not be present in all archives, and it may not be

tests/header/mod.rs

@@ -100,6 +100,18 @@ fn dev_major_minor() {
     assert_eq!(t!(h.device_major()), Some(1));
     assert_eq!(t!(h.device_minor()), Some(2));
 
+    // Verify this is idempotent
+    let onetwo_rdev = libc::makedev(1, 2);
+    t!(h.set_rdev(onetwo_rdev));
+    assert_eq!(t!(h.device_major()), Some(1));
+    assert_eq!(t!(h.device_minor()), Some(2));
+
+    // And verify that changing the device works
+    let nvme_rdev = libc::makedev(0x88, 0x6);
+    t!(h.set_rdev(nvme_rdev));
+    assert_eq!(t!(h.device_major()), Some(0x88));
+    assert_eq!(t!(h.device_minor()), Some(0x6));
+
     h = Header::new_ustar();
     t!(h.set_device_major(1));
     t!(h.set_device_minor(2));