blockdev.rs: --save-partindex will now check for MBR disk

If running `install` with the `----save-partindex` argument on a MBR drive
the listed partitions would be lost durring the install. To prevent the loss
of data the install proccess now checks to see if the drive is MBR, and if it
is it will bail fixes coreos#816

docs/release-notes.md

@@ -21,6 +21,7 @@ Minor changes:
 - Add release notes to documentation
 - iso: Detect incomplete ISO files
 - Warn if console kargs could have used `--console`/`--dest-console` instead
+- install: bail when `--save-partindex` is specified on a MBR disk
 
 Internal changes:
 

src/blockdev.rs

@@ -570,6 +570,10 @@ impl SavedPartitions {
         let gpt = match GPT::find_from(disk) {
             Ok(gpt) => gpt,
             Err(gptman::Error::InvalidSignature) => {
+                if disk_has_mbr(disk).context("checking if disk has an MBR")? {
+                    bail!("saving partitions from an MBR disk is not yet supported");
+                }
+
                 // no GPT on this disk, so no partitions to save
                 return Ok(Self {
                     sector_size,
@@ -1103,6 +1107,15 @@ pub fn get_gpt_size(file: &mut (impl Read + Seek)) -> Result<u64> {
     Ok(gpt.header.first_usable_lba * gpt.sector_size)
 }
 
+fn disk_has_mbr(file: &mut (impl Read + Seek)) -> Result<bool> {
+    let mut sig = [0u8; 2];
+    file.seek(SeekFrom::Start(510))
+        .context("seeking to MBR signature")?;
+    file.read_exact(&mut sig)
+        .context("reading MBR signature")?;
+    Ok(sig == [0x55, 0xaa])
+}
+
 pub fn udev_settle() -> Result<()> {
     // "udevadm settle" silently no-ops if the udev socket is missing, and
     // then lsblk can't find partition labels.  Catch this early.
@@ -1469,7 +1482,7 @@ mod tests {
             let saved = SavedPartitions::new_from_file(&mut base, 512, filter).unwrap();
             let mut disk = make_unformatted_disk();
             saved.overwrite(&mut disk).unwrap();
-            assert!(disk_has_mbr(&mut disk), "test {}", testnum);
+            assert!(disk_has_mbr(&mut disk).unwrap(), "test {}", testnum);
             let result = GPT::find_from(&mut disk).unwrap();
             assert_eq!(
                 get_gpt_size(&mut disk).unwrap(),
@@ -1481,7 +1494,7 @@ mod tests {
             let mut disk = make_disk(512, &merge_base_parts);
             saved.merge(&mut image, &mut disk).unwrap();
             assert!(
-                disk_has_mbr(&mut disk) == !expected_blank.is_empty(),
+                disk_has_mbr(&mut disk).unwrap() == !expected_blank.is_empty(),
                 "test {}",
                 testnum
             );
@@ -1515,7 +1528,7 @@ mod tests {
             let saved =
                 SavedPartitions::new_from_file(&mut disk, *sector_size as u64, &vec![]).unwrap();
             saved.overwrite(&mut disk).unwrap();
-            assert!(disk_has_mbr(&mut disk), "{}", *sector_size);
+            assert!(disk_has_mbr(&mut disk).unwrap(), "{}", *sector_size);
             disk.seek(SeekFrom::Start(0)).unwrap();
             let mut buf = vec![0u8; *sector_size + 1];
             disk.read_exact(&mut buf).unwrap();
@@ -1549,6 +1562,16 @@ mod tests {
             err
         );
 
+        // test trying to save partitions from a MBR disk
+        let mut disk = make_unformatted_disk();
+        gptman::GPT::write_protective_mbr_into(&mut disk, 512).unwrap();
+        assert_eq!(
+            SavedPartitions::new(&mut disk, 512, &vec![label("*i*")])
+                .unwrap_err()
+                .to_string(),
+            "saving partitions from an MBR disk is not yet supported"
+        );
+
         // test sector size mismatch
         let saved = SavedPartitions::new_from_file(&mut base, 512, &vec![label("*i*")]).unwrap();
         let mut image_4096 = make_disk(4096, &image_parts);
@@ -1679,13 +1702,6 @@ mod tests {
         guid
     }
 
-    fn disk_has_mbr(disk: &mut File) -> bool {
-        let mut sig = [0u8; 2];
-        disk.seek(SeekFrom::Start(510)).unwrap();
-        disk.read_exact(&mut sig).unwrap();
-        sig == [0x55, 0xaa]
-    }
-
     fn assert_partitions_eq(expected: &Vec<(u32, GPTPartitionEntry)>, found: &GPT, message: &str) {
         assert_eq!(
             expected