efi: update the ESP by creating a tmpdir and RENAME_EXCHANGE

See Timothée's comment #454 (comment)
Also make some tunes for the logic:
- `cp -a EFI .EFI.tmp`
- We start with a copy to make sure to keep all other files that we do not explicitly track in bootupd
- Update the content of `.EFI.tmp` with the new binaries
- Exchange `.EFI.tmp -> EFI`
- Remove now "old" .EFI.tmp

Fixes #454

src/efi.rs

@@ -26,6 +26,9 @@ use crate::{component::*, packagesystem};
 /// Well-known paths to the ESP that may have been mounted external to us.
 pub(crate) const ESP_MOUNTS: &[&str] = &["boot/efi", "efi", "boot"];
 
+/// Temp EFI dir
+const TEMP_EFI_DIR: &str = ".EFI.tmp";
+
 /// The binary to change EFI boot ordering
 const EFIBOOTMGR: &str = "efibootmgr";
 #[cfg(target_arch = "aarch64")]
@@ -349,12 +352,35 @@ impl Component for Efi {
             .context("opening update dir")?;
         let updatef = filetree::FileTree::new_from_dir(&updated).context("reading update dir")?;
         let diff = currentf.diff(&updatef)?;
-        self.ensure_mounted_esp(Path::new("/"))?;
-        let destdir = self.open_esp().context("opening EFI dir")?;
-        validate_esp(&destdir)?;
+        let mountdir = self.ensure_mounted_esp(Path::new("/"))?;
+
+        let efipath = self.esp_path()?;
+        let tmp_efipath = mountdir.join(TEMP_EFI_DIR);
+        // remove a previous directory if it exists in order to handle being interrupted in the middle.
+        if tmp_efipath.exists() {
+            std::fs::remove_dir_all(&tmp_efipath)?;
+        }
+        copy_dir(&efipath, &tmp_efipath).with_context(|| "copying existing files to temp dir")?;
+
+        let tmpdir = sysroot.sub_dir(&tmp_efipath)?;
+        validate_esp(&tmpdir)?;
         log::trace!("applying diff: {}", &diff);
-        filetree::apply_diff(&updated, &destdir, &diff, None)
-            .context("applying filesystem changes")?;
+        filetree::apply_diff(&updated, &tmpdir, &diff, None)
+            .context("applying filesystem changes to temp EFI")?;
+        {
+            let esp = sysroot.sub_dir(&mountdir)?;
+            log::trace!(
+                "doing local exchange for {} and {}",
+                tmp_efipath.display(),
+                efipath.display()
+            );
+
+            esp.local_exchange(&tmp_efipath, &efipath)
+                .with_context(|| format!("exchange for {:?} and {:?}", tmp_efipath, efipath))?;
+            // finally remove the temp dir
+            log::trace!("cleanup: {}", tmp_efipath.display());
+            std::fs::remove_dir_all(&tmp_efipath).context("clean up temp")?;
+        }
         let adopted_from = None;
         Ok(InstalledContent {
             meta: updatemeta,
@@ -584,6 +610,18 @@ fn find_file_recursive<P: AsRef<Path>>(dir: P, target_file: &str) -> Result<Vec<
     Ok(result)
 }
 
+fn copy_dir(src: &Path, dst: &Path) -> Result<()> {
+    let r = std::process::Command::new("cp")
+        .args(["-a"])
+        .arg(src)
+        .arg(dst)
+        .status()?;
+    if !r.success() {
+        anyhow::bail!("Failed to copy");
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;