efi: update the ESP by creating a tmpdir and RENAME_EXCHANGE

See Timothée's comment coreos#454 (comment)

Fixes coreos#454

src/efi.rs

@@ -348,12 +348,45 @@ 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)?;
+
+        /* copy "lowest" directory that we need to make the change
+         * e.g. we only affect EFI/fedora for example and not all of EFI
+         */
+        let vendor = if let Some(v) = self.get_efi_vendor(&sysroot)? {
+            v
+        } else {
+            bail!("Failed to get vendor dir");
+        };
+
+        let esp = self.esp_path()?;
+        let vendordir = esp.join(&vendor);
+        let tmp_vendordir = esp.join(format!(".{vendor}.tmp"));
+        // remove a previous directory if it exists in order to handle being interrupted in the middle.
+        if tmp_vendordir.exists() {
+            std::fs::remove_dir_all(&tmp_vendordir)?;
+        }
+        copy_dir(&vendordir, &tmp_vendordir)
+            .with_context(|| "copying existing files to temp dir")?;
+        assert!(tmp_vendordir.exists());
+
+        let tmpdir = sysroot.sub_dir(&vendordir)?;
+        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 = self.open_esp()?;
+            log::trace!(
+                "doing local exchange for {} and {}",
+                tmp_vendordir.display(),
+                vendordir.display()
+            );
+            esp.local_exchange(&tmp_vendordir, &vendordir)
+                .with_context(|| format!("exchange for {:?} and {:?}", tmp_vendordir, vendordir))?;
+            // finally remove the temp dir
+            std::fs::remove_dir_all(&tmp_vendordir)
+                .with_context(|| format!("clean up {:?}", tmp_vendordir))?;
+        }
         let adopted_from = None;
         Ok(InstalledContent {
             meta: updatemeta,
@@ -580,6 +613,18 @@ 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::*;