efi: update the ESP by creating a tmpdir and RENAME_EXCHANGE

See Timothée's comment coreos#454 (comment)

Fixes coreos#454

Author: HuijingHei

src/efi.rs

@@ -72,6 +72,17 @@ impl Efi {
         Ok(esp)
     }
 
+    fn esp_path_tmp(&self) -> Result<PathBuf> {
+        self.ensure_mounted_esp(Path::new("/"))
+            .map(|v| v.join(".EFI.tmp"))
+    }
+
+    fn open_esp_tmp_optional(&self) -> Result<Option<openat::Dir>> {
+        let sysroot = openat::Dir::open("/")?;
+        let esp = sysroot.sub_dir_optional(&self.esp_path_tmp()?)?;
+        Ok(esp)
+    }
+
     pub(crate) fn ensure_mounted_esp(&self, root: &Path) -> Result<PathBuf> {
         let mut mountpoint = self.mountpoint.borrow_mut();
         if let Some(mountpoint) = mountpoint.as_deref() {
@@ -348,12 +359,40 @@ 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("/"))?;
+
+        // copy esp dir to temp to do apply diff
+        let esp = &self.esp_path()?;
+        let tmpesp = &self.esp_path_tmp()?;
+        copy_dir_all(esp, tmpesp).context("copying esp dir to temp dir")?;
+        assert!(tmpesp.exists());
+
+        let tmpdir = if let Some(p) = self.open_esp_tmp_optional()? {
+            p
+        } else {
+            bail!("Failed to open temp efi dir");
+        };
+        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")?;
+        {
+            // do local exchange of the temp dir and esp dir
+            let parentdir = if let Some(p) = sysroot.sub_dir_optional(&mountdir)? {
+                p
+            } else {
+                bail!("Failed to get parent dir");
+            };
+            parentdir.local_exchange(tmpesp, esp).with_context(|| {
+                format!(
+                    "local exchange for {} and {}",
+                    tmpesp.display(),
+                    esp.display()
+                )
+            })?;
+            // finally remove the temp dir
+            std::fs::remove_dir_all(tmpesp)?;
+        }
         let adopted_from = None;
         Ok(InstalledContent {
             meta: updatemeta,
@@ -580,6 +619,28 @@ fn find_file_recursive<P: AsRef<Path>>(dir: P, target_file: &str) -> Result<Vec<
     Ok(result)
 }
 
+fn copy_dir_all(src: &Path, dst: &Path) -> Result<()> {
+    // Create the destination directory if it doesn't exist
+    if !dst.exists() {
+        std::fs::create_dir_all(dst)?;
+    }
+
+    // Iterate over directory entries
+    for entry in std::fs::read_dir(src)? {
+        let entry = entry?;
+        let path = entry.path();
+        let relative_path = path.strip_prefix(src)?;
+        let destination = dst.join(relative_path);
+
+        if path.is_dir() {
+            copy_dir_all(&path, &destination)?;
+        } else {
+            std::fs::copy(&path, &destination)?;
+        }
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -655,4 +716,25 @@ Boot0003* test";
         );
         Ok(())
     }
+
+    #[test]
+    fn test_copy_dir_all() -> Result<()> {
+        env_logger::init();
+        let td = tempfile::tempdir()?;
+        let tdp = td.path();
+        let src = tdp.join("efi");
+        std::fs::create_dir_all(&src)?;
+        std::fs::create_dir_all(tdp.join("efi/BOOT"))?;
+        std::fs::create_dir_all(tdp.join("efi/fedora"))?;
+        std::fs::write(tdp.join("efi/BOOT").join("fbx64.efi"), "fall back data")?;
+        std::fs::write(tdp.join("efi/fedora").join(crate::efi::SHIM), "shim data")?;
+        std::fs::write(tdp.join("efi/fedora").join("grub.cfg"), "grub config data")?;
+        let dest = tdp.join("tmpefi");
+        copy_dir_all(src.as_path(), dest.as_path())?;
+        assert_eq!(dest.join("BOOT/fbx64.efi").exists(), true);
+        assert_eq!(dest.join("fedora").join(crate::efi::SHIM).exists(), true);
+        assert_eq!(dest.join("fedora/grub.cfg").exists(), true);
+
+        Ok(())
+    }
 }