blockdev: add PARENT_DEVICES

As `PARENT_DEVICES` is used multiple times, initialize once at the
begining and read them then.

Author: HuijingHei

src/blockdev.rs

@@ -1,10 +1,23 @@
 use camino::Utf8Path;
 use std::path::Path;
+use std::sync::OnceLock;
 
 use anyhow::{bail, Context, Result};
 use bootc_blockdev::PartitionTable;
 use fn_error_context::context;
 
+static PARENT_DEVICES: OnceLock<Vec<String>> = OnceLock::new();
+
+// Initialize once
+pub fn init_parent_devices(strings: Vec<String>) {
+    PARENT_DEVICES.set(strings).expect("Already initialized");
+}
+
+// Read many times (lock-free)
+pub fn get_parent_devices() -> &'static [String] {
+    PARENT_DEVICES.get().expect("Not initialized")
+}
+
 #[context("get parent devices from mount point boot")]
 pub fn get_devices<P: AsRef<Path>>(target_root: P) -> Result<Vec<String>> {
     let target_root = target_root.as_ref();