add legacy mode and some cleanup

BackstageInjection-Rust/build.rs

@@ -31,16 +31,27 @@ fn main() {
 }
 
 fn generate_loader_entry() {
-    let seed = SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .unwrap()
-        .as_nanos() as u64
-        ^ (std::process::id() as u64).wrapping_mul(0x9E37_79B9_7F4A_7C15);
-    let name = format!("x{:06x}", seed & 0xFFFF_FF);
+    let name = match std::env::var("BACKSTAGE_LOADER_EXPORT").ok().as_deref() {
+        Some("ReflectiveLoader") => "ReflectiveLoader".to_string(),
+        Some(value) => panic!("unsupported BACKSTAGE_LOADER_EXPORT value: {value}"),
+        None => {
+            let seed = std::env::var("BACKSTAGE_LOADER_SEED")
+                .ok()
+                .and_then(|value| value.parse::<u64>().ok())
+                .unwrap_or_else(|| {
+                    SystemTime::now()
+                        .duration_since(UNIX_EPOCH)
+                        .unwrap()
+                        .as_nanos() as u64
+                        ^ (std::process::id() as u64).wrapping_mul(0x9E37_79B9_7F4A_7C15)
+                });
+            format!("x{:06x}", seed & 0xFFFF_FF)
+        }
+    };
 
     let src = format!(
         "// Generated by build.rs — do not edit.\n\
-         // Randomized per-build reflective loader export name.\n\
+         // Version-selected reflective loader export name.\n\
          pub(crate) const LOADER_EXPORT: &[u8] = b\"{name}\\0\";\n\
          \n\
          #[unsafe(no_mangle)]\n\
@@ -52,5 +63,6 @@ fn generate_loader_entry() {
     let out_dir = std::env::var("OUT_DIR").unwrap();
     std::fs::write(std::path::Path::new(&out_dir).join("loader_entry.rs"), src)
         .expect("failed to write loader_entry.rs");
-    println!("cargo:rerun-if-changed=loader_entry.rs");
-}
+    println!("cargo:rerun-if-env-changed=BACKSTAGE_LOADER_EXPORT");
+    println!("cargo:rerun-if-env-changed=BACKSTAGE_LOADER_SEED");
+}