Make compaction transitions recoverable

src/engine/metal.rs

@@ -14,7 +14,7 @@ use std::ptr::NonNull;
 use std::time::UNIX_EPOCH;
 
 const CHECKPOINT_MAGIC: &[u8; 8] = b"DS4RKV01";
-const CHECKPOINT_VERSION: u32 = 1;
+const CHECKPOINT_VERSION: u32 = 2;
 const CHECKPOINT_IO_CHUNK: usize = 8 * 1024 * 1024;
 const DEFAULT_PREFILL_CHUNK: u32 = 4096;
 
@@ -571,6 +571,7 @@ pub(super) struct Executor {
     quality: bool,
     checkpoint_tag: [u8; 32],
     model_modified: (u64, u32),
+    model_identity: [u8; 32],
     _context: Context,
     model: Model,
 }
@@ -599,6 +600,7 @@ impl Executor {
             .and_then(|modified| modified.duration_since(UNIX_EPOCH).ok())
             .map(|duration| (duration.as_secs(), duration.subsec_nanos()))
             .unwrap_or_default();
+        let model_identity = model.checkpoint_identity();
         Ok(Self {
             weights,
             session,
@@ -607,6 +609,7 @@ impl Executor {
             quality,
             checkpoint_tag: [0; 32],
             model_modified,
+            model_identity,
             _context: context_handle,
             model,
         })