Drop cache_loaded_ flag; gate init on metadata.empty() (pytorch#20245)

Differential Revision: D108178967

Pull Request resolved: pytorch#20245

Author: doggeral

backends/xnnpack/runtime/XNNWeightsCache.cpp

@@ -141,16 +141,19 @@ Error XNNWeightsCache::initialize_for_runtime(
     return Error::Ok;
   }
 
-  // Already loaded earlier this session; just reopen the write fd that
-  // save_packed_index() closed. Subsequent reserve_space can extend the
-  // file for any entries not in the saved trailer.
-  if (cache_loaded_) {
+  // Entries already in memory (from a prior load_packed_cache or a prior
+  // fresh-write session). Just reopen the write fd that save_packed_index
+  // closed; subsequent reserve_space can extend the file. Using metadata
+  // emptiness (not a separate flag) as the gate avoids a latent bug
+  // where fresh-write→save→re-init re-enters load_packed_cache and
+  // double-mmaps the same file.
+  if (!name_to_packed_data_metadata_.empty()) {
     packed_file_fd_ = open_locked(packed_cache_path_, O_RDWR);
     return Error::Ok;
   }
 
-  // First init for this path: try to load the saved trailer; on success
-  // open a write fd for any new entries. If load fails, fall through to
+  // No in-memory entries: try to load the saved trailer; on success open
+  // a write fd for any new entries. If load fails, fall through to
   // fresh-write below.
   if (load_packed_cache()) {
     ET_LOG(
@@ -280,7 +283,6 @@ void XNNWeightsCache::full_unload() {
   packed_data_ptrs_.clear();
   ptr_to_file_offset_.clear();
   file_ptr_to_region_index_.clear();
-  cache_loaded_ = false;
   if (packed_file_fd_ >= 0) {
     close(packed_file_fd_);
     packed_file_fd_ = -1;
@@ -721,7 +723,6 @@ bool XNNWeightsCache::load_packed_cache() {
     name_to_packed_data_metadata_[name] = meta;
   }
 
-  cache_loaded_ = true;
   packed_file_used_ = index_start;
   // In-memory state matches the on-disk trailer; the next save would be
   // a no-op. Initialize watermark so save_packed_index short-circuits.

backends/xnnpack/runtime/XNNWeightsCache.h

@@ -39,10 +39,8 @@ struct PackedDataMeta {
   bool in_current_runtime{};
   // True if this entry's bytes are persisted in the on-disk cache file
   // (either originally loaded via load_packed_cache, or freshly packed
-  // and then save_packed_index-ed). Used by delete_packed_data to
-  // detect when all persistent entries are gone, at which point
-  // cache_loaded_ is auto-invalidated so the next init re-enters
-  // load_packed_cache and reuses the saved file instead of re-packing.
+  // and then save_packed_index-ed). delete_packed_data preserves these
+  // entries so the next init reuses the saved file instead of re-packing.
   bool from_load{false};
   // Per-ukernel seed from xnn_weights_cache_look_up_key.seed. XNNPACK
   // guarantees this is consistent across runs of the same ukernel; when
@@ -195,10 +193,6 @@ class XNNWeightsCache {
   std::string packed_cache_path_;
   int packed_file_fd_{-1};
   size_t packed_file_used_{0};
-  // True once load_packed_cache() has populated metadata from a saved
-  // index, OR once a fresh-write session has been persisted to disk via
-  // save_packed_index() (so subsequent inits can load from it).
-  bool cache_loaded_{false};
   // Tracks file offset of each file-backed allocation. Used by
   // save_packed_index() to serialize (name → offset, size) index.
   std::unordered_map<void*, size_t> ptr_to_file_offset_;