refactor: lazily initialize block cache workers

include/dwarfs/reader/block_cache_options.h

@@ -43,7 +43,6 @@ struct block_cache_options {
   size_t num_workers{0};
   double decompress_ratio{1.0};
   bool mm_release{true};
-  bool init_workers{true};
   bool disable_block_integrity_check{false};
   size_t sequential_access_detector_threshold{0};
   block_cache_allocation_mode allocation_mode{

src/reader/block_cache_options.cpp

@@ -37,9 +37,9 @@ namespace dwarfs::reader {
 std::ostream& operator<<(std::ostream& os, block_cache_options const& opts) {
   os << fmt::format(
       "max_bytes={}, num_workers={}, decompress_ratio={}, mm_release={}, "
-      "init_workers={}, disable_block_integrity_check={}",
+      "disable_block_integrity_check={}",
       opts.max_bytes, opts.num_workers, opts.decompress_ratio, opts.mm_release,
-      opts.init_workers, opts.disable_block_integrity_check);
+      opts.disable_block_integrity_check);
   return os;
 }
 

src/reader/internal/block_cache.cpp

@@ -244,13 +244,6 @@ class block_cache_ final : public block_cache::impl {
             options.sequential_access_detector_threshold)}
       , os_{os}
       , options_(options) {
-    if (options.init_workers) {
-      wg_ = worker_group(lgr, os_, "blkcache",
-                         std::max(options.num_workers > 0
-                                      ? options.num_workers
-                                      : hardware_concurrency(),
-                                  static_cast<size_t>(1)));
-    }
     cache_.set_prune_hook(
         [this](size_t block_no, std::shared_ptr<cached_block>&& block) {
           on_block_removed("evicted", block_no, std::move(block));
@@ -263,8 +256,12 @@ class block_cache_ final : public block_cache::impl {
 
     tidy_runner_.stop();
 
-    if (wg_) {
-      wg_.stop();
+    {
+      std::unique_lock lock(mx_wg_);
+
+      if (wg_) {
+        wg_.stop();
+      }
     }
 
     if (!blocks_created_.load()) {
@@ -604,7 +601,22 @@ class block_cache_ final : public block_cache::impl {
                                  std::memory_order_relaxed);
   }
 
+  void init_worker_group() const {
+    std::unique_lock lock(mx_wg_);
+
+    if (!wg_) {
+      wg_ = worker_group(LOG_GET_LOGGER, os_, "blkcache",
+                         std::max(options_.num_workers > 0
+                                      ? options_.num_workers
+                                      : hardware_concurrency(),
+                                  static_cast<size_t>(1)));
+    }
+  }
+
   void enqueue_job(std::shared_ptr<block_request_set> brs) const {
+    // lazy initialization of worker group
+    std::call_once(wg_init_flag_, [this] { init_worker_group(); });
+
     std::shared_lock lock(mx_wg_);
 
     // Lambda needs to be mutable so we can actually move out of it
@@ -789,6 +801,7 @@ class block_cache_ final : public block_cache::impl {
 
   mutable std::shared_mutex mx_wg_;
   mutable worker_group wg_;
+  mutable std::once_flag wg_init_flag_;
   std::vector<fs_section> block_;
   std::shared_ptr<mmif> mm_;
   byte_buffer_factory buffer_factory_;

tools/src/dwarfs_main.cpp

@@ -1492,7 +1492,6 @@ void load_filesystem(dwarfs_userdata& userdata) {
   fsopts.block_cache.num_workers = opts.workers;
   fsopts.block_cache.decompress_ratio = opts.decompress_ratio;
   fsopts.block_cache.mm_release = !opts.cache_image;
-  fsopts.block_cache.init_workers = false;
   fsopts.block_cache.sequential_access_detector_threshold =
       opts.seq_detector_threshold;
   fsopts.block_cache.allocation_mode = opts.block_allocator;