Add readonly mode

README.md

@@ -57,6 +57,7 @@ Bootsnap.setup(
   load_path_cache:      true,                 # Optimize the LOAD_PATH with a cache
   compile_cache_iseq:   true,                 # Compile Ruby code into ISeq cache, breaks coverage reporting.
   compile_cache_yaml:   true,                 # Compile YAML into a cache
+  readonly:             true,                 # Use the caches but don't update them on miss or stale entries.
 )
 ```
 
@@ -77,6 +78,7 @@ well together, and are both included in a newly-generated Rails applications by
 - `DISABLE_BOOTSNAP` allows to entirely disable bootsnap.
 - `DISABLE_BOOTSNAP_LOAD_PATH_CACHE` allows to disable load path caching.
 - `DISABLE_BOOTSNAP_COMPILE_CACHE` allows to disable ISeq and YAML caches.
+- `BOOTSNAP_READONLY` configure bootsnap to not update the cache on miss or stale entries.
 - `BOOTSNAP_LOG` configure bootsnap to log all caches misses to STDERR.
 - `BOOTSNAP_IGNORE_DIRECTORIES` a comma separated list of directories that shouldn't be scanned.
   Useful when you have large directories of non-ruby files inside `$LOAD_PATH`.

ext/bootsnap/bootsnap.c

@@ -90,9 +90,11 @@ static ID instrumentation_method;
 static VALUE sym_miss;
 static VALUE sym_stale;
 static bool instrumentation_enabled = false;
+static bool readonly = false;
 
 /* Functions exposed as module functions on Bootsnap::CompileCache::Native */
 static VALUE bs_instrumentation_enabled_set(VALUE self, VALUE enabled);
+static VALUE bs_readonly_set(VALUE self, VALUE enabled);
 static VALUE bs_compile_option_crc32_set(VALUE self, VALUE crc32_v);
 static VALUE bs_rb_fetch(VALUE self, VALUE cachedir_v, VALUE path_v, VALUE handler, VALUE args);
 static VALUE bs_rb_precompile(VALUE self, VALUE cachedir_v, VALUE path_v, VALUE handler);
@@ -166,6 +168,7 @@ Init_bootsnap(void)
   rb_global_variable(&sym_stale);
 
   rb_define_module_function(rb_mBootsnap, "instrumentation_enabled=", bs_instrumentation_enabled_set, 1);
+  rb_define_module_function(rb_mBootsnap_CompileCache_Native, "readonly=", bs_readonly_set, 1);
   rb_define_module_function(rb_mBootsnap_CompileCache_Native, "coverage_running?", bs_rb_coverage_running, 0);
   rb_define_module_function(rb_mBootsnap_CompileCache_Native, "fetch", bs_rb_fetch, 4);
   rb_define_module_function(rb_mBootsnap_CompileCache_Native, "precompile", bs_rb_precompile, 3);
@@ -182,6 +185,13 @@ bs_instrumentation_enabled_set(VALUE self, VALUE enabled)
   return enabled;
 }
 
+static VALUE
+bs_readonly_set(VALUE self, VALUE enabled)
+{
+  readonly = RTEST(enabled);
+  return enabled;
+}
+
 /*
  * Bootsnap's ruby code registers a hook that notifies us via this function
  * when compile_option changes. These changes invalidate all existing caches.
@@ -945,12 +955,17 @@ try_input_to_storage(VALUE arg)
 static int
 bs_input_to_storage(VALUE handler, VALUE args, VALUE input_data, VALUE pathval, VALUE * storage_data)
 {
-  int state;
-  struct i2s_data i2s_data = {
-    .handler    = handler,
-    .input_data = input_data,
-    .pathval    = pathval,
-  };
-  *storage_data = rb_protect(try_input_to_storage, (VALUE)&i2s_data, &state);
-  return state;
+  if (readonly) {
+    *storage_data = rb_cBootsnap_CompileCache_UNCOMPILABLE;
+    return 0;
+  } else {
+    int state;
+    struct i2s_data i2s_data = {
+      .handler    = handler,
+      .input_data = input_data,
+      .pathval    = pathval,
+    };
+    *storage_data = rb_protect(try_input_to_storage, (VALUE)&i2s_data, &state);
+    return state;
+  }
 }

lib/bootsnap.rb

@@ -40,6 +40,7 @@ def setup(
       development_mode: true,
       load_path_cache: true,
       ignore_directories: nil,
+      readonly: false,
       compile_cache_iseq: true,
       compile_cache_yaml: true,
       compile_cache_json: true
@@ -49,6 +50,7 @@ def setup(
           cache_path: "#{cache_dir}/bootsnap/load-path-cache",
           development_mode: development_mode,
           ignore_directories: ignore_directories,
+          readonly: readonly,
         )
       end
 
@@ -57,6 +59,7 @@ def setup(
         iseq: compile_cache_iseq,
         yaml: compile_cache_yaml,
         json: compile_cache_json,
+        readonly: readonly,
       )
     end
 
@@ -101,6 +104,7 @@ def default_setup
           compile_cache_iseq: !ENV["DISABLE_BOOTSNAP_COMPILE_CACHE"],
           compile_cache_yaml: !ENV["DISABLE_BOOTSNAP_COMPILE_CACHE"],
           compile_cache_json: !ENV["DISABLE_BOOTSNAP_COMPILE_CACHE"],
+          readonly: !!ENV["BOOTSNAP_READONLY"],
           ignore_directories: ignore_directories,
         )
 

lib/bootsnap/compile_cache.rb

@@ -10,7 +10,7 @@ def UNCOMPILABLE.inspect
     Error = Class.new(StandardError)
     PermissionError = Class.new(Error)
 
-    def self.setup(cache_dir:, iseq:, yaml:, json:)
+    def self.setup(cache_dir:, iseq:, yaml:, json:, readonly: false)
       if iseq
         if supported?
           require_relative("compile_cache/iseq")
@@ -37,6 +37,10 @@ def self.setup(cache_dir:, iseq:, yaml:, json:)
           warn("[bootsnap/setup] JSON parsing caching is not supported on this implementation of Ruby")
         end
       end
+
+      if supported? && defined?(Bootsnap::CompileCache::Native)
+        Bootsnap::CompileCache::Native.readonly = readonly
+      end
     end
 
     def self.permission_error(path)

lib/bootsnap/load_path_cache.rb

@@ -28,13 +28,13 @@ class << self
       alias_method :enabled?, :enabled
       remove_method(:enabled)
 
-      def setup(cache_path:, development_mode:, ignore_directories:)
+      def setup(cache_path:, development_mode:, ignore_directories:, readonly: false)
         unless supported?
           warn("[bootsnap/setup] Load path caching is not supported on this implementation of Ruby") if $VERBOSE
           return
         end
 
-        store = Store.new(cache_path)
+        store = Store.new(cache_path, readonly: readonly)
 
         @loaded_features_index = LoadedFeaturesIndex.new
 

lib/bootsnap/load_path_cache/store.rb

@@ -13,10 +13,11 @@ class Store
       NestedTransactionError = Class.new(StandardError)
       SetOutsideTransactionNotAllowed = Class.new(StandardError)
 
-      def initialize(store_path)
+      def initialize(store_path, readonly: false)
         @store_path = store_path
         @txn_mutex = Mutex.new
         @dirty = false
+        @readonly = readonly
         load_data
       end
 
@@ -63,7 +64,7 @@ def mark_for_mutation!
       end
 
       def commit_transaction
-        if @dirty
+        if @dirty && !@readonly
           dump_data
           @dirty = false
         end

test/compile_cache_test.rb

@@ -5,6 +5,11 @@
 class CompileCacheTest < Minitest::Test
   include(TmpdirHelper)
 
+  def teardown
+    super
+    Bootsnap::CompileCache::Native.readonly = false
+  end
+
   def test_compile_option_crc32
     # Just assert that this works.
     Bootsnap::CompileCache::Native.compile_option_crc32 = 0xffffffff
@@ -112,6 +117,32 @@ def test_recache_when_size_different
     load(path)
   end
 
+  def test_dont_store_cache_after_a_miss_when_readonly
+    Bootsnap::CompileCache::Native.readonly = true
+
+    path = Help.set_file("a.rb", "a = a = 3", 100)
+    output = RubyVM::InstructionSequence.compile_file(path)
+    Bootsnap::CompileCache::ISeq.expects(:input_to_storage).never
+    Bootsnap::CompileCache::ISeq.expects(:storage_to_output).never
+    Bootsnap::CompileCache::ISeq.expects(:input_to_output).once.returns(output)
+
+    load(path)
+  end
+
+  def test_dont_store_cache_after_a_stale_when_readonly
+    path = Help.set_file("a.rb", "a = a = 3", 100)
+    load(path)
+
+    Bootsnap::CompileCache::Native.readonly = true
+
+    output = RubyVM::InstructionSequence.compile_file(path)
+    Bootsnap::CompileCache::ISeq.expects(:input_to_storage).never
+    Bootsnap::CompileCache::ISeq.expects(:storage_to_output).once.returns(output)
+    Bootsnap::CompileCache::ISeq.expects(:input_to_output).never
+
+    load(path)
+  end
+
   def test_invalid_cache_file
     path = Help.set_file("a.rb", "a = a = 3", 100)
     cp = Help.cache_path("#{@tmp_dir}-iseq", path)

test/setup_test.rb

@@ -23,6 +23,7 @@ def test_default_setup
         compile_cache_yaml: true,
         compile_cache_json: true,
         ignore_directories: nil,
+        readonly: false,
       )
 
       Bootsnap.default_setup
@@ -39,6 +40,7 @@ def test_default_setup_with_ENV_not_dev
         compile_cache_yaml: true,
         compile_cache_json: true,
         ignore_directories: nil,
+        readonly: false,
       )
 
       Bootsnap.default_setup
@@ -55,6 +57,7 @@ def test_default_setup_with_DISABLE_BOOTSNAP_LOAD_PATH_CACHE
         compile_cache_yaml: true,
         compile_cache_json: true,
         ignore_directories: nil,
+        readonly: false,
       )
 
       Bootsnap.default_setup
@@ -71,6 +74,7 @@ def test_default_setup_with_DISABLE_BOOTSNAP_COMPILE_CACHE
         compile_cache_yaml: false,
         compile_cache_json: false,
         ignore_directories: nil,
+        readonly: false,
       )
 
       Bootsnap.default_setup
@@ -94,6 +98,7 @@ def test_default_setup_with_BOOTSNAP_LOG
         compile_cache_yaml: true,
         compile_cache_json: true,
         ignore_directories: nil,
+        readonly: false,
       )
       Bootsnap.expects(:logger=).with($stderr.method(:puts))
 
@@ -111,6 +116,7 @@ def test_default_setup_with_BOOTSNAP_IGNORE_DIRECTORIES
         compile_cache_yaml: true,
         compile_cache_json: true,
         ignore_directories: %w[foo bar],
+        readonly: false,
       )
 
       Bootsnap.default_setup