Ractor support (supersedes #365)

RACTOR.md

@@ -0,0 +1,85 @@
+# Ractor support in SQLite3-Ruby
+
+SQLite3 has [3 different modes of threading support](https://www.sqlite.org/threadsafe.html).
+
+1. Single-thread
+2. Multi-thread
+3. Serialized
+
+"Single thread" mode means there are no mutexes, so the library itself is not
+thread safe.  In other words if two threads do `SQLite3::Database.new` on the
+same file, it will have thread safety problems.
+
+"Multi thread" mode means that SQLite3 puts mutexes in place, but it does not
+mean that the SQLite3 API itself is thread safe.  In other words, in this mode
+it is SAFE for two threads to do `SQLite3::Database.new` on the same file, but
+it is NOT SAFE to share that database object between two threads.
+
+"Serialized" mode is like "Multi thread" mode except that there are mutexes in
+place such that it IS SAFE to share the same database object between threads.
+
+## Ractor Safety
+
+When a C extension claims to be Ractor safe by calling `rb_ext_ractor_safe`,
+it's merely claiming that its C API is thread safe.  This _does not_ mean that
+objects allocated from said C extension are allowed to cross between Ractor
+boundaries.
+
+In other words, `rb_ext_ractor_safe` matches the expectations of the
+"multi-thread" mode of SQLite3.  We can detect the multithread mode via the
+`sqlite3_threadsafe` function.  In other words, it's fine to declare this
+extension is Ractor safe, but only if `sqlite3_threadsafe` returns true.
+`SQLite3.ractor_safe?` reports whether the extension made that declaration.
+
+The supported pattern is that each Ractor opens and uses its own connections.
+Everything works inside a Ractor, including user-defined functions,
+aggregators, collations, the authorizer, `trace`, and `busy_handler`.  Several
+Ractors may open connections to the same database file; SQLite's own locking
+coordinates them, just as it does for separate processes.
+
+```ruby
+require "sqlite3"
+
+ractors = 4.times.map do |i|
+  Ractor.new(i) do |i|
+    db = SQLite3::Database.new("app.db")
+    db.busy_timeout = 5000
+    db.execute("INSERT INTO log (worker) VALUES (?)", [i])
+    db.close
+  end
+end
+ractors.each(&:value)
+```
+
+## Database objects can't be passed between Ractors
+
+A `SQLite3::Database` belongs to the Ractor that opened it.  Passing one to
+another Ractor raises a `Ractor::Error`:
+
+```ruby
+require "sqlite3"
+
+r = Ractor.new { Ractor.receive }
+
+db = SQLite3::Database.new ":memory:"
+
+begin
+  r.send db
+  puts "unreachable"
+rescue Ractor::Error
+end
+```
+
+This is true even for a connection opened in "Serialized" mode
+(`SQLite3::Constants::Open::FULLMUTEX`).  Serialized mode makes the SQLite
+calls safe, but a `Database` also holds Ruby state: the procs and objects
+registered as functions, aggregators, collations, and handlers.  Those can't
+be shared between Ractors.
+
+## Fork Safety
+
+Fork safety is restricted to database objects that were created on the main
+Ractor.  When a process forks, the child process shuts down all Ractors, so
+any database connections that are inside a Ractor should be released.
+Because a connection can't be passed between Ractors, every connection the main
+Ractor can reach is tracked.

ext/sqlite3/extconf.rb

@@ -125,6 +125,9 @@ def configure_extension
         # Functions defined in 2.1 but not 2.0
         have_func("rb_integer_pack")
 
+        # Functions defined in 3.0 but not 2.7
+        have_func("rb_ext_ractor_safe")
+
         # These functions may not be defined
         have_func("sqlite3_initialize")
         have_func("sqlite3_backup_init")

ext/sqlite3/sqlite3.c

@@ -127,6 +127,12 @@ init_sqlite3_constants(void)
     VALUE mSqlite3Constants;
     VALUE mSqlite3Open;
 
+#ifdef HAVE_RB_EXT_RACTOR_SAFE
+    if (sqlite3_threadsafe()) {
+        rb_ext_ractor_safe(true);
+    }
+#endif
+
     mSqlite3Constants = rb_define_module_under(mSqlite3, "Constants");
 
     /* sqlite3_open_v2 flags for Database::new */

ext/sqlite3/statement.c

@@ -534,35 +534,31 @@ static VALUE stmt_stat_symbols[stmt_stat_sym_last];
 static void
 setup_stmt_stat_symbols(void)
 {
-    if (stmt_stat_symbols[0] == 0) {
 #define S(s) stmt_stat_symbols[stmt_stat_sym_##s] = ID2SYM(rb_intern_const(#s))
-        S(fullscan_steps);
-        S(sorts);
-        S(autoindexes);
-        S(vm_steps);
+    S(fullscan_steps);
+    S(sorts);
+    S(autoindexes);
+    S(vm_steps);
 #ifdef SQLITE_STMTSTATUS_REPREPARE
-        S(reprepares);
+    S(reprepares);
 #endif
 #ifdef SQLITE_STMTSTATUS_RUN
-        S(runs);
+    S(runs);
 #endif
 #ifdef SQLITE_STMTSTATUS_FILTER_MISS
-        S(filter_misses);
+    S(filter_misses);
 #endif
 #ifdef SQLITE_STMTSTATUS_FILTER_HIT
-        S(filter_hits);
+    S(filter_hits);
 #endif
 #undef S
-    }
 }
 
 static size_t
 stmt_stat_internal(VALUE hash_or_sym, sqlite3_stmt *stmt)
 {
     VALUE hash = Qnil, key = Qnil;
 
-    setup_stmt_stat_symbols();
-
     if (RB_TYPE_P(hash_or_sym, T_HASH)) {
         hash = hash_or_sym;
     } else if (SYMBOL_P(hash_or_sym)) {
@@ -726,6 +722,9 @@ init_sqlite3_statement(void)
 {
     cSqlite3Statement = rb_define_class_under(mSqlite3, "Statement", rb_cObject);
 
+    /* populate eagerly: a lazy fill would race when the extension is used from multiple Ractors */
+    setup_stmt_stat_symbols();
+
     rb_define_alloc_func(cSqlite3Statement, allocate);
     rb_define_method(cSqlite3Statement, "close", sqlite3_rb_close, 0);
     rb_define_method(cSqlite3Statement, "closed?", closed_p, 0);

lib/sqlite3.rb

@@ -14,6 +14,11 @@ module SQLite3
   def self.threadsafe?
     threadsafe > 0
   end
+
+  # Is the gem's C extension marked as Ractor-safe?
+  def self.ractor_safe?
+    threadsafe? && !defined?(Ractor).nil?
+  end
 end
 
 require "sqlite3/version_info"

lib/sqlite3/database.rb

@@ -180,7 +180,7 @@ def initialize file, options = {}, zvfs = nil
 
       initialize_extensions(options[:extensions])
 
-      ForkSafety.track(self)
+      ForkSafety.track(self) if !defined?(Ractor) || (Ractor.main == Ractor.current)
 
       if block_given?
         begin
@@ -592,22 +592,15 @@ def finalize
     def define_aggregator(name, aggregator)
       # Previously, this has been implemented in C. Now this is just yet
       # another compatibility shim
+      #
+      # The template and name are captured in closures rather than class
+      # instance variables, which can't be set outside the main Ractor.
       proxy = Class.new do
-        @template = aggregator
-        @name = name
-
-        def self.template
-          @template
-        end
+        define_singleton_method(:template) { aggregator }
+        define_singleton_method(:name) { name }
 
-        def self.name
-          @name
-        end
-
-        def self.arity
-          # this is what sqlite3_obj_method_arity did before
-          @template.method(:step).arity
-        end
+        # this is what sqlite3_obj_method_arity did before
+        define_singleton_method(:arity) { aggregator.method(:step).arity }
 
         def initialize
           @klass = self.class.template.clone

lib/sqlite3/version_info.rb

@@ -1,6 +1,5 @@
 module SQLite3
-  # a hash of descriptive metadata about the current version of the sqlite3 gem
-  VERSION_INFO = {
+  version_info = {
     ruby: RUBY_DESCRIPTION,
     gem: {
       version: SQLite3::VERSION
@@ -14,4 +13,7 @@ module SQLite3
       threadsafe: SQLite3.threadsafe?
     }
   }
+  # a hash of descriptive metadata about the current version of the sqlite3 gem,
+  # deeply frozen so that it can be read from any Ractor
+  VERSION_INFO = defined?(Ractor.make_shareable) ? Ractor.make_shareable(version_info) : version_info.freeze
 end

test/test_integration_ractor.rb

@@ -0,0 +1,139 @@
+# frozen_string_literal: true
+
+require "helper"
+require "open3"
+require "rbconfig"
+
+# Each Ractor test runs in a child process. Once a process has started a
+# Ractor, some process-wide behavior changes for good (for example,
+# ObjectSpace.each_object stops seeing objects), which would leak into
+# unrelated tests. A child process also lets us fail on a hang instead of
+# stalling the suite.
+class IntegrationRactorTestCase < SQLite3::TestCase
+  TIMEOUT = 60
+
+  PRELUDE = <<~RUBY
+    require "sqlite3"
+    Warning[:experimental] = false
+    def ractor_value(r) = r.respond_to?(:value) ? r.value : r.take
+  RUBY
+
+  def setup
+    super
+    skip("Requires a Ractor-safe build") unless SQLite3.ractor_safe?
+
+    # valgrind follows the child processes, and a Ruby process that has created a Ractor can't
+    # exit cleanly under it: with RUBY_FREE_AT_EXIT, Ruby 4.0.7 reads freed memory in
+    # ractor_free (RUBY_FREE_AT_EXIT=1 valgrind ruby -e 'Ractor.new { 1 }.value'); without it,
+    # everything Ruby allocated is reported as leaked.
+    skip("Ractor processes can't exit cleanly under valgrind") if i_am_running_in_valgrind
+  end
+
+  def test_ractor_safe
+    assert_predicate SQLite3, :ractor_safe?
+  end
+
+  def test_open_and_query_in_ractors
+    assert_ractor_script(<<~RUBY, "4950,4950,4950,4950")
+      ractors = 4.times.map do
+        Ractor.new do
+          db = SQLite3::Database.new(":memory:")
+          begin
+            db.execute "CREATE TABLE t (a INTEGER)"
+            db.transaction do
+              stmt = db.prepare("INSERT INTO t VALUES (?)")
+              100.times { |n| stmt.execute(n) }
+              stmt.close
+            end
+            db.get_first_value("SELECT sum(a) FROM t")
+          ensure
+            db.close
+          end
+        end
+      end
+      print ractors.map { ractor_value(_1) }.join(",")
+    RUBY
+  end
+
+  def test_callbacks_in_ractor
+    assert_ractor_script(<<~RUBY, "20,b,true,true")
+      class Summer
+        def initialize = @sum = 0
+        def step(v) = @sum += v
+        def finalize = @sum
+      end
+
+      class Reverse
+        def compare(a, b) = b <=> a
+      end
+
+      r = Ractor.new do
+        db = SQLite3::Database.new(":memory:")
+        begin
+          traced = false
+          db.trace { traced = true }
+          db.authorizer = ->(*) { true }
+          db.create_function("double", 1) { |fn, x| fn.result = x * 2 }
+          db.define_aggregator("summer", Summer.new)
+          db.collation("rev", Reverse.new)
+
+          db.execute "CREATE TABLE t (a INTEGER, s TEXT)"
+          db.execute "INSERT INTO t VALUES (1, 'a'), (2, 'b'), (3, 'a'), (4, 'b')"
+          stmt = db.prepare("SELECT s FROM t ORDER BY s COLLATE rev LIMIT 1")
+          first = stmt.execute.to_a.flatten.first
+          stats = stmt.stat.key?(:vm_steps)
+          stmt.close
+
+          [db.get_first_value("SELECT summer(double(a)) FROM t"), first, stats, traced].join(",")
+        ensure
+          db.close
+        end
+      end
+      print ractor_value(r)
+    RUBY
+  end
+
+  def test_version_info_is_shareable
+    assert_ractor_script(<<~RUBY, SQLite3::VERSION)
+      print ractor_value(Ractor.new { SQLite3::VERSION_INFO[:gem][:version] })
+    RUBY
+  end
+
+  def test_database_cannot_be_sent_to_another_ractor
+    # Before ruby/ruby@ce47ee00, sending a T_DATA object made a shallow copy instead of raising
+    skip("Requires Ruby 3.3 or later") if RUBY_VERSION < "3.3"
+
+    assert_ractor_script(<<~RUBY, "Ractor::Error")
+      db = SQLite3::Database.new(":memory:")
+      r = Ractor.new { Ractor.receive }
+      begin
+        r.send(db)
+        print "sent"
+      rescue Ractor::Error => e
+        print e.class
+      end
+    RUBY
+  end
+
+  private
+
+  def assert_ractor_script(script, expected)
+    load_path = $LOAD_PATH.select { |dir| File.directory?(dir) }.flat_map { |dir| ["-I", dir] }
+    cmd = [RbConfig.ruby, *load_path, "-e", PRELUDE + script]
+
+    Open3.popen3(*cmd) do |stdin, out, err, wait_thr|
+      stdin.close
+      out_reader = Thread.new { out.read }
+      err_reader = Thread.new { err.read }
+
+      unless wait_thr.join(TIMEOUT)
+        Process.kill(:KILL, wait_thr.pid)
+        flunk "Ractor script did not finish within #{TIMEOUT}s:\n#{script}"
+      end
+
+      output = out_reader.value
+      assert_predicate wait_thr.value, :success?, "Ractor script failed:\n#{output}#{err_reader.value}"
+      assert_equal expected, output
+    end
+  end
+end