Fix async callbacks in conjunction with fork()

After fork() the dispatcher thread is no longer running, so it needs to be restarted in the child process.

Fixes #884

CHANGELOG.md

@@ -3,6 +3,7 @@
 
 Fixed:
 * Fix MSVC build
+* Fix async callbacks in conjunction with fork(). #884
 
 Added:
 * Allow to pass callbacks in varargs. #885

ext/ffi_c/Function.c

@@ -282,6 +282,17 @@ rbffi_Function_ForProc(VALUE rbFunctionInfo, VALUE proc)
     return callback;
 }
 
+#if !defined(_WIN32) && defined(DEFER_ASYNC_CALLBACK)
+static void
+after_fork_callback(void)
+{
+    /* Ensure that a new dispatcher thread is started in a forked process */
+    async_cb_thread = Qnil;
+    pthread_mutex_init(&async_cb_mutex, NULL);
+    pthread_cond_init(&async_cb_cond, NULL);
+}
+#endif
+
 static VALUE
 function_init(VALUE self, VALUE rbFunctionInfo, VALUE rbProc)
 {
@@ -309,6 +320,13 @@ function_init(VALUE self, VALUE rbFunctionInfo, VALUE rbProc)
 
 #if defined(DEFER_ASYNC_CALLBACK)
         if (async_cb_thread == Qnil) {
+
+#if !defined(_WIN32)
+            if( pthread_atfork(NULL, NULL, after_fork_callback) ){
+                rb_warn("FFI: unable to register fork callback");
+            }
+#endif
+
             async_cb_thread = rb_thread_create(async_cb_event, NULL);
             /* Name thread, for better debugging */
             rb_funcall(async_cb_thread, rb_intern("name="), 1, rb_str_new2("FFI Callback Dispatcher"));

spec/ffi/fork_spec.rb

@@ -8,32 +8,59 @@
 if Process.respond_to?(:fork)
   describe "Callback in conjunction with fork()" do
 
-    module LibTest
+    libtest = Module.new do
       extend FFI::Library
       ffi_lib TestLibrary::PATH
 
       callback :cbVrL, [ ], :long
       attach_function :testCallbackVrL, :testClosureVrL, [ :cbVrL ], :long
+      AsyncIntCallback = callback [ :int ], :void
+      attach_function :testAsyncCallback, [ AsyncIntCallback, :int ], :void, blocking: true
     end
 
     it "works with forked process and GC" do
-      expect(LibTest.testCallbackVrL { 12345 }).to eq(12345)
+      expect(libtest.testCallbackVrL { 12345 }).to eq(12345)
       fork do
-        expect(LibTest.testCallbackVrL { 12345 }).to eq(12345)
+        expect(libtest.testCallbackVrL { 12345 }).to eq(12345)
+        Process.exit 42
       end
-      expect(LibTest.testCallbackVrL { 12345 }).to eq(12345)
+      expect(libtest.testCallbackVrL { 12345 }).to eq(12345)
       GC.start
+
+      expect(Process.wait2[1].exitstatus).to eq(42)
     end
 
     it "works with forked process and free()" do
       cbf = FFI::Function.new(FFI::Type::LONG, []) { 234 }
 
       fork do
         cbf.free
+        Process.exit 43
       end
 
-      expect(LibTest.testCallbackVrL(cbf)).to eq(234)
+      expect(libtest.testCallbackVrL(cbf)).to eq(234)
       cbf.free
+
+      expect(Process.wait2[1].exitstatus).to eq(43)
+    end
+
+    def run_async_callback(libtest)
+      recv = nil
+      libtest.testAsyncCallback(proc { |r| recv = r }, 23)
+      expect(recv).to eq(23)
+    end
+
+    it "async thread dispatch works after forking" do
+      run_async_callback(libtest)
+
+      fork do
+        run_async_callback(libtest)
+        Process.exit 44
+      end
+
+      run_async_callback(libtest)
+
+      expect(Process.wait2[1].exitstatus).to eq(44)
     end
   end
 end