Fix race condition causing crash when setting content filter during spin (#1372)

Investigated and resolved an intermittent crash (`SIGABRT` / "pure virtual method called") occurring in the underlying FastDDS layer.

**Crash log**
```bash
✔ setContentFilter (2003ms)
pure virtual method called
terminate called without an active exception
Aborted (core dumped)
Error: Process completed with exit code 134.
```

**Callstack**
```bash
#0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3  0x00007cfa4744527e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007cfa474288ff in __GI_abort () at ./stdlib/abort.c:79
#5  0x00007cfa478a5ff5 in __gnu_cxx::__verbose_terminate_handler () at ../../../../src/libstdc++-v3/libsupc++/vterminate.cc:95
#6  0x00007cfa478bb0da in __cxxabiv1::__terminate (handler=<optimized out>) at ../../../../src/libstdc++-v3/libsupc++/eh_terminate.cc:48
#7  0x00007cfa478a5a55 in std::terminate () at ../../../../src/libstdc++-v3/libsupc++/eh_terminate.cc:58
#8  0x00007cfa478bbfc3 in __cxxabiv1::__cxa_pure_virtual () at ../../../../src/libstdc++-v3/libsupc++/pure.cc:50
#9  0x00007cf9f689e916 in eprosima::fastdds::dds::detail::DataReaderHistory::get_first_untaken_info (this=0x2d825968, info=...) at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/eProsima/Fast-DDS/src/cpp/fastdds/subscriber/history/DataReaderHistory.cpp:364
#10 0x00007cf9f688d9fb in eprosima::fastdds::dds::DataReaderImpl::get_first_untaken_info (this=<optimized out>, info=info@entry=0x7cf9e57f8d80) at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/eProsima/Fast-DDS/src/cpp/fastdds/subscriber/DataReaderImpl.cpp:793
#11 0x00007cf9f688a2dd in eprosima::fastdds::dds::DataReader::get_first_untaken_info (this=<optimized out>, info=info@entry=0x7cf9e57f8d80) at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/eProsima/Fast-DDS/src/cpp/fastdds/subscriber/DataReader.cpp:278
#12 0x00007cfa44644c46 in rmw_fastrtps_shared_cpp::__rmw_wait (identifier=<optimized out>, subscriptions=<optimized out>, guard_conditions=<optimized out>, services=0x7cf9dc000bc0, clients=0x7cf9dc000ba8, events=0x7cf9dc000bd8, wait_set=0x7cf9dc000f60, wait_timeout=0x7cf9e57f8ea0)
    at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/ros2/rmw_fastrtps/rmw_fastrtps_shared_cpp/src/rmw_wait.cpp:235
#13 0x00007cfa446ac27f in rmw_wait (subscriptions=<optimized out>, guard_conditions=<optimized out>, services=<optimized out>, clients=<optimized out>, events=<optimized out>, wait_set=<optimized out>, wait_timeout=0x7cf9e57f8ea0)
    at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/ros2/rmw_fastrtps/rmw_fastrtps_cpp/src/rmw_wait.cpp:33
#14 0x00007cfa44adf742 in rcl_wait (wait_set=0x7cf9e57f9890, timeout=<optimized out>) at /home/jenkins-agent/workspace/ci_packaging_linux/ws/src/ros2/rcl/rcl/src/rcl/wait.c:670
#15 0x00007cfa44b54625 in rclnodejs::Executor::WaitForReadyCallbacks(rcl_wait_set_s*, int) () from /home/minggang/proj/rclnodejs/build/Release/rclnodejs.node
#16 0x00007cfa44b561aa in rclnodejs::Executor::Run(void*) () from /home/minggang/proj/rclnodejs/build/Release/rclnodejs.node
#17 0x00007cfa4749caa4 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:447
#18 0x00007cfa47529c6c in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78
```

**Root Cause Analysis:**
The crash was caused by a race condition between the main JavaScript thread and the background worker thread:
1. The node creates a background thread when `node.spin()` is called to run the `rcl` executor and wait for messages.
2. Calling `subscription.setContentFilter()` from the main thread invokes `rcl_subscription_set_content_filter`, which modifies the underlying DDS DataReader.
3. If the background thread is simultaneously accessing that same DataReader (e.g., in `rcl_wait` or checking for new data), it leads to memory corruption or accessing an object in an invalid state, resulting in the "pure virtual method called" termination.

**Fix Implementation:**
Updated tests, examples, and documentation to enforce a thread-safe pattern. The node must now explicitly stop the background executor before modifying the content filter, and restart it afterwards:

```javascript
node.stop(); // Stop background thread
subscription.setContentFilter(filter); // Safe modification
node.spin(); // Restart background thread
```

Fix: #1368

Author: minggangw

src/rcl_subscription_bindings.cpp

@@ -283,18 +283,22 @@ Napi::Value SetContentFilter(const Napi::CallbackInfo& info) {
     free(argv);
   }
 
+  std::string error_string = "";
+  if (ret != RCL_RET_OK) {
+    error_string = rcl_get_error_string().str;
+    rcl_reset_error();
+  }
+
   rcl_ret_t fini_ret =
       rcl_subscription_content_filter_options_fini(subscription, &options);
 
   if (ret != RCL_RET_OK) {
-    std::string error_string = rcl_get_error_string().str;
-    rcl_reset_error();
     Napi::Error::New(env, error_string).ThrowAsJavaScriptException();
     return env.Undefined();
   }
 
   if (fini_ret != RCL_RET_OK) {
-    std::string error_string = rcl_get_error_string().str;
+    error_string = rcl_get_error_string().str;
     rcl_reset_error();
     Napi::Error::New(env, error_string).ThrowAsJavaScriptException();
     return env.Undefined();

test/test-subscription-content-filter.js

@@ -295,7 +295,10 @@ describe('subscription content-filtering', function () {
         const contentFilter5 = {
           expression: 'data = 5',
         };
+        // Stop spinning before changing content filter to avoid race condition
+        this.subscriberNode.stop();
         subscription.setContentFilter(contentFilter5);
+        this.subscriberNode.spin();
         resolve(msgCnt);
       }, SUBSCRIBER_WAIT_TIME)
     );
@@ -364,7 +367,10 @@ describe('subscription content-filtering', function () {
     const p1 = new Promise((resolve) =>
       setTimeout(() => {
         const result = !msgCnt0 && msgCnt5 && !fail;
+        // Stop spinning before changing content filter to avoid race condition
+        this.subscriberNode.stop();
         subscription.setContentFilter();
+        this.subscriberNode.spin();
         resolve(result);
       }, SUBSCRIBER_WAIT_TIME)
     );