File system watcher returns zero events despite successful FSEvents API initialization

[cbenhagen]

Summary

Dart's FSEvents implementation on macOS fails to deliver file system events on some machines. To investigate this issue, I added debug logging to runtime/bin/file_system_watcher_macos.cc. The debug output shows errno=32 (Broken pipe) errors when FSEvents callbacks attempt to write event data to the internal pipe. This results in zero events being delivered to applications despite FSEvents API functioning correctly.

Environment

• Platform: macOS 15.7
• Architecture: ARM64 (Apple Silicon)
• Dart SDK versions: Several versions from 2.12 to 3.9. Still present in latest version built from source (latest main branch)
• Affected Component: runtime/bin/file_system_watcher_macos.cc

Observed Behavior

Debug Output Analysis

On failing machines, the debug output shows:

1. FSEvents API initializes successfully
2. FSEvents callbacks are triggered
3. Event processing completes normally
4. Pipe write operation fails with errno=32 (Broken pipe)
5. No events reach the Dart application (0 events received)

On working machines, all pipe write operations succeed and events are delivered normally.

Reproduction

Test Case

The test script performs normal file operations (create, modify, delete) and waits for events. On some machines, no events are received.

Reproduction Script

import 'dart:async';
import 'dart:io';

void main() async {
  print('=== FSEvents Debug Test ===');
  
  var tempDir = await Directory.systemTemp.createTemp('fsevents_test_');
  print('Created temp directory: ${tempDir.path}');
  
  print('Starting native directory watcher...');
  var watcher = tempDir.watch();
  var eventsReceived = 0;
  
  var subscription = watcher.listen((event) {
    eventsReceived++;
    print('DART EVENT #$eventsReceived: ${event.type.index} - ${event.path}');
  });
  
  print('Waiting for watcher to initialize...');
  await Future.delayed(Duration(milliseconds: 100));
  
  print('Creating test file...');
  var testFile = File('${tempDir.path}/test_file.txt');
  await testFile.writeAsString('test content');
  
  print('Modifying test file...');
  await testFile.writeAsString('modified content', mode: FileMode.append);
  
  print('Deleting test file...');
  await testFile.delete();
  
  print('Waiting for final events...');
  await Future.delayed(Duration(milliseconds: 100));
  
  print('Cancelling watcher...');
  await subscription.cancel();
  
  print('\n=== Test Results ===');
  print('Events received: $eventsReceived');
  
  if (eventsReceived > 0) {
    print('✅ SUCCESS: FSEvents are working - received $eventsReceived events');
  } else {
    print('❌ FAILURE: No FSEvents received - file watcher is not working');
  }
  
  print('\nCleaning up temp directory...');
  try {
    await tempDir.delete(recursive: true);
  } catch (e) {
    print('Cleanup error: $e');
  }
  
  print('\n=== Test Complete ===');
  print('Final result: ${eventsReceived > 0 ? "PASSED" : "FAILED"}');
}

Test Results

Working Machine

=== FSEvents Debug Test ===
Created temp directory: /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h
Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h', events=15, recursive=1
[FSEVENTS DEBUG] Resolved path: '/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h' -> '/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x12c604150
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h', flags=0x00028100
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] Writing event to pipe: path='', flags=0x00028100, exists=1
[FSEVENTS DEBUG] Write result: 1032 bytes (expected 1032) - SUCCESS
DART EVENT #1: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h
DART EVENT #2: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h
Creating test file...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt', flags=0x00019100
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt -> true
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] Writing event to pipe: path='test_file.txt', flags=0x00019100, exists=1
[FSEVENTS DEBUG] Write result: 1032 bytes (expected 1032) - SUCCESS
DART EVENT #3: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #4: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #5: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
Modifying test file...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt', flags=0x00019500
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt -> true
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] Writing event to pipe: path='test_file.txt', flags=0x00019500, exists=1
[FSEVENTS DEBUG] Write result: 1032 bytes (expected 1032) - SUCCESS
DART EVENT #6: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #7: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #8: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
Deleting test file...
Waiting for final events...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt', flags=0x00019700
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt -> false
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] Writing event to pipe: path='test_file.txt', flags=0x00019700, exists=0
[FSEVENTS DEBUG] Write result: 1032 bytes (expected 1032) - SUCCESS
DART EVENT #9: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #10: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #11: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
DART EVENT #12: 4 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_Ibwa5h/test_file.txt
Cancelling watcher...

=== Test Results ===
Events received: 12
✅ SUCCESS: FSEvents are working - received 12 events

Cleaning up temp directory...

=== Test Complete ===

Failing Machine

=== FSEvents Debug Test ===
Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un
Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un', events=15, recursive=1
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x152304280
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_8Rs9un -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] Writing event to pipe: path='', flags=0x00020100, exists=1
Creating test file...
Modifying test file...
Deleting test file...
Waiting for final events...
Cancelling watcher...
[FSEVENTS DEBUG] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)

=== Test Results ===
Events received: 0
❌ FAILURE: No FSEvents received - file watcher is not working

Cleaning up temp directory...

=== Test Complete ===
Final result: FAILED

Debug Infrastructure Added

To investigate this issue, comprehensive debug logging was added to runtime/bin/file_system_watcher_macos.cc. The modifications include:

Debug Output Added

• FSEvents stream creation and initialization status
• Callback invocation with event counts
• Detailed event processing (paths, flags, existence checks)
• Pipe write operations with success/failure status and errno details
• Path resolution and processing steps

Key Debug Modifications

diff --git a/runtime/bin/file_system_watcher_macos.cc b/runtime/bin/file_system_watcher_macos.cc
index 4e4fb47ac15..d50605ba2d6 100644
--- a/runtime/bin/file_system_watcher_macos.cc
+++ b/runtime/bin/file_system_watcher_macos.cc
@@ -86,6 +86,11 @@ class FSEventsWatcher {
     void set_ref(FSEventStreamRef ref) { ref_ = ref; }
 
     void Start() {
+      // DEBUG: Log start of FSEvents setup
+      printf("[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=%ld, recursive=%d\n", 
+             base_path_length_, recursive_);
+      fflush(stdout);
+      
       FSEventStreamContext context;
       memset(&context, 0, sizeof(context));
       context.info = reinterpret_cast<void*>(this);
@@ -99,13 +104,37 @@ class FSEventsWatcher {
           0.10, kFSEventStreamCreateFlagFileEvents);
       CFRelease(array);
 
+      // DEBUG: Log FSEventStream creation result
+      if (ref == nullptr) {
+        printf("[FSEVENTS DEBUG] CRITICAL: FSEventStreamCreate returned NULL!\n");
+        fflush(stdout);
+        return;
+      } else {
+        printf("[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=%p\n", ref);
+        fflush(stdout);
+      }
+
       set_ref(ref);
 
       FSEventStreamScheduleWithRunLoop(ref_, watcher_->run_loop_,
                                        kCFRunLoopDefaultMode);
-
-      FSEventStreamStart(ref_);
+      
+      // DEBUG: Log FSEventStream scheduling result
+      printf("[FSEVENTS DEBUG] FSEventStream scheduled with run loop\n");
+      fflush(stdout);
+
+      Boolean started = FSEventStreamStart(ref_);
+      
+      // DEBUG: Log FSEventStream start result
+      printf("[FSEVENTS DEBUG] FSEventStreamStart result: %s\n", 
+             started ? "SUCCESS" : "FAILED");
+      fflush(stdout);
+      
       FSEventStreamFlushSync(ref_);
+      
+      // DEBUG: Log completion
+      printf("[FSEVENTS DEBUG] FSEventStream setup completed\n");
+      fflush(stdout);
     }
 
     void Stop() {
@@ -205,6 +234,11 @@ class FSEventsWatcher {
   }
 
   Node* AddPath(const char* path, int events, bool recursive) {
+    // DEBUG: Log path addition
+    printf("[FSEVENTS DEBUG] AddPath called: path='%s', events=%d, recursive=%d\n", 
+           path, events, recursive);
+    fflush(stdout);
+    
     int fds[2];
     VOID_NO_RETRY_EXPECTED(pipe(fds));
     FDUtils::SetNonBlocking(fds[0]);
@@ -212,6 +246,11 @@ class FSEventsWatcher {
 
     char base_path[PATH_MAX];
     realpath(path, base_path);
+    
+    // DEBUG: Log resolved path
+    printf("[FSEVENTS DEBUG] Resolved path: '%s' -> '%s'\n", path, base_path);
+    printf("[FSEVENTS DEBUG] Creating Node with read_fd=%d, write_fd=%d\n", fds[0], fds[1]);
+    fflush(stdout);
 
     return new Node(this, base_path, fds[0], fds[1], recursive);
   }
@@ -223,6 +262,10 @@ class FSEventsWatcher {
                        void* event_paths,
                        const FSEventStreamEventFlags event_flags[],
                        const FSEventStreamEventId event_ids[]) {
+    // DEBUG: Log callback entry
+    printf("[FSEVENTS DEBUG] Callback triggered with %zu events\n", num_events);
+    fflush(stdout);
+    
     if (FileSystemWatcher::delayed_filewatch_callback()) {
       // Used in tests to highlight race between callback invocation
       // and unwatching the file path, Node destruction
@@ -231,22 +274,63 @@ class FSEventsWatcher {
     Node* node = static_cast<Node*>(client);
     RELEASE_ASSERT(node->watcher() != nullptr);
     DEBUG_ASSERT(node->watcher()->owner_.IsOwnedByCurrentThread());
+    
+    // DEBUG: Log node info
+    printf("[FSEVENTS DEBUG] Node info: base_path_length=%ld, recursive=%d\n", 
+           node->base_path_length(), node->recursive());
+    fflush(stdout);
+    
     for (size_t i = 0; i < num_events; i++) {
       char* path = reinterpret_cast<char**>(event_paths)[i];
       FSEvent event;
+      
+      // DEBUG: Log original path and flags
+      printf("[FSEVENTS DEBUG] Event %zu: original_path='%s', flags=0x%08x\n", 
+             i, path, event_flags[i]);
+      fflush(stdout);
+      
       event.data.exists =
           File::GetType(nullptr, path, false) != File::kDoesNotExist;
+      
+      // DEBUG: Log file existence check
+      printf("[FSEVENTS DEBUG] File exists check: %s -> %s\n", 
+             path, event.data.exists ? "true" : "false");
+      fflush(stdout);
+      
       path += node->base_path_length();
       // If path is longer the base, skip next character ('/').
       if (path[0] != '\0') {
         path += 1;
       }
+      
+      // DEBUG: Log processed path
+      printf("[FSEVENTS DEBUG] Processed relative path: '%s'\n", path);
+      fflush(stdout);
+      
       if (!node->recursive() && (strstr(path, "/") != nullptr)) {
+        printf("[FSEVENTS DEBUG] Skipping non-recursive path: '%s'\n", path);
+        fflush(stdout);
         continue;
       }
       event.data.flags = event_flags[i];
       memmove(event.data.path, path, strlen(path) + 1);
-      write(node->write_fd(), event.bytes, sizeof(event));
+      
+      // DEBUG: Log write attempt
+      printf("[FSEVENTS DEBUG] Writing event to pipe: path='%s', flags=0x%08x, exists=%d\n",
+             event.data.path, event.data.flags, event.data.exists);
+      fflush(stdout);
+      
+      ssize_t written = write(node->write_fd(), event.bytes, sizeof(event));
+      
+      // DEBUG: Log write result with errno
+      if (written == sizeof(event)) {
+        printf("[FSEVENTS DEBUG] Write result: %ld bytes (expected %zu) - SUCCESS\n", 
+               written, sizeof(event));
+      } else {
+        printf("[FSEVENTS DEBUG] Write result: %ld bytes (expected %zu) - ERROR: errno=%d (%s)\n", 
+               written, sizeof(event), errno, strerror(errno));
+      }
+      fflush(stdout);
     }
   }

[mraleph]

Hmm. Hard to say what is really wrong here. I noticed that there is no error checking after pipe in AddPath. Could you add a check for return value and errno and dump an error there if it occurs? Could you also dump fd numbers for both read and write direction?

[cbenhagen]

@mraleph thanks for the quick feedback! Here is the output with the following changes:

    int pipe_result = pipe(fds);
    
    if (pipe_result != 0) {
      printf("[FSEVENTS DEBUG] ERROR: pipe() failed with return value %d, errno=%d (%s)\n", 
             pipe_result, errno, strerror(errno));
      fflush(stdout);
      return nullptr;
    } else {
      printf("[FSEVENTS DEBUG] pipe() succeeded: read_fd=%d, write_fd=%d\n", fds[0], fds[1]);
      fflush(stdout);
    }
    

=== FSEvents Debug Test ===
Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B
Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x110f04080
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_CA802B -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] Writing event to pipe: path='', flags=0x00020100, exists=1
Creating test file...
Modifying test file...
Deleting test file...
Waiting for final events...
Cancelling watcher...
[FSEVENTS DEBUG] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)

=== Test Results ===
Events received: 0
❌ FAILURE: No FSEvents received - file watcher is not working

Cleaning up temp directory...

=== Test Complete ===
Final result: FAILED

[cbenhagen]

Adding some more debug output before writing to the pipe. Here is the full diff and output:

diff --git a/runtime/bin/file_system_watcher_macos.cc b/runtime/bin/file_system_watcher_macos.cc
index 4e4fb47ac15..5c281697077 100644
--- a/runtime/bin/file_system_watcher_macos.cc
+++ b/runtime/bin/file_system_watcher_macos.cc
@@ -86,6 +86,11 @@ class FSEventsWatcher {
     void set_ref(FSEventStreamRef ref) { ref_ = ref; }
 
     void Start() {
+      // DEBUG: Log start of FSEvents setup
+      printf("[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=%ld, recursive=%d\n", 
+             base_path_length_, recursive_);
+      fflush(stdout);
+      
       FSEventStreamContext context;
       memset(&context, 0, sizeof(context));
       context.info = reinterpret_cast<void*>(this);
@@ -99,13 +104,37 @@ class FSEventsWatcher {
           0.10, kFSEventStreamCreateFlagFileEvents);
       CFRelease(array);
 
+      // DEBUG: Log FSEventStream creation result
+      if (ref == nullptr) {
+        printf("[FSEVENTS DEBUG] CRITICAL: FSEventStreamCreate returned NULL!\n");
+        fflush(stdout);
+        return;
+      } else {
+        printf("[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=%p\n", ref);
+        fflush(stdout);
+      }
+
       set_ref(ref);
 
       FSEventStreamScheduleWithRunLoop(ref_, watcher_->run_loop_,
                                        kCFRunLoopDefaultMode);
-
-      FSEventStreamStart(ref_);
+      
+      // DEBUG: Log FSEventStream scheduling result
+      printf("[FSEVENTS DEBUG] FSEventStream scheduled with run loop\n");
+      fflush(stdout);
+
+      Boolean started = FSEventStreamStart(ref_);
+      
+      // DEBUG: Log FSEventStream start result
+      printf("[FSEVENTS DEBUG] FSEventStreamStart result: %s\n", 
+             started ? "SUCCESS" : "FAILED");
+      fflush(stdout);
+      
       FSEventStreamFlushSync(ref_);
+      
+      // DEBUG: Log completion
+      printf("[FSEVENTS DEBUG] FSEventStream setup completed\n");
+      fflush(stdout);
     }
 
     void Stop() {
@@ -205,13 +234,34 @@ class FSEventsWatcher {
   }
 
   Node* AddPath(const char* path, int events, bool recursive) {
+    // DEBUG: Log path addition
+    printf("[FSEVENTS DEBUG] AddPath called: path='%s', events=%d, recursive=%d\n", 
+           path, events, recursive);
+    fflush(stdout);
+    
     int fds[2];
-    VOID_NO_RETRY_EXPECTED(pipe(fds));
+    int pipe_result = pipe(fds);
+    
+    if (pipe_result != 0) {
+      printf("[FSEVENTS DEBUG] ERROR: pipe() failed with return value %d, errno=%d (%s)\n", 
+             pipe_result, errno, strerror(errno));
+      fflush(stdout);
+      return nullptr;  // Return nullptr to indicate failure
+    } else {
+      printf("[FSEVENTS DEBUG] pipe() succeeded: read_fd=%d, write_fd=%d\n", fds[0], fds[1]);
+      fflush(stdout);
+    }
+    
     FDUtils::SetNonBlocking(fds[0]);
     FDUtils::SetBlocking(fds[1]);
 
     char base_path[PATH_MAX];
     realpath(path, base_path);
+    
+    // DEBUG: Log resolved path
+    printf("[FSEVENTS DEBUG] Resolved path: '%s' -> '%s'\n", path, base_path);
+    printf("[FSEVENTS DEBUG] Creating Node with read_fd=%d, write_fd=%d\n", fds[0], fds[1]);
+    fflush(stdout);
 
     return new Node(this, base_path, fds[0], fds[1], recursive);
   }
@@ -223,6 +273,10 @@ class FSEventsWatcher {
                        void* event_paths,
                        const FSEventStreamEventFlags event_flags[],
                        const FSEventStreamEventId event_ids[]) {
+    // DEBUG: Log callback entry
+    printf("[FSEVENTS DEBUG] Callback triggered with %zu events\n", num_events);
+    fflush(stdout);
+    
     if (FileSystemWatcher::delayed_filewatch_callback()) {
       // Used in tests to highlight race between callback invocation
       // and unwatching the file path, Node destruction
@@ -231,22 +285,78 @@ class FSEventsWatcher {
     Node* node = static_cast<Node*>(client);
     RELEASE_ASSERT(node->watcher() != nullptr);
     DEBUG_ASSERT(node->watcher()->owner_.IsOwnedByCurrentThread());
+    
+    // DEBUG: Log node info
+    printf("[FSEVENTS DEBUG] Node info: base_path_length=%ld, recursive=%d\n", 
+           node->base_path_length(), node->recursive());
+    fflush(stdout);
+    
     for (size_t i = 0; i < num_events; i++) {
       char* path = reinterpret_cast<char**>(event_paths)[i];
       FSEvent event;
+      
+      // DEBUG: Log original path and flags
+      printf("[FSEVENTS DEBUG] Event %zu: original_path='%s', flags=0x%08x\n", 
+             i, path, event_flags[i]);
+      fflush(stdout);
+      
       event.data.exists =
           File::GetType(nullptr, path, false) != File::kDoesNotExist;
+      
+      // DEBUG: Log file existence check
+      printf("[FSEVENTS DEBUG] File exists check: %s -> %s\n", 
+             path, event.data.exists ? "true" : "false");
+      fflush(stdout);
+      
       path += node->base_path_length();
       // If path is longer the base, skip next character ('/').
       if (path[0] != '\0') {
         path += 1;
       }
+      
+      // DEBUG: Log processed path
+      printf("[FSEVENTS DEBUG] Processed relative path: '%s'\n", path);
+      fflush(stdout);
+      
       if (!node->recursive() && (strstr(path, "/") != nullptr)) {
+        printf("[FSEVENTS DEBUG] Skipping non-recursive path: '%s'\n", path);
+        fflush(stdout);
         continue;
       }
       event.data.flags = event_flags[i];
       memmove(event.data.path, path, strlen(path) + 1);
-      write(node->write_fd(), event.bytes, sizeof(event));
+      
+      // DEBUG: Get current timestamp
+      struct timespec ts;
+      clock_gettime(CLOCK_MONOTONIC, &ts);
+      
+      // DEBUG: Check pipe status before writing
+      int write_flags = fcntl(node->write_fd(), F_GETFL);
+      int write_errno = errno;
+      
+      printf("[FSEVENTS DEBUG] [%ld.%06ld] Pre-write check: write_fd=%d, fcntl_flags=%d, errno=%d\n", 
+             ts.tv_sec, ts.tv_nsec / 1000, node->write_fd(), write_flags, write_errno);
+      
+      // DEBUG: Log write attempt
+      printf("[FSEVENTS DEBUG] [%ld.%06ld] Writing event to pipe: path='%s', flags=0x%08x, exists=%d\n",
+             ts.tv_sec, ts.tv_nsec / 1000, event.data.path, event.data.flags, event.data.exists);
+      fflush(stdout);
+      
+      ssize_t written = write(node->write_fd(), event.bytes, sizeof(event));
+      int write_result_errno = errno;
+      
+      // DEBUG: Get timestamp after write
+      clock_gettime(CLOCK_MONOTONIC, &ts);
+      
+      // DEBUG: Log write result with errno and timestamp
+      if (written == sizeof(event)) {
+        printf("[FSEVENTS DEBUG] [%ld.%06ld] Write result: %ld bytes (expected %zu) - SUCCESS\n", 
+               ts.tv_sec, ts.tv_nsec / 1000, written, sizeof(event));
+      } else {
+        printf("[FSEVENTS DEBUG] [%ld.%06ld] Write result: %ld bytes (expected %zu) - ERROR: errno=%d (%s)\n", 
+               ts.tv_sec, ts.tv_nsec / 1000, written, sizeof(event), write_result_errno, strerror(write_result_errno));
+      }
+      fflush(stdout);
     }
   }

=== FSEvents Debug Test ===
Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N
Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x12ac04540
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_q6Cn1N -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] [180797.729831] Pre-write check: write_fd=7, fcntl_flags=1, errno=0
[FSEVENTS DEBUG] [180797.729831] Writing event to pipe: path='', flags=0x00020100, exists=1
Creating test file...
Modifying test file...
Deleting test file...
Waiting for final events...
Cancelling watcher...
[FSEVENTS DEBUG] [180801.133004] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)

=== Test Results ===
Events received: 0
❌ FAILURE: No FSEvents received - file watcher is not working

Cleaning up temp directory...

=== Test Complete ===
Final result: FAILED

Happy to run whatever other tests that could help you analyze the issue.

[mraleph]

So I think based on the output the only option I can think about is the race between Callback firing and you cancelling the subscription to file system events. In which case I think the code is working as expected: cancellation is synchronous (e.g. once you call cancel on the subscription the underlying Stream is not supposed to emit any more events).

Can you try increasing timeout in your test (e.g. to 1 second) and see what happens?

[mraleph]

Adding some more debug output before writing to the pipe. Here is the full diff and output:

I wrote my previous comment before seeing additional output you have provided. So it does look like a race between Callback and cancel. We are cancelling before we receive the event so we don't get it.

[cbenhagen]

@mraleph There already was a 2 second wait before we cancel. I increased it to 30s

// Wait for any final events
print('Waiting for final events...');
await Future.delayed(Duration(seconds: 30));

print('Cancelling watcher...');
await eventSubscription.cancel();

Also added more timestamps to the output:

[1758540899.434266] === FSEvents Debug Test ===
[1758540899.434768] Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx
[1758540899.434782] Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x142c04210
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
[1758540899.435824] Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_LAIZdx -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] [1758540899.549186] Pre-write check: write_fd=7, fcntl_flags=1, errno=0
[FSEVENTS DEBUG] [1758540899.549186] Writing event to pipe: path='', flags=0x00020100, exists=1
[1758540899.939383] Creating test file...
[1758540900.441171] Modifying test file...
[1758540900.945448] Deleting test file...
[1758540900.946027] Waiting for final events...
[1758540930.949338] Cancelling watcher...
[FSEVENTS DEBUG] [1758540930.949800] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)
[1758540930.950005]
=== Test Results ===
[1758540930.950031] Events received: 0
[1758540930.950039] ❌ FAILURE: No FSEvents received - file watcher is not working
[1758540930.950053]
Cleaning up temp directory...
[1758540930.950858]
=== Test Complete ===
[1758540930.950879] Final result: FAILED

[mraleph]

[FSEVENTS DEBUG] [1758540899.549186] Writing event to pipe: path='', flags=0x00020100, exists=1
...
[1758540900.946027] Waiting for final events...
[1758540930.949338] Cancelling watcher...
[FSEVENTS DEBUG] [1758540930.949800] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)

Ok so from the looks of it write just blocked until we closed the whole thing... There should be enough space to write 1032 bytes without blocking.

I wonder what is happening on the read side? Can you try compiling with #define KQUEUE_DEBUG 1 at the top of eventhandler_macos.cc and run it again.

Is there anything in common between all failing machines? e.g. all failing machines run the same kernel version and all working machines run a different kernel version?

I am wondering if this is some wonky bug in kqueue implementation for pipes/fifos on Mac OS X.

[cbenhagen]

@mraleph I currently only have access to one failing machine but remember that this happened on others. Both this and my dev machine run the exact same macOS version. When updating from macOS 15.6 to 15.7 it worked until some reboots later it stopped working again. Using fswatch or similar tools work without issues.

Here is the new debug output:

[1758543258.678290] === FSEvents Debug Test ===
[1758543258.678819] Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK
[1758543258.678833] Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x139704080
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
[1758543258.679998] Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_pVzLTK -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] [1758543258.793132] Pre-write check: write_fd=7, fcntl_flags=1, errno=0
[FSEVENTS DEBUG] [1758543258.793132] Writing event to pipe: path='', flags=0x00020100, exists=1
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 512 (available 512)
[1758543259.181693] Creating test file...
[1758543259.685064] Modifying test file...
[1758543260.189170] Deleting test file...
[1758543260.189749] Waiting for final events...
[1758543262.192937] Cancelling watcher...
[FSEVENTS DEBUG] [1758543262.193265] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)
[1758543262.193480]
=== Test Results ===
[1758543262.193508] Events received: 0
[1758543262.193515] ❌ FAILURE: No FSEvents received - file watcher is not working
[1758543262.193530]
Cleaning up temp directory...
[1758543262.194315]
=== Test Complete ===
[1758543262.194339] Final result: FAILED

[cbenhagen]

Output from the working machine:

[1758543596.002392] === FSEvents Debug Test ===
[1758543596.002696] Created temp directory: /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG
[1758543596.002704] Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG' -> '/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x159010360
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
[1758543596.003297] Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG', flags=0x00028100
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] [1758543596.115022] Pre-write check: write_fd=7, fcntl_flags=1, errno=0
[FSEVENTS DEBUG] [1758543596.115022] Writing event to pipe: path='', flags=0x00028100, exists=1
[FSEVENTS DEBUG] [1758543596.115029] Write result: 1032 bytes (expected 1032) - SUCCESS
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 1032 (available 1032) 
[1758543596.115167] DART EVENT #1: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG
[1758543596.115177] DART EVENT #2: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG
[1758543596.504823] Creating test file...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt', flags=0x00019100
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt -> true
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] [1758543596.616701] Pre-write check: write_fd=7, fcntl_flags=65537, errno=0
[FSEVENTS DEBUG] [1758543596.616701] Writing event to pipe: path='test_file.txt', flags=0x00019100, exists=1
[FSEVENTS DEBUG] [1758543596.616710] Write result: 1032 bytes (expected 1032) - SUCCESS
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 1032 (available 1032) 
[1758543596.616791] DART EVENT #3: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543596.616800] DART EVENT #4: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543596.616802] DART EVENT #5: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.005872] Modifying test file...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt', flags=0x00019500
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt -> true
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] [1758543597.118506] Pre-write check: write_fd=7, fcntl_flags=65537, errno=0
[FSEVENTS DEBUG] [1758543597.118506] Writing event to pipe: path='test_file.txt', flags=0x00019500, exists=1
[FSEVENTS DEBUG] [1758543597.118513] Write result: 1032 bytes (expected 1032) - SUCCESS
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 1032 (available 1032) 
[1758543597.118583] DART EVENT #6: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.118593] DART EVENT #7: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.118595] DART EVENT #8: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.507612] Deleting test file...
[1758543597.507911] Waiting for final events...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt', flags=0x00019700
[FSEVENTS DEBUG] File exists check: /private/var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt -> false
[FSEVENTS DEBUG] Processed relative path: 'test_file.txt'
[FSEVENTS DEBUG] [1758543597.611902] Pre-write check: write_fd=7, fcntl_flags=65537, errno=2
[FSEVENTS DEBUG] [1758543597.611902] Writing event to pipe: path='test_file.txt', flags=0x00019700, exists=0
[FSEVENTS DEBUG] [1758543597.611909] Write result: 1032 bytes (expected 1032) - SUCCESS
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 1032 (available 1032) 
[1758543597.611990] DART EVENT #9: 1 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.611999] DART EVENT #10: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.612001] DART EVENT #11: 2 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543597.612002] DART EVENT #12: 4 - /var/folders/k8/wkdccllx2718jbgb_5qm5dsh0000gn/T/fsevents_test_3HZNpG/test_file.txt
[1758543599.510013] Cancelling watcher...
6 filter=0xffffffff:EVFILT_READ flags: 8021: EV_EOF EV_CLEAR EV_ADD - fflags: 0 - data: 0 (available 0) 
[1758543599.510235] 
=== Test Results ===
[1758543599.510246] Events received: 12
[1758543599.510249] ✅ SUCCESS: FSEvents are working - received 12 events
[1758543599.510252] 
Cleaning up temp directory...
[1758543599.510552] 
=== Test Complete ===
[1758543599.510562] Final result: PASSED

[cbenhagen]

Adding some more debug output:

Dart_Handle FileSystemWatcher::ReadEvents(intptr_t id, intptr_t path_id) {
  intptr_t fd = GetSocketId(id, path_id);
  intptr_t avail = FDUtils::AvailableBytes(fd);
  int count = avail / sizeof(FSEvent);
  
  // DEBUG: Log ReadEvents call
  struct timespec ts;
  clock_gettime(CLOCK_REALTIME, &ts);
  printf("[FSEVENTS DEBUG] [%ld.%06ld] ReadEvents called: fd=%ld, avail=%ld bytes, sizeof(FSEvent)=%zu, count=%d\n", 
         ts.tv_sec, ts.tv_nsec / 1000, fd, avail, sizeof(FSEvent), count);
  fflush(stdout);
  
  if (count <= 0) {
    printf("[FSEVENTS DEBUG] [%ld.%06ld] ReadEvents returning empty list (count=%d)\n", 
           ts.tv_sec, ts.tv_nsec / 1000, count);
    fflush(stdout);
    return Dart_NewList(0);
  }

[1758544112.110553] === FSEvents Debug Test ===
[1758544112.111064] Created temp directory: /var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p
[1758544112.111081] Starting native directory watcher...
[FSEVENTS DEBUG] AddPath called: path='/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p', events=15, recursive=1
[FSEVENTS DEBUG] pipe() succeeded: read_fd=6, write_fd=7
[FSEVENTS DEBUG] Resolved path: '/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p' -> '/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p'
[FSEVENTS DEBUG] Creating Node with read_fd=6, write_fd=7
[FSEVENTS DEBUG] Starting FSEvents for path: base_path_length=77, recursive=1
[FSEVENTS DEBUG] FSEventStreamCreate succeeded, stream=0x148105250
[FSEVENTS DEBUG] FSEventStream scheduled with run loop
[FSEVENTS DEBUG] FSEventStreamStart result: SUCCESS
[FSEVENTS DEBUG] FSEventStream setup completed
[1758544112.112121] Waiting for watcher to initialize...
[FSEVENTS DEBUG] Callback triggered with 1 events
[FSEVENTS DEBUG] Node info: base_path_length=77, recursive=1
[FSEVENTS DEBUG] Event 0: original_path='/private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p', flags=0x00020100
[FSEVENTS DEBUG] File exists check: /private/var/folders/4l/qvvv84514t10q9br028s07xh0000gn/T/fsevents_test_iAz18p -> true
[FSEVENTS DEBUG] Processed relative path: ''
[FSEVENTS DEBUG] [1758544112.225497] Pre-write check: write_fd=7, fcntl_flags=1, errno=0
[FSEVENTS DEBUG] [1758544112.225497] Writing event to pipe: path='', flags=0x00020100, exists=1
6 filter=0xffffffff:EVFILT_READ flags: 21: EV_CLEAR EV_ADD - fflags: 0 - data: 512 (available 512)
[FSEVENTS DEBUG] [1758544112.225711] ReadEvents called: fd=6, avail=512 bytes, sizeof(FSEvent)=1032, count=0
[FSEVENTS DEBUG] [1758544112.225711] ReadEvents returning empty list (count=0)
[1758544112.614716] Creating test file...
[1758544113.119190] Modifying test file...
[1758544113.623303] Deleting test file...
[1758544113.623851] Waiting for final events...
[1758544115.627059] Cancelling watcher...
[FSEVENTS DEBUG] [1758544115.627429] Write result: -1 bytes (expected 1032) - ERROR: errno=32 (Broken pipe)
[1758544115.627638]
=== Test Results ===
[1758544115.627666] Events received: 0
[1758544115.627674] ❌ FAILURE: No FSEvents received - file watcher is not working
[1758544115.627690]
Cleaning up temp directory...
[1758544115.628433]
=== Test Complete ===
[1758544115.628455] Final result: FAILED

[cbenhagen]

I found something interesting by running pipebuf-constants as well as measure-pipebuf from https://github.com/afborchert/pipebuf on both machines.

Good machine

 ./pipebuf-constants
PIPE_BUF = 512
_PC_PIPE_BUF = 512

./measure-pipebuf
pipe buffer size = 65536

Bad machine

./pipebuf-constants
PIPE_BUF = 512
_PC_PIPE_BUF = 512

./measure-pipebuf
pipe buffer size = 512

[mraleph]

Yeah, now it makes sense. We should rewrite this code defensively to not expect pipe buffer to be large enough to supprot atomic reads.

[cbenhagen]

Sounds like a plan! Do you have time to come up with a something?

[mraleph]

@cbenhagen could you try applying patch (this is just a download link for the CL). Does it fix the problem?