BACKPORT: MessagePumpIOSForIO: Fix handling of read+write one-shot events (#7789)

rakuco · web-flow · commit 2e86ea6f942b · 2025-10-30T15:15:55.000-07:00
This is a clean backport of https://crrev.com/c/6633541, which fixes a corner case issue in the I/O message pump used by tvOS. Bug: 432824509 --- Original CL description: MessagePumpIOSForIO had the same problem that https://crrev.com/c/6043387 fixed in MessagePumpEpoll: when watching read and write events in one-shot mode, the code was still dispatching both events even though the contract is that only one will fire. Apply the fix from the CL above by taking into account whether we have a persistent watcher or not when deciding whether to fire both events. This fixes IOWatcherFdTest.ReadWriteUnifiedOneShot on tvOS. Change-Id: I0e7adf7e16a04af45b947f15bfc7dd616bb5408e Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6633541 Cr-Commit-Position: refs/heads/main@{#1472485}
diff --git a/base/message_loop/message_pump_io_ios.cc b/base/message_loop/message_pump_io_ios.cc
@@ -168,23 +168,32 @@ void MessagePumpIOSForIO::HandleFdIOEvent(CFFileDescriptorRef fdref,
     scoped_do_work_item = pump->delegate()->BeginWorkItem();
   }
 
-  if (callback_types & kCFFileDescriptorWriteCallBack) {
+  // When the watcher is in one-shot mode (i.e. `is_persistent` is false) and
+  // the FD watcher is watching both read and write events, the contract is that
+  // only one will be reported (which one is chosen does not matter).
+  // This implementation reports writes before reads, so `can_read` is true iff
+  // the watcher is not in one-shot mode or no write event is being reported.
+  const bool is_persistent = controller->is_persistent_;
+  const bool can_write = callback_types & kCFFileDescriptorWriteCallBack;
+  const bool can_read = callback_types & kCFFileDescriptorReadCallBack &&
+                        (is_persistent || !can_write);
+
+  if (can_write) {
     controller->OnFileCanWriteWithoutBlocking(fd, pump);
   }
 
   // Perform the read callback only if the file descriptor has not been
   // invalidated in the write callback. As |FdWatchController| invalidates
   // its file descriptor on destruction, the file descriptor being valid also
   // guarantees that |controller| has not been deleted.
-  if (callback_types & kCFFileDescriptorReadCallBack &&
-      CFFileDescriptorIsValid(fdref)) {
+  if (can_read && CFFileDescriptorIsValid(fdref)) {
     DCHECK_EQ(fdref, controller->fdref_.get());
     controller->OnFileCanReadWithoutBlocking(fd, pump);
   }
 
   // Re-enable callbacks after the read/write if the file descriptor is still
   // valid and the controller is persistent.
-  if (CFFileDescriptorIsValid(fdref) && controller->is_persistent_) {
+  if (CFFileDescriptorIsValid(fdref) && is_persistent) {
     DCHECK_EQ(fdref, controller->fdref_.get());
     CFFileDescriptorEnableCallBacks(fdref, callback_types);
   }
