[Lldb-commits] [lldb] SBDebugger: Add new APIs `AddDestroyCallback` and `RemoveDestroyCallback` (PR #89868)

[via lldb-commits]

================
@@ -743,9 +743,28 @@ DebuggerSP Debugger::CreateInstance(lldb::LogOutputCallback log_callback,
 }
 
 void Debugger::HandleDestroyCallback() {
-  if (m_destroy_callback) {
-    m_destroy_callback(GetID(), m_destroy_callback_baton);
-    m_destroy_callback = nullptr;
+  std::lock_guard<std::recursive_mutex> guard(m_destroy_callback_mutex);
+  const lldb::user_id_t user_id = GetID();
+  // This loop handles the case where callbacks are added/removed by existing
+  // callbacks during the loop, as the following:
+  // - Added callbacks will always be invoked.
+  // - Removed callbacks will never be invoked. That is *unless* the loop
+  //   happens to invoke the said callbacks first, before they get removed.
+  //   In this case, the callbacks gets invoked, and the removal return false.
+  //
+  // In the removal case, because the order of the container is random, it's
+  // wise to not depend on the order and instead implement logic inside the
+  // callbacks to decide if their work should be skipped.
+  while (m_destroy_callback_and_baton.size()) {
+    auto iter = m_destroy_callback_and_baton.begin();
+    const lldb::destroy_callback_token_t &token = iter->first;
+    const lldb_private::DebuggerDestroyCallback &callback = iter->second.first;
+    void *const &baton = iter->second.second;
+    callback(user_id, baton);
+    // Using `token` to erase, because elements may have been added/removed, and
+    // that will cause error "invalid iterator access!" if `iter` is used
+    // instead.
+    m_destroy_callback_and_baton.erase(token);
----------------
jeffreytan81 wrote:

The biggest concern is that, this implementation completely changes the semantics -- now, client using this API has to re-register the callbacks after destroying debugger which is weird and not expected because there is no removeCallback() called from client. For example, the following user case won't work anymore:
```
void MyDestroyCallback() {
...
}

SBDebugger::AddDestroyCallbac(MyDestroyCallback);
SBDebugger::Create();
...
SBDebugger::Destroy();

// Recreate another debugger
SBDebugger::Create();
SBDebugger::Destroy();   // Now the MyDestroyCallback won't be called even user did not call RemoveDestroyCallback() which is not expected
```

There are several ways to handle this issue without clearing `m_destroy_callback_and_baton`. One simply way is making a copy of `m_destroy_callback_and_baton`, and calling callbacks from the copy (by checking if it still exists in original `m_destroy_callback_and_baton`). And at the end, checking there is no new entries in `m_destroy_callback_and_baton`, otherwise, getting the delta of the local copy and original copy, and redo the process in a loop.

https://github.com/llvm/llvm-project/pull/89868