Fix T102292: deadlock in geometry nodes evaluation with task isolation

As described in the comment on `BLI_task_isolate`, deadlocks can happen
when isolation is used with threading primitives that separate spawning tasks
from executing them. All threads are waiting the tasks to complete but no
thread is able to continue working due to task isolation.

The fix is to not pass lazy-threading hints through task isolations. This way
isolated regions can't create new tasks in a scheduler further up the call stack.
This may lead to minor slowdowns because less threading may be used.
It's generally possible to get rid of the slowdown again by sending the
lazy-threading hint before entering the isolated region.

1 files changed, 25 insertions, 5 deletions

diff --git a/source/blender/blenlib/intern/lazy_threading.cc b/source/blender/blenlib/intern/lazy_threading.cc
index 803fd81a96d..4f6d3a75ecc 100644
--- a/source/blender/blenlib/intern/lazy_threading.cc
+++ b/source/blender/blenlib/intern/lazy_threading.cc
@@ -1,30 +1,50 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
 #include "BLI_lazy_threading.hh"
+#include "BLI_stack.hh"
 #include "BLI_vector.hh"
 
 namespace blender::lazy_threading {
 
 /**
- * This is a #RawVector so that it can be destructed after Blender checks for memory leaks.
+ * This uses a "raw" stack and vector so that it can be destructed after Blender checks for memory
+ * leaks. A new list of receivers is created whenever an isolated region is entered to avoid
+ * deadlocks.
  */
-thread_local RawVector<FunctionRef<void()>, 0> hint_receivers;
+using HintReceivers = RawStack<RawVector<FunctionRef<void()>, 0>, 0>;
+thread_local HintReceivers hint_receivers = []() {
+  HintReceivers receivers;
+  /* Make sure there is always at least one vector. */
+  receivers.push_as();
+  return receivers;
+}();
 
 void send_hint()
 {
-  for (const FunctionRef<void()> &fn : hint_receivers) {
+  for (const FunctionRef<void()> &fn : hint_receivers.peek()) {
     fn();
   }
 }
 
 HintReceiver::HintReceiver(const FunctionRef<void()> fn)
 {
-  hint_receivers.append(fn);
+  hint_receivers.peek().append(fn);
 }
 
 HintReceiver::~HintReceiver()
 {
-  hint_receivers.pop_last();
+  hint_receivers.peek().pop_last();
+}
+
+ReceiverIsolation::ReceiverIsolation()
+{
+  hint_receivers.push_as();
+}
+
+ReceiverIsolation::~ReceiverIsolation()
+{
+  BLI_assert(hint_receivers.peek().is_empty());
+  hint_receivers.pop();
 }
 
 }  // namespace blender::lazy_threading