Merge pull request #6052 from hercules-ci/issue-3294-fix-interruptCallback-deadlock

Fix deadlocked nix-daemon zombies on darwin #3294

src/libutil/util.cc

@@ -1565,7 +1565,22 @@ std::pair<unsigned short, unsigned short> getWindowSize()
 }
 
 
-static Sync<std::list<std::function<void()>>> _interruptCallbacks;
+/* We keep track of interrupt callbacks using integer tokens, so we can iterate
+   safely without having to lock the data structure while executing arbitrary
+   functions.
+ */
+struct InterruptCallbacks {
+    typedef int64_t Token;
+
+    /* We use unique tokens so that we can't accidentally delete the wrong
+       handler because of an erroneous double delete. */
+    Token nextToken = 0;
+
+    /* Used as a list, see InterruptCallbacks comment. */
+    std::map<Token, std::function<void()>> callbacks;
+};
+
+static Sync<InterruptCallbacks> _interruptCallbacks;
 
 static void signalHandlerThread(sigset_t set)
 {
@@ -1586,9 +1601,20 @@ void triggerInterrupt()
 {
     _isInterrupted = true;
 
+    {
+        InterruptCallbacks::Token i = 0;
+        while (true) {
+            std::function<void()> callback;
             {
                 auto interruptCallbacks(_interruptCallbacks.lock());
-        for (auto & callback : *interruptCallbacks) {
+                auto lb = interruptCallbacks->callbacks.lower_bound(i);
+                if (lb == interruptCallbacks->callbacks.end())
+                    break;
+
+                callback = lb->second;
+                i = lb->first + 1;
+            }
+
             try {
                 callback();
             } catch (...) {
@@ -1698,21 +1724,22 @@ void restoreProcessContext(bool restoreMounts)
 /* RAII helper to automatically deregister a callback. */
 struct InterruptCallbackImpl : InterruptCallback
 {
-    std::list<std::function<void()>>::iterator it;
+    InterruptCallbacks::Token token;
     ~InterruptCallbackImpl() override
     {
-        _interruptCallbacks.lock()->erase(it);
+        auto interruptCallbacks(_interruptCallbacks.lock());
+        interruptCallbacks->callbacks.erase(token);
     }
 };
 
 std::unique_ptr<InterruptCallback> createInterruptCallback(std::function<void()> callback)
 {
     auto interruptCallbacks(_interruptCallbacks.lock());
-    interruptCallbacks->push_back(callback);
+    auto token = interruptCallbacks->nextToken++;
+    interruptCallbacks->callbacks.emplace(token, callback);
 
     auto res = std::make_unique<InterruptCallbackImpl>();
-    res->it = interruptCallbacks->end();
+    res->token = token;
-    res->it--;
 
     return std::unique_ptr<InterruptCallback>(res.release());
 }