From 5c34248d00259fff352138e335dc2ab7c85279ce Mon Sep 17 00:00:00 2001
From: reuk <reuk@users.noreply.github.com>
Date: Tue, 3 Jun 2025 17:41:52 +0100
Subject: [PATCH] PluginListComponent: Wait for current scan to complete before
 calling owner.scanFinished()

I observed a deadlock when scanning AU plugins in-process in the
AudioPluginHost, and then clicking the "cancel" button in the scan
progress alert window.

The cause of the deadlock seems to be that JUCE uses async messages to
create and destroy AU plugins on the main thread. When running a plugin
scanner on a background thread, it was possible to end up in a situation
where the background thread was waiting on the message thread to process
a create/destroy message; and, at the same time, the main thread was
blocked waiting for all scan jobs to complete. This seemed to happen
because scanFinished() was called directly from the Scanner's
timerCallback as soon as the progress window was cancelled, even if
there was still a scan in progress at that point.

To avoid the deadlock, we now wait until the current scan has completely
finished before allowing the timerCallback to call scanFinished(). If no
scan is in progress, then the main thread can safely destroy the scanner
ThreadPool without needing to wait at that point.
---
 .../scanning/juce_PluginListComponent.cpp     | 24 ++++++++++++-------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/modules/juce_audio_processors/scanning/juce_PluginListComponent.cpp b/modules/juce_audio_processors/scanning/juce_PluginListComponent.cpp
index 81cdcfce47..336d4229f7 100644
--- a/modules/juce_audio_processors/scanning/juce_PluginListComponent.cpp
+++ b/modules/juce_audio_processors/scanning/juce_PluginListComponent.cpp
@@ -218,6 +218,12 @@ public:
     }
 
 private:
+    enum Flags
+    {
+        stopRequested   = 1 << 0,   // Set to indicate to the background scanner that it should stop asap
+        finished        = 1 << 1,   // Set by the scanner to indicate that it's done
+    };
+
     PluginListComponent& owner;
     AudioPluginFormat& formatToScan;
     StringArray filesOrIdentifiersToScan;
@@ -229,7 +235,7 @@ private:
     double progress = 0;
     const int numThreads;
     bool allowAsync, timerReentrancyCheck = false;
-    std::atomic<bool> finished { false };
+    std::atomic<int> flags { 0 };
     std::unique_ptr<ThreadPool> pool;
     std::set<String> initiallyBlacklistedFiles;
     ScopedMessageBox messageBox;
@@ -315,7 +321,10 @@ private:
 
         progressWindow.addButton (TRANS ("Cancel"), 0, KeyPress (KeyPress::escapeKey));
         progressWindow.addProgressBarComponent (progress);
-        progressWindow.enterModalState();
+        progressWindow.enterModalState (true, ModalCallbackFunction::create ([this] (auto)
+        {
+            flags |= stopRequested;
+        }));
 
         if (numThreads > 0)
         {
@@ -357,10 +366,7 @@ private:
                 startTimer (20);
         }
 
-        if (! progressWindow.isCurrentlyModal())
-            finished = true;
-
-        if (finished)
+        if ((flags & finished) != 0)
             finishedScan();
         else
             progressWindow.setMessage (TRANS ("Testing") + ":\n\n" + pluginBeingScanned);
@@ -368,16 +374,16 @@ private:
 
     bool doNextScan()
     {
-        if (scanner->scanNextFile (true, pluginBeingScanned))
+        if ((flags & stopRequested) == 0 && scanner->scanNextFile (true, pluginBeingScanned))
             return true;
 
-        finished = true;
+        flags |= finished;
         return false;
     }
 
     struct ScanJob final : public ThreadPoolJob
     {
-        ScanJob (Scanner& s)  : ThreadPoolJob ("pluginscan"), scanner (s) {}
+        explicit ScanJob (Scanner& s)  : ThreadPoolJob ("pluginscan"), scanner (s) {}
 
         JobStatus runJob() override
         {