Execute NSOperationQueue mainQueue on main thread

ChangeLog

@@ -1,3 +1,11 @@
+2026-03-19 Frederik Seiffert <frederik@algoriddim.com>
+
+	* Source/NSOperation.m:
+	Ensure NSOperationQueue mainQueue executes queued operations on the
+	main thread, using libdispatch main-queue fast path when available.
+	* Tests/base/NSOperation/threads.m:
+	Add test for enqueuing on mainQueue from a background thread.
+
 2026-03-12 Richard Frith-Macdonald <rfm@gnu.org>
 
 	* Source/GSHTTPURLHandle.m:

Source/NSOperation.m

@@ -66,6 +66,7 @@
 #import "Foundation/NSValue.h"
 #import "GNUstepBase/NSArray+GNUstepBase.h"
 #import "GSPrivate.h"
+#import "GSDispatch.h"
 
 #define	GSInternal	NSOperationInternal
 #include	"GSInternal.h"
@@ -609,6 +610,7 @@ - (void) main
 
 @interface	NSOperationQueue (Private)
 - (void) _execute;
+- (void) _main: (NSOperation *)op;
 - (void) _thread: (NSNumber *) threadNumber;
 - (void) observeValueForKeyPath: (NSString *)keyPath
 		       ofObject: (id)object
@@ -632,6 +634,17 @@ - (void) observeValueForKeyPath: (NSString *)keyPath
 static NSString	*threadKey = @"NSOperationQueue";
 static NSOperationQueue *mainQueue = nil;
 
+#if GS_USE_LIBDISPATCH == 1
+static void
+mainQueueExecuteOperation(void *context)
+{
+  NSOperation	*op = (NSOperation *)context;
+
+  [mainQueue _main: op];
+  RELEASE(op);
+}
+#endif
+
 @implementation NSOperationQueue
 
 + (id) currentQueue
@@ -648,6 +661,7 @@ + (void) initialize
   if (nil == mainQueue)
     {
       mainQueue = [self new];
+      [mainQueue setMaxConcurrentOperationCount: 1];
     }
 }
 
@@ -886,6 +900,10 @@ - (NSArray *) operations
 
 - (void) setMaxConcurrentOperationCount: (NSInteger)cnt
 {
+  if (self == mainQueue)
+    {
+      cnt = 1;
+    }
   if (cnt < 0
     && cnt != NSOperationQueueDefaultMaxConcurrentOperationCount)
     {
@@ -1102,11 +1120,35 @@ - (void) _thread: (NSNumber *) threadNumber
   [NSThread exit];
 }
 
+- (void) _main: (NSOperation *)op
+{
+  BOOL	concurrent;
+
+  concurrent = [op isConcurrent];
+  NS_DURING
+    {
+      ENTER_POOL
+      [op start];
+      LEAVE_POOL
+    }
+  NS_HANDLER
+    {
+      NSLog(@"Problem running operation %@ ... %@",
+	op, localException);
+    }
+  NS_ENDHANDLER
+  if (NO == concurrent)
+    {
+      [op _finish];
+    }
+}
+
 /* Check for operations which can be executed and start them.
  */
 - (void) _execute
 {
   NSInteger	max;
+  NSMutableArray *mainQueueOperations = nil;
 
   [internal->lock lock];
 
@@ -1136,9 +1178,17 @@ - (void) _execute
 	      options: NSKeyValueObservingOptionNew
 	      context: isFinishedCtxt];
       internal->executing++;
-      if (YES == [op isConcurrent])
+      if (self == mainQueue)
+	{
+	  if (nil == mainQueueOperations)
+	    {
+	      mainQueueOperations = [NSMutableArray new];
+	    }
+	  [mainQueueOperations addObject: op];
+	}
+      else if (YES == [op isConcurrent])
 	{
-          [op start];
+	  [op start];
 	}
       else
 	{
@@ -1178,6 +1228,23 @@ - (void) _execute
     }
   NS_ENDHANDLER
   [internal->lock unlock];
+
+  if (nil != mainQueueOperations)
+    {
+      GS_FOR_IN(NSOperation *, op, mainQueueOperations)
+	{
+#if GS_USE_LIBDISPATCH == 1
+	  dispatch_async_f(dispatch_get_main_queue(), RETAIN(op),
+	    mainQueueExecuteOperation);
+#else
+	  [self performSelectorOnMainThread: @selector(_main:)
+				 withObject: op
+			      waitUntilDone: NO];
+#endif
+	}
+      GS_END_FOR(mainQueueOperations)
+      RELEASE(mainQueueOperations);
+    }
 }
 
 @end

Tests/base/NSOperation/threads.m

@@ -4,6 +4,8 @@
 #import <Foundation/NSThread.h>
 #import <Foundation/NSNotification.h>
 #import <Foundation/NSAutoreleasePool.h>
+#import <Foundation/NSDate.h>
+#import <Foundation/NSRunLoop.h>
 #import "ObjectTesting.h"
 
 
@@ -135,6 +137,28 @@ - (void) main
 }
 @end
 
+static BOOL
+waitForOperation(NSOperation *op, NSTimeInterval timeout)
+{
+  NSDate	*end;
+
+  end = [NSDate dateWithTimeIntervalSinceNow: timeout];
+  while (NO == [op isFinished] && [end timeIntervalSinceNow] > 0.0)
+    {
+      NSDate	*next;
+      BOOL	handled;
+
+      next = [NSDate dateWithTimeIntervalSinceNow: 0.01];
+      handled = [[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode
+					 beforeDate: next];
+      if (NO == handled)
+	{
+	  [NSThread sleepForTimeInterval: 0.01];
+	}
+    }
+  return [op isFinished];
+}
+
 int main()
 {
   ThreadCounter         *cnt;
@@ -225,6 +249,19 @@ int main()
   PASS(([NSOperationQueue currentQueue] == [NSOperationQueue mainQueue]), "current queue outside -main is main queue");
   PASS(([NSOperationQueue mainQueue] != nil), "main queue is not nil");
 
+  obj = [OpFlag new];
+  [NSThread detachNewThreadSelector: @selector(addOperation:)
+			   toTarget: [NSOperationQueue mainQueue]
+			 withObject: obj];
+  PASS((YES == waitForOperation(obj, 2.0)),
+    "main queue runs enqueued operations");
+  PASS(([obj ran] == YES), "main queue operation ran");
+  PASS(([obj thread] == [NSThread mainThread]),
+    "main queue operation ran in main thread");
+  PASS(([obj queue] == [NSOperationQueue mainQueue]),
+    "main queue operation reported main queue");
+  RELEASE(obj);
+
   obj = [OpFlag new];
   [q addOperation: obj];
   [q waitUntilAllOperationsAreFinished];