fix(macos): avoid camera-permission deadlock when opening off the main thread

src/ccap_apple_async.h

@@ -0,0 +1,58 @@
+/**
+ * @file ccap_apple_async.h
+ * @author wysaid (this@wysaid.org)
+ * @brief Run a callback-based asynchronous request and block until it completes,
+ *        without bouncing the request onto the main dispatch queue.
+ *
+ * macOS callback APIs such as `AVCaptureDevice requestAccessForMediaType:` deliver
+ * their completion on an internal queue, not on the caller's run loop. ccap used to
+ * dispatch the permission request onto the main queue for non-main-thread callers,
+ * which deadlocks whenever nothing is servicing that queue -- e.g. a ccap::Provider
+ * opened from a worker thread in a process that has no CFRunLoop on its main thread
+ * (a Node.js / Electron addon, a head-less multi-threaded service, ...).
+ *
+ * runBlockingAsyncRequest() starts the request on the *calling* thread and blocks on a
+ * portable condition variable until the supplied continuation is invoked, so it is
+ * safe to call from any thread regardless of run-loop state.
+ *
+ * Covered by tests/test_apple_permission.cpp.
+ */
+
+#pragma once
+
+#if defined(__APPLE__)
+
+#include <condition_variable>
+#include <functional>
+#include <mutex>
+
+namespace ccap
+{
+
+/**
+ * Invoke @p start on the current thread and block until the continuation that
+ * @p start receives (its `done` argument) is called. @p start may invoke `done` from
+ * any thread or queue. The request is never dispatched to the main queue, so this
+ * cannot deadlock when no run loop is servicing it.
+ */
+inline void runBlockingAsyncRequest(const std::function<void(const std::function<void()>& done)>& start)
+{
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool finished = false;
+
+    start([&mutex, &cv, &finished]() {
+        {
+            std::lock_guard<std::mutex> lock(mutex);
+            finished = true;
+        }
+        cv.notify_one();
+    });
+
+    std::unique_lock<std::mutex> lock(mutex);
+    cv.wait(lock, [&finished]() { return finished; });
+}
+
+} // namespace ccap
+
+#endif // __APPLE__

src/ccap_imp_apple.mm

@@ -11,6 +11,7 @@
 #include "ccap_imp_apple.h"
 #include "ccap_file_reader_apple.h"
 
+#include "ccap_apple_async.h"
 #include "ccap_convert.h"
 #include "ccap_convert_frame.h"
 
@@ -19,6 +20,7 @@
 #import <Foundation/Foundation.h>
 #include <cassert>
 #include <cmath>
+#include <functional>
 
 #if _CCAP_LOG_ENABLED_
 #include <deque>
@@ -255,23 +257,22 @@ - (instancetype)initWithProvider:(ProviderApple*)provider {
 - (BOOL)open {
     AVAuthorizationStatus authStatus = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
     if (authStatus == AVAuthorizationStatusNotDetermined) {
-        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
-        void (^requestAccess)(void) = ^(void) {
-            [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL granted) {
-                CCAP_NSLOG_I(@"ccap: Camera access %@", granted ? @"granted" : @"denied");
-                dispatch_semaphore_signal(sema);
-            }];
-        };
-
-        // Permission must be requested on the main thread
-        if (![NSThread isMainThread]) {
-            dispatch_async(dispatch_get_main_queue(), ^{ requestAccess(); });
-        } else {
-            requestAccess();
-        }
-
         CCAP_NSLOG_I(@"ccap: Waiting for camera access permission...");
-        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
+        // Request authorization on the calling thread and block until the system's
+        // completion handler fires. We deliberately do NOT dispatch the request onto the
+        // main queue: requestAccessForMediaType: may be called from any thread and
+        // delivers its completion on an internal queue, so bouncing to the main queue
+        // would deadlock whenever no run loop is servicing it (e.g. a ccap::Provider
+        // opened from a worker thread in a process without a CFRunLoop). See
+        // tests/test_apple_permission.cpp.
+        ccap::runBlockingAsyncRequest([](const std::function<void()>& done) {
+            std::function<void()> notifyDone = done; // outlive the async completion
+            [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo
+                                     completionHandler:^(BOOL granted) {
+                                         CCAP_NSLOG_I(@"ccap: Camera access %@", granted ? @"granted" : @"denied");
+                                         notifyDone();
+                                     }];
+        });
         authStatus = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
     }
 

tests/CMakeLists.txt

@@ -203,6 +203,7 @@ add_executable(
         test_frame_conversions.cpp
         test_boundary_conditions.cpp
         test_grab_timeout.cpp
+        test_apple_permission.cpp  # macOS camera-permission deadlock regression (empty TU elsewhere)
 )
 
 target_link_libraries(

tests/test_apple_permission.cpp

@@ -0,0 +1,94 @@
+/**
+ * @file test_apple_permission.cpp
+ * @brief Regression test for the macOS camera-permission request deadlock.
+ *
+ * ccap::runBlockingAsyncRequest() (used by ProviderApple::open) must run the
+ * permission request on the calling thread and must NOT bounce it onto the main
+ * dispatch queue. Otherwise Provider::open() hangs forever when called from a worker
+ * thread in a process whose main thread is not running a run loop -- exactly the
+ * situation a Node.js / Electron addon or any head-less multi-threaded embedder
+ * creates.
+ *
+ * We exercise the real helper with a *simulated* asynchronous request: a short
+ * countdown that fires the completion from a background thread, just like
+ * AVCaptureDevice requestAccessForMediaType: delivers its completion off the caller's
+ * run loop. No camera is required, so this runs deterministically in CI.
+ *
+ * On non-Apple platforms this file compiles to an empty translation unit.
+ */
+
+#if defined(__APPLE__)
+
+#include <gtest/gtest.h>
+
+#include <chrono>
+#include <functional>
+#include <future>
+#include <thread>
+
+#include "ccap_apple_async.h"
+
+namespace
+{
+
+// Stand-in for AVCaptureDevice requestAccessForMediaType:completionHandler:: it fires
+// the completion asynchronously from a *background* thread after a short countdown,
+// never touching the caller's main run loop.
+void simulateAsyncPermissionRequest(const std::function<void()>& done)
+{
+    std::function<void()> completion = done; // must outlive this call
+    std::thread([completion]() {
+        std::this_thread::sleep_for(std::chrono::milliseconds(50)); // countdown
+        completion();
+    }).detach();
+}
+
+// Runs runBlockingAsyncRequest (optionally on a worker thread) and reports whether it
+// returned within the timeout. A timeout means it deadlocked.
+bool completesWithoutDeadlock(bool onWorkerThread, std::chrono::milliseconds timeout)
+{
+    std::promise<void> donePromise;
+    std::future<void> doneFuture = donePromise.get_future();
+
+    auto body = [&donePromise]() {
+        ccap::runBlockingAsyncRequest(&simulateAsyncPermissionRequest);
+        donePromise.set_value();
+    };
+
+    std::thread worker;
+    if (onWorkerThread) {
+        worker = std::thread(body);
+    } else {
+        body();
+    }
+
+    const bool completed = doneFuture.wait_for(timeout) == std::future_status::ready;
+    if (worker.joinable()) {
+        if (completed) {
+            worker.join();
+        } else {
+            worker.detach(); // leave the hung thread; the process exits regardless
+        }
+    }
+    return completed;
+}
+
+} // namespace
+
+// The regression: open() called off the main thread with no run loop servicing the
+// main queue. This deadlocked with the old dispatch-to-main-queue implementation.
+TEST(AppleCameraPermission, OffMainThreadWithoutRunLoopDoesNotDeadlock)
+{
+    EXPECT_TRUE(completesWithoutDeadlock(/*onWorkerThread=*/true, std::chrono::seconds(5)))
+        << "runBlockingAsyncRequest() deadlocked off the main thread -- the request was "
+           "likely bounced onto an unserviced main dispatch queue.";
+}
+
+// Sanity: the common main-thread path must also complete promptly.
+TEST(AppleCameraPermission, MainThreadDoesNotDeadlock)
+{
+    EXPECT_TRUE(completesWithoutDeadlock(/*onWorkerThread=*/false, std::chrono::seconds(5)))
+        << "runBlockingAsyncRequest() deadlocked on the main thread.";
+}
+
+#endif // __APPLE__