main-loop: Disable block backend global state assertion on Cocoa

Since commit 0439c5a ("block/block-backend.c: assertions for
block-backend") QEMU crashes when using Cocoa on Darwin hosts.

Example on macOS:

  $ qemu-system-i386
  Assertion failed: (qemu_in_main_thread()), function blk_all_next, file block-backend.c, line 552.
  Abort trap: 6

Looking with lldb:

  Assertion failed: (qemu_in_main_thread()), function blk_all_next, file block-backend.c, line 552.
  Process 76914 stopped
  * thread TrungNguyen1909#1, queue = 'com.apple.main-thread', stop reason = hit program assert
     frame TrungNguyen1909#4: 0x000000010057c2d4 qemu-system-i386`blk_all_next.cold.1
  at block-backend.c:552:5 [opt]
      549    */
      550   BlockBackend *blk_all_next(BlockBackend *blk)
      551   {
  --> 552       GLOBAL_STATE_CODE();
      553       return blk ? QTAILQ_NEXT(blk, link)
      554                  : QTAILQ_FIRST(&block_backends);
      555   }
  Target 1: (qemu-system-i386) stopped.

  (lldb) bt
  * thread TrungNguyen1909#1, queue = 'com.apple.main-thread', stop reason = hit program assert
     frame #0: 0x00000001908c99b8 libsystem_kernel.dylib`__pthread_kill + 8
     frame TrungNguyen1909#1: 0x00000001908fceb0 libsystem_pthread.dylib`pthread_kill + 288
     frame TrungNguyen1909#2: 0x000000019083a314 libsystem_c.dylib`abort + 164
     frame TrungNguyen1909#3: 0x000000019083972c libsystem_c.dylib`__assert_rtn + 300
   * frame TrungNguyen1909#4: 0x000000010057c2d4 qemu-system-i386`blk_all_next.cold.1 at block-backend.c:552:5 [opt]
     frame TrungNguyen1909#5: 0x00000001003c00b4 qemu-system-i386`blk_all_next(blk=<unavailable>) at block-backend.c:552:5 [opt]
     frame TrungNguyen1909#6: 0x00000001003d8f04 qemu-system-i386`qmp_query_block(errp=0x0000000000000000) at qapi.c:591:16 [opt]
     frame TrungNguyen1909#7: 0x000000010003ab0c qemu-system-i386`main [inlined] addRemovableDevicesMenuItems at cocoa.m:1756:21 [opt]
     frame TrungNguyen1909#8: 0x000000010003ab04 qemu-system-i386`main(argc=<unavailable>, argv=<unavailable>) at cocoa.m:1980:5 [opt]
     frame TrungNguyen1909#9: 0x00000001012690f4 dyld`start + 520

As we are in passed release 7.0 hard freeze, disable the block
backend assertion which, while being valuable during development,
is not helpful to users. We'll restore this assertion immediately
once 7.0 is released and work on a fix.

Suggested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220325183707.85733-1-philippe.mathieu.daude@gmail.com>

include/qemu/main-loop.h

@@ -270,10 +270,23 @@ bool qemu_mutex_iothread_locked(void);
 bool qemu_in_main_thread(void);
 
 /* Mark and check that the function is part of the global state API. */
+#ifdef CONFIG_COCOA
+/*
+ * When using the Cocoa UI, addRemovableDevicesMenuItems() is called from
+ * a thread different from the QEMU main thread and can not take the BQL,
+ * triggering this assertions in the block layer (commit 0439c5a462).
+ * As the Cocoa fix is not trivial, disable this assertion for the v7.0.0
+ * release (when using Cocoa); we will restore it immediately after the
+ * release.
+ * This issue is tracked as https://gitlab.com/qemu-project/qemu/-/issues/926
+ */
+#define GLOBAL_STATE_CODE()
+#else
 #define GLOBAL_STATE_CODE()                                         \
     do {                                                            \
         assert(qemu_in_main_thread());                              \
     } while (0)
+#endif /* CONFIG_COCOA */
 
 /* Mark and check that the function is part of the I/O API. */
 #define IO_CODE()                                                   \