Merge pull request #43701 from JuliaLang/jn/43578

workaround a dyld/libunwind deadlock issue, since macOS 12.1

src/Makefile

@@ -145,6 +145,7 @@ endif
 CLANG_LDFLAGS := $(LLVM_LDFLAGS)
 ifeq ($(OS), Darwin)
 CLANG_LDFLAGS += -Wl,-undefined,dynamic_lookup
+OSLIBS += $(SRCDIR)/mach_dyld_atfork.tbd
 endif
 
 COMMON_LIBPATHS := -L$(build_libdir) -L$(build_shlibdir)

src/mach_dyld_atfork.tbd

@@ -0,0 +1,25 @@
+--- !tapi-tbd
+# copied from XCode's libSystem.tbd (current-version: 1311)
+# to provide weak-linkage info for new symbols on old systems
+tbd-version: 4
+targets: [ x86_64-macos, x86_64-maccatalyst, arm64-macos, arm64-maccatalyst,
+ arm64e-macos, arm64e-maccatalyst ]
+uuids:
+ - target: x86_64-macos
+ value: AFE6C76A-B47A-35F5-91D0-4E9FC439E90D
+ - target: x86_64-maccatalyst
+ value: AFE6C76A-B47A-35F5-91D0-4E9FC439E90D
+ - target: arm64-macos
+ value: 2EA09BDB-811B-33AA-BB58-4B53AA2DB522
+ - target: arm64-maccatalyst
+ value: 2EA09BDB-811B-33AA-BB58-4B53AA2DB522
+ - target: arm64e-macos
+ value: 09AB3723-C26D-3762-93BA-98E9C38B89C1
+ - target: arm64e-maccatalyst
+ value: 09AB3723-C26D-3762-93BA-98E9C38B89C1
+install-name: '/usr/lib/libSystem.B.dylib'
+exports:
+ - targets: [ arm64-macos, arm64e-macos, x86_64-macos, x86_64-maccatalyst,
+ arm64-maccatalyst, arm64e-maccatalyst ]
+ symbols: [ __dyld_atfork_parent, __dyld_atfork_prepare ]
+...

src/signals-mach.c

@@ -31,6 +31,11 @@ extern void *_keymgr_get_and_lock_processwide_ptr(unsigned int key);
 extern int _keymgr_get_and_lock_processwide_ptr_2(unsigned int key, void **result);
 extern int _keymgr_set_lockmode_processwide_ptr(unsigned int key, unsigned int mode);
 
+// private dyld3/dyld4 stuff
+extern void _dyld_atfork_prepare(void) __attribute__((weak_import));
+extern void _dyld_atfork_parent(void) __attribute__((weak_import));
+//extern void _dyld_fork_child(void) __attribute__((weak_import));
+
 static void attach_exception_port(thread_port_t thread, int segv_only);
 
 // low 16 bits are the thread id, the next 8 bits are the original gc_state
@@ -521,6 +526,31 @@ static kern_return_t profiler_segv_handler
 }
 #endif
 
+// WARNING: we are unable to handle sigsegv while the dlsymlock is held
+static int jl_lock_profile_mach(int dlsymlock)
+{
+ jl_lock_profile();
+ // workaround for old keymgr bugs
+ void *unused = NULL;
+ int keymgr_locked = _keymgr_get_and_lock_processwide_ptr_2(KEYMGR_GCC3_DW2_OBJ_LIST, &unused) == 0;
+ // workaround for new dlsym4 bugs (API and bugs introduced in macOS 12.1)
+ if (dlsymlock && _dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL)
+ _dyld_atfork_prepare();
+ return keymgr_locked;
+}
+
+static void jl_unlock_profile_mach(int dlsymlock, int keymgr_locked)
+{
+ if (dlsymlock && _dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL) \
+ _dyld_atfork_parent(); \
+ if (keymgr_locked)
+ _keymgr_unlock_processwide_ptr(KEYMGR_GCC3_DW2_OBJ_LIST);
+ jl_unlock_profile();
+}
+
+#define jl_lock_profile() int keymgr_locked = jl_lock_profile_mach(1)
+#define jl_unlock_profile() jl_unlock_profile_mach(1, keymgr_locked)
+
 void *mach_profile_listener(void *arg)
 {
  (void)arg;
@@ -537,9 +567,7 @@ void *mach_profile_listener(void *arg)
  HANDLE_MACH_ERROR("mach_msg", ret);
  // sample each thread, round-robin style in reverse order
  // (so that thread zero gets notified last)
- jl_lock_profile();
- void *unused = NULL;
- int keymgr_locked = _keymgr_get_and_lock_processwide_ptr_2(KEYMGR_GCC3_DW2_OBJ_LIST, &unused) == 0;
+ int keymgr_locked = jl_lock_profile_mach(0);
  jl_shuffle_int_array_inplace(profile_round_robin_thread_order, jl_n_threads, &profile_cong_rng_seed);
  for (int idx = jl_n_threads; idx-- > 0; ) {
  // Stop the threads in the random round-robin order.
@@ -550,9 +578,13 @@ void *mach_profile_listener(void *arg)
  break;
  }
 
+ if (_dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL)
+ _dyld_atfork_prepare(); // briefly acquire the dlsym lock
  host_thread_state_t state;
  jl_thread_suspend_and_get_state2(i, &state);
  unw_context_t *uc = (unw_context_t*)&state;
+ if (_dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL)
+ _dyld_atfork_parent(); // quickly release the dlsym lock
 
  if (running) {
 #ifdef LLVMLIBUNWIND
@@ -609,9 +641,7 @@ void *mach_profile_listener(void *arg)
  // We're done! Resume the thread.
  jl_thread_resume(i, 0);
  }
- if (keymgr_locked)
- _keymgr_unlock_processwide_ptr(KEYMGR_GCC3_DW2_OBJ_LIST);
- jl_unlock_profile();
+ jl_unlock_profile_mach(0, keymgr_locked);
  if (running) {
  // Reset the alarm
  kern_return_t ret = clock_alarm(clk, TIME_RELATIVE, timerprof, profile_port);