Fix thread safety in atexit(f): Lock access to atexit_hooks (#49774)

- atexit(f) mutates global shared state.
- atexit(f) can be called anytime by any thread.
- Accesses & mutations to global shared state must be locked if they can
  be accessed from multiple threads.

Add unit test for thread safety of adding many atexit functions in
parallel

(cherry picked from commit 9dd3090)

base/initdefs.jl

@@ -353,6 +353,7 @@ end
 const atexit_hooks = Callable[
     () -> Filesystem.temp_cleanup_purge(force=true)
 ]
+const _atexit_hooks_lock = ReentrantLock()
 
 """
     atexit(f)
@@ -374,7 +375,7 @@ calls `exit(n)`, then Julia will exit with the exit code corresponding to the
 last called exit hook that calls `exit(n)`. (Because exit hooks are called in
 LIFO order, "last called" is equivalent to "first registered".)
 """
-atexit(f::Function) = (pushfirst!(atexit_hooks, f); nothing)
+atexit(f::Function) = Base.@lock _atexit_hooks_lock (pushfirst!(atexit_hooks, f); nothing)
 
 function _atexit(exitcode::Cint)
     while !isempty(atexit_hooks)

test/threads_exec.jl

@@ -1067,3 +1067,23 @@ end
         popfirst!(LOAD_PATH)
     end
 end
+
+# issue #49746, thread safety in `atexit(f)`
+@testset "atexit thread safety" begin
+    f = () -> nothing
+    before_len = length(Base.atexit_hooks)
+    @sync begin
+        for _ in 1:1_000_000
+            Threads.@spawn begin
+                atexit(f)
+            end
+        end
+    end
+    @test length(Base.atexit_hooks) == before_len + 1_000_000
+    @test all(hook -> hook === f, Base.atexit_hooks[1 : 1_000_000])
+
+    # cleanup
+    Base.@lock Base._atexit_hooks_lock begin
+        deleteat!(Base.atexit_hooks, 1:1_000_000)
+    end
+end