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

- 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

Author: NHDaly

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

@@ -1070,4 +1070,24 @@ end
     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
+
 end # main testset