Fix use-after-free in FileWatching (#59017)

We observe an abort on Windows on Revise master CI, where a free'd
handle is passed to jl_close_uv. The root cause is that uv_fseventscb_file
called uvfinalize earlier, but did not set the handle to NULL, so when the
actual finalizer ran later, it would see corrupted state.

Author: Keno

stdlib/FileWatching/src/FileWatching.jl

@@ -516,17 +516,19 @@ end
 
 function uvfinalize(uv::Union{FileMonitor, FolderMonitor})
     iolock_begin()
-    if uv.handle != C_NULL
-        disassociate_julia_struct(uv) # close (and free) without notify
-        ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), uv.handle)
+    handle = @atomicswap :monotonic uv.handle = C_NULL
+    if handle != C_NULL
+        disassociate_julia_struct(handle) # close (and free) without notify
+        ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), handle)
     end
     iolock_end()
 end
 
 function close(t::Union{FileMonitor, FolderMonitor})
     iolock_begin()
-    if t.handle != C_NULL
-        ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), t.handle)
+    handle = t.handle
+    if handle != C_NULL
+        ccall(:jl_close_uv, Cvoid, (Ptr{Cvoid},), handle)
     end
     iolock_end()
 end