block-copy: Fix missing graph lock

The graph lock needs to be held when calling bdrv_co_pdiscard(). Fix
block_copy_task_entry() to take it for the call.

WITH_GRAPH_RDLOCK_GUARD() was implemented in a weak way because of
limitations in clang's Thread Safety Analysis at the time, so that it
only asserts that the lock is held (which allows calling functions that
require the lock), but we never deal with the unlocking (so even after
the scope of the guard, the compiler assumes that the lock is still
held). This is why the compiler didn't catch this locking error.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240627181245.281403-2-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-copy.c

@@ -595,7 +595,9 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)
     if (s->discard_source && ret == 0) {
         int64_t nbytes =
             MIN(t->req.offset + t->req.bytes, s->len) - t->req.offset;
-        bdrv_co_pdiscard(s->source, t->req.offset, nbytes);
+        WITH_GRAPH_RDLOCK_GUARD() {
+            bdrv_co_pdiscard(s->source, t->req.offset, nbytes);
+        }
     }
 
     return ret;