bug#60237: 30.0.50; tree sitter core dumps when I edebug view a node

[Yuan Fu]

> GC has historically never called xmalloc, so the profiler will likely
> crash upon growing the mark stack as well.  I guess another important
> question is why ts_delete_parser is calling xmalloc.
> 

> As you see, when we call ts_tree_delete, it calls ts_subtree_release,
> which in turn calls malloc (redirected into our xmalloc).  Is this
> expected?  Can you look in the tree-sitter sources and verify that
> this is OK?

I had a look, and it seems legit. In tree-sitter, a TSTree (or more precisely, 
a Subtree) is just some inlined data plus a refcounted pointer to the complete 
data. This way multiple trees share common subtrees/nodes. Eg, when 
incrementally parsing, you pass in an old tree and get a new tree, these two 
trees will share the unchanged part of the tree. 

Therefore, deleting a tree is not simply free(). Instead, you decrement the 
refcount of the subtree, and if the count == 0, free the data and traverse the 
subtree and decrementing each children’s refcount, and delete them if the count 
== 0, and so on. To traverse the tree, the function uses an array as a stack, 
which calls array_push to push new elements, which may call malloc.

Yuan