Re: git fsck segmentation fault

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 11, 2008, 02:33 UTC
Message-ID: <7vljunwidr.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <20081210075338.GA7776@auto.tuwien.ac.at>

mkoegler@auto.tuwien.ac.at (Martin Koegler) writes:

> Maybe something like this could help:

Show 7 quoted lines

>>From 32be177cbb0825fc019200b172f3d79117b28140 Mon Sep 17 00:00:00 2001
> From: Martin Koegler <mkoegler@auto.tuwien.ac.at>
> Date: Wed, 10 Dec 2008 08:42:08 +0100
> Subject: [PATCH] fsck: use fewer stack
>
> This patch moves the state while traversing the tree
> from the stack to the heap.

Hmm, after the change:

	* mark_object() marks the object as reachable, and pushes the
	  objects to the objectstack;

	* mark_object_reachable() marks the object using mark_object(),
          and repeatedly calls mark_child_object() until the objectstack
          is fully drained;

	* mark_child_object() inspects the object taken from the
          objectstack, calls fsck_walk() on it, with mark_object as the
          callback;

	  * fsck_walk() calls the callback function (i.e. mark_object) on
            the object given, and the objects immediately reachable from
            it;

            * mark_object() does not recurse, so these immediately
              reachable objects are left in the objectstack, without a
              deep recursion.
        
That seems to be what is going on, and this should be a good fix.

A similar change would be needed for other callers of fsck_walk(), no? There seem to be one in builtin-unpack-objects.c (check_object calls fsck_walk as itself as the callback).

Another caller is in index-pack.c (sha1_object() calls fsck_walk with mark_link as the callback), but I do not think it would recurse for the depth of the history, so we are safe there.

I initially expected that the fix would be to introduce this "userspace work queue" (i.e. your objectstack) to be maintained on the fsck.c:fsck_walk() side (perhaps as an extra parameter to an actual queue for reentrancy), not by making the callee not to recurse, though.