Subject: Re: git fsck segmentation fault

From: Junio C Hamano <[email-masked]>
Date: Wed, 10 Dec 2008 18:33:20 -0800

[email-masked] (Martin Koegler) writes:

> Maybe something like this could help:

>>>From 32be177cbb0825fc019200b172f3d79117b28140 Mon Sep 17 00:00:00 2001
> From: Martin Koegler <[email-masked]>
> 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.