[PATCH] Don't dereference NULL upon lookup_tree failure.

While Matthew Farrellee was working on converting the Condor repository
from cvs to git, he managed to make git segfault (with help from parsecvs)
while producing a 100MB .git repository.  He found the single offending
,v file that led parsecvs to generate a bogus repository, and deduced
that adding a single well-placed branch tag[*] was enough to avoid
the problem.  I fixed the bug in git along with a few in parsecvs that
were exposed while paring the ,v file down from a 150KB monstrosity to
something manageable.

[*] Adding this tag avoids the problem: FOO:1.30.2.5.0.8

First, here's how to reproduce the git segfault, using the file, "min,v"
included below.  Run this in an empty directory:

  rm -rf k .git .git-cvs
  parsecvs min,v >& log
  git clone -q . k

I noticed that while I get a segfault both on x86 and x86_64, I see
clear evidence of it only on x86:

  0 blocks
  error: Object 0d57588da39d10795486bd5451bc2660832228e6 is a commit, not a tree
  Segmentation fault
  fatal: The remote end hung up unexpectedly
  [Exit 1]

When running on an x86_64 system (either debian unstable or rawhide)
I see only this:

  0 blocks
  error: Object 0d57588da39d10795486bd5451bc2660832228e6 is a commit, not a tree
  fatal: The remote end hung up unexpectedly

Of course, valgrind shows you the segfault in both cases.

The above was using git version 1.5.4.rc1.3.gec692
and the latest parsecvs from here:
git://people.freedesktop.org/~keithp/parsecvs

    commit 2b0113ffb0055620193397c025d6f6bca3b110cd
    Author: Finn Arne Gangstad [off-list ref]
    Date:   Sun Nov 18 15:26:35 2007 -0800

This patch avoids the NULL dereference by treating a failed lookup_tree the
same way an invalid "type" is handled in the "else" block just below.
The only difference is that for a failed lookup_tree, the failing
function has already produced a diagnostic.

-----------------------------------------------------
From 4cd649160d8174b23727b3d7276f1bd7246d0aff Mon Sep 17 00:00:00 2001
From: Jim Meyering <redacted>
Date: Fri, 21 Dec 2007 11:56:32 +0100
Subject: [PATCH] Don't dereference NULL upon lookup_tree failure.


Signed-off-by: Jim Meyering <redacted>
---
 object.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/object.c b/object.c
index 16793d9..eb59550 100644
--- a/object.c
+++ b/object.c

@@ -142,10 +142,14 @@ struct object *parse_object_buffer(const unsigned char *sha1, enum object_type t
 		obj = &blob->object;
 	} else if (type == OBJ_TREE) {
 		struct tree *tree = lookup_tree(sha1);
-		obj = &tree->object;
-		if (!tree->object.parsed) {
-			parse_tree_buffer(tree, buffer, size);
-			eaten = 1;
+		if (!tree)
+		    obj = NULL;
+		else {
+		    obj = &tree->object;
+		    if (!tree->object.parsed) {
+			    parse_tree_buffer(tree, buffer, size);
+			    eaten = 1;
+		    }
 		}
 	} else if (type == OBJ_COMMIT) {
 		struct commit *commit = lookup_commit(sha1);
--

1.5.4.rc0.76.g55ee


Here's the contents of min,v
------------------------------------------------
head	1.31;
access;
symbols
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	q:1.30.2.18.4.14.4.24.0.10
	r:1.30.2.18.4.14.4.24.0.8
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	t:1.30.2.18.4.14.4.24.0.4
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	a0:1.30.2.18.4.14.4.23.0.10
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	a105:1.30.0.2
	new-syscall-branch:1.14.0.2
	V6_0-branch:1.6.0.2;
locks; strict;
comment	@ * @;


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