Add public API for fsck, use it before loading metadata

A while ago I did `truncate -s 0 /path/to/repo/00/123.commit`, and expected a
checksum error, but I actually got a validation error due to us loading the
commit into a variant and trying to parse out the parent checksum, etc.

I first started by changing the `load_and_fsck_one_object()` function to
checksum before loading, but the problem is that we do a traverse of all objects
first. Fixing this is going to require an `OSTREE_REPO_COMMIT_TRAVER_FLAG_FSCK`
or something.

In the meantime at least though, let's add a public API to fsck a single object
which *does* checksum cleanly before parsing the object, and change the `fsck`
command to use it.

We then change the fsck binary to do this while iterating over the refs
and finding the commit object.  This way we'll at least get a checksum
first for commit objects, even if not dirtree/dirmeta.

Closes: #1364
Approved by: jlebon

tests/pull-test.sh

@@ -183,7 +183,7 @@ if ! skip_one_without_user_xattrs; then
         if ${CMD_PREFIX} ostree --repo=cacherepo fsck 2>err.txt; then
             fatal "corrupt repo fsck?"
         fi
-        assert_file_has_content err.txt "corrupted.*${checksum}"
+        assert_file_has_content err.txt "Corrupted.*${checksum}"
         rm ostree-srv/corruptrepo -rf
         ostree_repo_init ostree-srv/corruptrepo --mode=archive
         ${CMD_PREFIX} ostree --repo=ostree-srv/corruptrepo pull-local cacherepo main

tests/test-corruption.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 #
-# Copyright (C) 2011 Colin Walters <walters@verbum.org>
+# Copyright (C) 2011,2017 Colin Walters <walters@verbum.org>
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..3"
+echo "1..4"
 
 . $(dirname $0)/libtest.sh
 
@@ -35,6 +35,18 @@ $OSTREE fsck -q
 
 echo "ok chmod"
 
+cd ${test_tmpdir}
+rm repo files -rf
+setup_test_repository "bare"
+rev=$($OSTREE rev-parse test2)
+echo -n > repo/objects/${rev:0:2}/${rev:2}.commit
+if $OSTREE fsck -q 2>err.txt; then
+    fatal "fsck unexpectedly succeeded"
+fi
+assert_file_has_content_literal err.txt "Corrupted commit object; checksum expected"
+
+echo "ok metadata checksum"
+
 cd ${test_tmpdir}
 rm repo files -rf
 setup_test_repository "bare"

tests/test-pull-corruption.sh

@@ -76,7 +76,7 @@ if ! skip_one_without_user_xattrs; then
     if ${CMD_PREFIX} ostree --repo=ostree-srv/gnomerepo fsck 2>err.txt; then
         assert_not_reached "fsck with corrupted commit worked?"
     fi
-    assert_file_has_content err.txt "corrupted object ${corruptrev}\.commit"
+    assert_file_has_content_literal err.txt "Corrupted commit object; checksum expected='${corruptrev}' actual='${rev}'"
 
     # Do a pull-local; this should succeed since we don't verify checksums
     # for local repos by default.