Merge branch 'fc/remote-bzr'
[git/git.git] / Documentation / howto / recover-corrupted-blob-object.txt
1 Date: Fri, 9 Nov 2007 08:28:38 -0800 (PST)
2 From: Linus Torvalds <torvalds@linux-foundation.org>
3 Subject: corrupt object on git-gc
4 Abstract: Some tricks to reconstruct blob objects in order to fix
5 a corrupted repository.
6 Content-type: text/asciidoc
7
8 How to recover a corrupted blob object
9 ======================================
10
11 -----------------------------------------------------------
12 On Fri, 9 Nov 2007, Yossi Leybovich wrote:
13 >
14 > Did not help still the repository look for this object?
15 > Any one know how can I track this object and understand which file is it
16 -----------------------------------------------------------
17
18 So exactly *because* the SHA1 hash is cryptographically secure, the hash
19 itself doesn't actually tell you anything, in order to fix a corrupt
20 object you basically have to find the "original source" for it.
21
22 The easiest way to do that is almost always to have backups, and find the
23 same object somewhere else. Backups really are a good idea, and git makes
24 it pretty easy (if nothing else, just clone the repository somewhere else,
25 and make sure that you do *not* use a hard-linked clone, and preferably
26 not the same disk/machine).
27
28 But since you don't seem to have backups right now, the good news is that
29 especially with a single blob being corrupt, these things *are* somewhat
30 debuggable.
31
32 First off, move the corrupt object away, and *save* it. The most common
33 cause of corruption so far has been memory corruption, but even so, there
34 are people who would be interested in seeing the corruption - but it's
35 basically impossible to judge the corruption until we can also see the
36 original object, so right now the corrupt object is useless, but it's very
37 interesting for the future, in the hope that you can re-create a
38 non-corrupt version.
39
40 -----------------------------------------------------------
41 So:
42
43 > ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../
44 -----------------------------------------------------------
45
46 This is the right thing to do, although it's usually best to save it under
47 it's full SHA1 name (you just dropped the "4b" from the result ;).
48
49 Let's see what that tells us:
50
51 -----------------------------------------------------------
52 > ib]$ git-fsck --full
53 > broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
54 > to blob 4b9458b3786228369c63936db65827de3cc06200
55 > missing blob 4b9458b3786228369c63936db65827de3cc06200
56 -----------------------------------------------------------
57
58 Ok, I removed the "dangling commit" messages, because they are just
59 messages about the fact that you probably have rebased etc, so they're not
60 at all interesting. But what remains is still very useful. In particular,
61 we now know which tree points to it!
62
63 Now you can do
64
65 git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
66
67 which will show something like
68
69 100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore
70 100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap
71 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING
72 100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453 CREDITS
73 040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6 Documentation
74 100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32 Kbuild
75 100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9 MAINTAINERS
76 ...
77
78 and you should now have a line that looks like
79
80 10064 blob 4b9458b3786228369c63936db65827de3cc06200 my-magic-file
81
82 in the output. This already tells you a *lot* it tells you what file the
83 corrupt blob came from!
84
85 Now, it doesn't tell you quite enough, though: it doesn't tell what
86 *version* of the file didn't get correctly written! You might be really
87 lucky, and it may be the version that you already have checked out in your
88 working tree, in which case fixing this problem is really simple, just do
89
90 git hash-object -w my-magic-file
91
92 again, and if it outputs the missing SHA1 (4b945..) you're now all done!
93
94 But that's the really lucky case, so let's assume that it was some older
95 version that was broken. How do you tell which version it was?
96
97 The easiest way to do it is to do
98
99 git log --raw --all --full-history -- subdirectory/my-magic-file
100
101 and that will show you the whole log for that file (please realize that
102 the tree you had may not be the top-level tree, so you need to figure out
103 which subdirectory it was in on your own), and because you're asking for
104 raw output, you'll now get something like
105
106 commit abc
107 Author:
108 Date:
109 ..
110 :100644 100644 4b9458b... newsha... M somedirectory/my-magic-file
111
112
113 commit xyz
114 Author:
115 Date:
116
117 ..
118 :100644 100644 oldsha... 4b9458b... M somedirectory/my-magic-file
119
120 and this actually tells you what the *previous* and *subsequent* versions
121 of that file were! So now you can look at those ("oldsha" and "newsha"
122 respectively), and hopefully you have done commits often, and can
123 re-create the missing my-magic-file version by looking at those older and
124 newer versions!
125
126 If you can do that, you can now recreate the missing object with
127
128 git hash-object -w <recreated-file>
129
130 and your repository is good again!
131
132 (Btw, you could have ignored the fsck, and started with doing a
133
134 git log --raw --all
135
136 and just looked for the sha of the missing object (4b9458b..) in that
137 whole thing. It's up to you - git does *have* a lot of information, it is
138 just missing one particular blob version.
139
140 Trying to recreate trees and especially commits is *much* harder. So you
141 were lucky that it's a blob. It's quite possible that you can recreate the
142 thing.
143
144 Linus