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[RFC/PATCH] svnadmin: recover/hotcopy erroring out for old FSFS repositories

From: Evgeny Kotkov <evgeny.kotkov_at_visualsvn.com>
Date: Thu, 23 Jan 2014 14:00:18 +0400

Hi,

I've noticed that svnadmin recover and hotcopy commands are broken for old
repositories around trunk_at_1560210. The problem can be reproduced by creating
a new --compatible-version=1.3 / 1.2 / 1.1 FSFS repository, doing something
simple with it (say, running svn mkdir or svn checkout / add file / commit) and
attempting to recover or hotcopy it:
[[[
  (svnadmin-trunk recover REPOS)
   svnadmin: E200002: Serialized hash missing terminator

  (svnadmin-trunk hotcopy REPOS REPOS_BACKUP)
   svnadmin: E160006: No such revision 1
]]]

Alternatively, this can be reproduced by running svnadmin_tests.py with, for
example, --server-minor-version=3. This problem exists in both 1.8.x and
trunk. In 1.8.x, however, the errors for both hotcopy and recover are visually
identical:
[[[
  (svnadmin-1.8.x recover REPOS)
   svnadmin: E200002: Serialized hash missing terminator

  (svnadmin-1.8.x hotcopy REPOS REPOS_BACKUP)
   svnadmin: E200002: Serialized hash missing terminator
]]]

Overall, this looks like a regression since 1.7.x, where hotcopy works just
fine. The 1.7.x recover also fails for these repositories with an
"svnadmin: E000002: Can't open file 'X/db/revs/2': No such file or directory"
error, but this is a well-known issue 4213 [1], which should be fixed by
r1367683 [2].

Being unable to recover and hotcopy old repositories with the most recent
svnadmin version seems like an important issue, so I think it's worth a couple
of tests. [See the attached patch #1]

I did a quick investigation and spotted a couple of odd moments in the
related code:

1) First of all, there is a reason for the different error messages in trunk
   and in the 1.8.x branch.

   This is a regression introduced in r1503742 [3], because now calling the
   svn_fs_fs__find_max_ids() routine during hotcopy creates a cyclic
   dependency. Hotcopy needs to update the db/current file, so it uses this
   routine to get the NEXT_NODE_ID / NEXT_COPY_ID for old repositories.
   However, this currently behaves the following way:

   hotcopy_update_current()
     ...
     (wants to update the db/current file, but doesn't know the next ids)

      svn_fs_fs__find_max_ids()
        svn_fs_fs__rev_get_root()
          svn_fs_fs__ensure_revision_exists()
          ...
          (reads the most recent db/current file in order to check the presence
           of this revision. it is reported missing, because we're right in the
           middle of updating the db/current file and cannot do that yet,
           because we still do not know the next ids)

   So, basically, we're calling a routine that assumes the db/current file is
   up-to-date in order to get the information on how to update the outdated
   db/current file. This ends with an "E160006: No such revision 1" error.

2) The reason for the "hash missing terminator" error is a bit trickier.

   The NEXT_NODE_ID / NEXT_COPY_ID recovery code recursively crawls
   through the dir representations in the db, however, it assumes that the
   EXPANDED_SIZE in the rep description always is non-zero. According to
   the docstring in subversion/libsvn_fs_fs/fs.h, this assumption is incorrect:
   [[[
     /* The size of the representation in bytes as seen in the revision
        file. */
     svn_filesize_t size;

     /* The size of the fulltext of the representation. If this is 0,
      * the fulltext size is equal to representation size in the rev file, */
     svn_filesize_t expanded_size;
   ]]]

   I did a small experiment with different svn (1.7 / 1.8 / trunk) versions
   writing dir reps to repositories with different --compatible-versions...
   [[[
     # text: REVISION OFFSET SIZE EXPANDED_SIZE
     svn 1.7, --compatible-version=1.3 repository (db/revs/1)
      text: 1 57 28 28
     svn 1.8, --compatible-version=1.3 repository (db/revs/1)
      text: 1 57 28 0
   ]]]

   ...and it looks like svn 1.7.x did have the EXPANDED_SIZE equal to SIZE, but
   this behavior changed with 1.8.x. In 1.8.x, EXPANDED_SIZE is set to 0 for
   these representations, so, attemping to read them ends with an
   "E200002: Serialized hash missing terminator" error.

   See the attached 'rep-description-dumps' file.

   NOTE: This code is a part of the recover_find_max_ids() routine and, worth
   mentioning, has some history. There is r1404675 [4], which added the "pick
   the right one from EXPANDED_SIZE and SIZE" logic, however, it was reverted
   in r1404708 [5]. Unfortunately, I do not yet understand the reasoning
   behind this revert, so, I guess, I'll just have to continue investigating.

3) There is an "uniquifier" concept for the representations that makes the rep
   caching possible. Each uniquifier requires a new NODE_ID, so, for example,
   a transaction which requires 5 new NODE_IDs ends up with taking 10 of them.
   This assumption is not shared by the recovery procedure, which recovers the
   NEXT_NODE_ID / NEXT_COPY_ID for old repositories by finding the maximum
   NODE_ID / COPY_ID and bumping them by one.

   For example, for the old format Greek tree repository with db/current
   containing "1 14 1" the recovery procedure will replace the db/current
   contents with "1 13 1". '13' in this example is the recovered NEW_NODE_ID
   (but it also is the NODE_ID reserved for the uniquifier of the last "real"
   NODE_ID). As a consequence, hotcopying this repository (which utilizes the
   recover_find_max_ids() routine) will end up with *mismatching db/current*
   files in the hotcopy source and destination.

   It looks like making these uniquifiers for old repositories is unnecessary
   in the first place (they are only required for rep caching, which is only
   available for the newer >= SVN_FS_FS__MIN_REP_SHARING_FORMAT
   repositories).

   See set_uniquifier() in subversion/libsvn_fs_fs/transaction.c

4) The hotcopy_update_current() does something quite odd for old repositories.

   It grabs the MAX_NODE_ID / MAX_COPY_ID from svn_fs_fs__find_max_ids(),
   claims them to be the NEXT_NODE_ID / NEXT_COPY_ID and writes them into the
   db/current file. The maximum values found by the svn_fs_fs__find_max_ids()
   routine should be bumped by one (the same way it is done in recover_body(),
   subversion/libsvn_fs_fs/recovery.c).

   Not doing so will result in the hotcopy destination having incorrect
   NEXT_NODE_ID and NEXT_COPY_ID in the db/current file (pointing to
   already taken ids).

If this might help to understand what is going on, I've made a quick-and-dirty
patch for 1.8.x branch, which explicitly points the problems and rushes toward
the green test suite. Doing this for 1.8.x allows to postpone the problem 1)
and focus on 2-4), because, as far as I suspect, fixing 1) might get a bit
messy. [See the attached patch #2]

An appropriate fix for this issue undoubtedly requires a lot more work.
I think I might be able to come up with a patch for the trunk in the nearby
future (in case someone does not fix it earlier).

[1] http://subversion.tigris.org/issues/show_bug.cgi?id=4213
[2] http://svn.apache.org/viewvc?view=revision&revision=r1367683
[3] http://svn.apache.org/viewvc?view=revision&revision=r1503742
[4] http://svn.apache.org/viewvc?view=revision&revision=r1404675
[5] http://svn.apache.org/viewvc?view=revision&revision=r1404708

Thanks and regards,
Evgeny Kotkov

Received on 2014-01-23 11:01:12 CET

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