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Re: svn commit: r1555350 -/subversion/trunk/subversion/libsvn_client/mtcc.c

From: Bert Huijben <bert_at_qqmail.nl>
Date: Sat, 4 Jan 2014 23:07:25 +0000

In the case of svnmucc the passed scratch pool is a subpool of the pool that the Ra session id created it (where the mtcc is created in). This is a perfectly valid case of using pools, just like the common iterpool pattern.

If this shows a bug in the caching of the fs layer we should fix that bug instead of requiring the rest of our code to just use a single pool for everything. So your fs changes +- requires users to stop using iterpool and scratch pool everywhere.

The fs layer should manage its own memory and not require API users to manage it for this library.

The cache pattern of allocating pointers in an unrelated pool, pointing to data sorted centrally with different lifetime is broken. To make this work you need cleanup handlers both ways… or a separate allocation strategy (refcounts, global allocator?)

(I'm not able to look at the svnmucc code in detail now, so there might be another bug that you fixed… but by reading your log message and change this looks like covering up for a design problem)

We can't fix all code using our APIs until 2.0. Until then our existing API surface should cover for requirements changes.


Sent from Windows Mail

From: Stefan Fuhrmann
Sent: ‎Saturday‎, ‎January‎ ‎4‎, ‎2014 ‎10‎:‎26‎ ‎PM
To: Bert Huijben
Cc: Subversion Development, commits_at_subversion.apache.org

On Sat, Jan 4, 2014 at 9:47 PM, Bert Huijben <bert_at_qqmail.nl> wrote:

You only know this for the success path. The fs api should hide these details..

With all due respect, the fault is svn_client_mtcc_commit

destroying mtcc->pool before destroying mtcc itself (via

its owning pool). Hence, the caller has no control over pool
lifetimes and any allocation in sibling pools, e.g. scratch_pool,
is potentially invalid.

There is no way we can document these guarantees for public APIs and really assume that all existing and future users follow all these new requirements for a new Fs layer cache.

We had similar caches in the past, but hid the ugly details by installing cleanup handlers...

We should really look at this well before 1.9, as adding such requirements is a breaking change. And the commit API already existed since well before 1.0 without these pool requirements. Strict dual pools are only common since 1.7, but especially generated bindings (like swig) have strange pool handling much longer.

I leave it to the author of the mucc code to simply go back

to standard pool handling by destroying the mtcc with its
parent pool instead of partly destroying it in some API function.

-- Stefan^2.

From: stefan2_at_apache.org
Sent: 4-1-2014 15:05
To: commits_at_subversion.apache.org
Subject: svn commit: r1555350 -/subversion/trunk/subversion/libsvn_client/mtcc.c

Author: stefan2
Date: Sat Jan 4 14:04:36 2014
New Revision: 1555350

URL: http://svn.apache.org/r1555350
Fix segfault in svnmucc.

* subversion/libsvn_client/mtcc.c
  (svn_client_mtcc_commit): We explicitly destroy the MTCC pool and
                             have no knowledge about its relation to
                             SCRATCH_POOL. Thus, we can't use the
                             for anything non-trivial.


Modified: subversion/trunk/subversion/libsvn_client/mtcc.c
URL: http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_client/mtcc.c?rev=1555350&r1=1555349&r2=1555350&view=diff
--- subversion/trunk/subversion/libsvn_client/mtcc.c (original)
+++ subversion/trunk/subversion/libsvn_client/mtcc.c Sat Jan 4 14:04:36 2014
@@ -1337,12 +1337,15 @@ svn_client_mtcc_commit(apr_hash_t *revpr
                                "is not a directory"),

+ /* Beware that the editor object must not live longer than the MTCC.
+ Otherwise, txn objects etc. in EDITOR may live longer than their
+ respective FS objects. So, we can't use SCRATCH_POOL here. */
   SVN_ERR(svn_ra_get_commit_editor3(mtcc->ra_session, &editor, &edit_baton,
                                     commit_callback, commit_baton,
                                     NULL /* lock_tokens */,
                                     FALSE /* keep_locks */,
- scratch_pool));
+ mtcc->pool));

   err = editor->open_root(edit_baton, mtcc->base_revision, scratch_pool, &root_baton);
Received on 2014-01-05 00:16:52 CET

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