Subject: Git-SVN on Cygwin: svn+ssh good, https awkward - msg#02003

Hi Eric,

=== Short story ===

git-svn on Cygwin works well with svn+ssh://, but needs some fixes around tempfile and/or subprocess handling for https:// - please help.


=== Long story ===

First the good news: git-svn on Cygwin seems to work just fine with svn+ssh://... style Subversion URLs. I've been doing this for quite a while now with various repos, never had issues: I can init, clone, fetch rebase, dcommit just fine.

I'm looking for some help https:// access on Cygwin (Windows XP SP3 underneath; more version info at the end of the message).

I've debugged it a little, but am not sufficiently acquainted with the git-svn Perl source to do changes myself; I hope I can describe the issue well enough that you (or somebody else) can suggest code changes to improve the experience.


0. PROBLEM/SYMPTOM:

login@mycomp ~/compnet2.gitsvn
$ git svn fetch
M file1
A file2
A file3
A file4
r15 = 70a56f818a057442e29161170e28c5df4b38a811 (git-svn)
Permission denied: Can't open '/cygdrive/c/DOKUME~1/login/LOKALE~1/Temp/tempfile.tmp': Permission denied at /usr/local/libexec/git-core/git-svn line 2540

which is, in my version:

2537 }
2538 $ed = SVN::Git::Fetcher->new($self);
2539 }
+ 2540 unless ($self->ra->gs_do_update($last_rev, $rev, $self, $ed)) {
2541 die "SVN connection failed somewhere...\n";
2542 }
2543 $self->make_log_entry($rev, \@parents, $ed);


Apparently, we die when fetching a new revision for the 2nd time. I can restart git svn fetch and it will pick up where it left off. It will make progress, but is painful.

git svn rebase has the same issues. I haven't tested other operations such as dcommit yet.


1. WORKAROUND for fetch/rebase:

while ! git svn fetch ; do : ; done
git svn rebase


2. A SHORT LOG OF FINDING THE CAUSE:

$ perl -d /usr/local/libexec/git-core/git-svn fetch
...
main::(/usr/local/libexec/git-core/git-svn:10):
10: $AUTHOR = 'Eric Wong <normalperson@xxxxxxxx>';
DB<1> c 2540
Git::SVN::do_fetch(/usr/local/libexec/git-core/git-svn:2540):
2540: unless ($self->ra->gs_do_update($last_rev, $rev, $self, $ed)) {
DB<2> c 2540
...
r16 = d7829595222a013e703fe4377996984b52921789 (git-svn)
Git::SVN::do_fetch(/usr/local/libexec/git-core/git-svn:2540):
2540: unless ($self->ra->gs_do_update($last_rev, $rev, $self, $ed)) {

Now watch this (we're immediately before the problem):

$ ls -l /cygdrive/c/DOKUME~1/login/LOKALE~1/Temp/
??????????? ? ? ? ? ? tempfile.tmp
$ ls -l /cygdrive/c/DOKUME~1/login/LOKALE~1/Temp/tempfile.tmp
ls: cannot access /cygdrive/c/DOKUME~1/login/LOKALE~1/Temp/tempfile.tmp: No such file or directory

This zombie behaviour of trashing the file flags happens if Cygwin is asked to unlink() a file to which there are still open handles. It can't delete the file right away (due to the open handles), so it marks it delete-on-close instead, which then happens when the process(es) terminate.

$ ~/Desktop/handle.exe tempfile.tmp
...
git.exe pid: 2236 62C: C:\DOKUME~1\mandree\LOKALE~1\Temp\tempfile.tmp
git.exe pid: 5552 62C: C:\DOKUME~1\mandree\LOKALE~1\Temp\tempfile.tmp
git-hash-object.exe pid: 5320 62C: C:\DOKUME~1\mandree\LOKALE~1\Temp\tempfile.tmp

So we have three processes that have the file open. I cannot say if they mean the same file, or a different one, but this is the problem.

Here's the pstree dump to help identify the users of this file, with cmdline arguments:

perl,5660 -d -I... /usr/local/libexec/git-core/git-svn fetch
|-git,2236 --batch
|-git,5552 git-hash-object -w --stdin-paths
| `-git-hash-object,5320
`-git,1128 update-index -z --index-info

Let's continue in the Perl debugger and crash:

DB<3> c
Permission denied: Can't open '/cygdrive/c/DOKUME~1/login/LOKALE~1/Temp/tempfile.tmp': Permission denied at /usr/local/libexec/git-core/git-svn line 2540


Bottom line: the second time we hit this "unless ($self->ra->gs_do_update($last_rev, $rev, $self, $ed))" line, we die because there are still processes that have the tempfile open and thus the former unlink() on the tempfile.tmp couldn't become effective.



3. UNDERLYING CYGWIN LIMITATIONS

<http://wiki.osdev.org/Cygwin_Issues#unlink.28.29> documents now (2009-04-23 16:15 UTC):
* unlink isn't atomic (as on other platforms also)
...
On cygwin unlink() is deferred to the end of the process, when unlink fails because of locks or delete on close (ERROR_SHARING_VIOLATION).

* unlink a file and subsequent creation of a new file in the same location is not supported.

* "delete on close" (unlinking an open file) is supported.


I can provide a short C program to demonstrate this behaviour.



4. FIX SUGGESTIONS AND QUESTIONS

Can we defer unlink()ing the tempfile until all handles to it are closed?

Can we let complete and wait for all processes that hold handles to the tempfile.tmp file before attempting to create a new copy?

Can we use unique tempfile names (timestamps, counter, random number generator) instead of the hardcoded "tempfile.tmp"? This is probably a good idea anyways to evade symlink attacks.


Either should work to fix the issue.


I'm willing to test patches.


5. VERSION INFORMATION:

Cygwin:
CYGWIN_NT-5.1 balu 1.5.25(0.156/4/2) 2008-06-12 19:34 i686 Cygwin

Git:
git version 1.6.3.rc1.51.gea0b7

Subversion:
svn, version 1.6.1 (r37116)
compiled Apr 11 2009, 12:07:06

Copyright (C) 2000-2009 CollabNet.
Subversion is open source software, see http://subversion.tigris.org/
This product includes software developed by CollabNet (http://www.Collab.Net/).

The following repository access (RA) modules are available:

* ra_neon : Module for accessing a repository via WebDAV protocol using Neon.
- handles 'http' scheme
- handles 'https' scheme
* ra_svn : Module for accessing a repository using the svn network protocol.
- with Cyrus SASL authentication
- handles 'svn' scheme
* ra_local : Module for accessing a repository on local disk.
- handles 'file' scheme
* ra_serf : Module for accessing a repository via WebDAV protocol using serf.
- handles 'http' scheme
- handles 'https' scheme



Thanks for any efforts to help!

--
Matthias Andree
--
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html