git-svn: handling merge-base failures

> One of my projects is failing to clone because merge-base is failing
> on one of the revisions; the branch is a partial branch, so merge-base
> can't find a common ancestor with trunk.  I'd like to catch the
> exception that command_oneline should throw when merge-base fails, but
> my perl is very rusty and I'm struggling to get git-svn.perl to grok
> the Git::Error::Command class.  What is the appropriate way to import
> that class?  Or more generally, is there a better solution to handling
> this error case?

> Andrew Myrick <amyrick@apple.com> wrote:
>> One of my projects is failing to clone because merge-base is failing
>> on one of the revisions; the branch is a partial branch, so merge-base
>> can't find a common ancestor with trunk.  I'd like to catch the
>> exception that command_oneline should throw when merge-base fails, but
>> my perl is very rusty and I'm struggling to get git-svn.perl to grok
>> the Git::Error::Command class.  What is the appropriate way to import
>> that class?  Or more generally, is there a better solution to handling
>> this error case?
> 
> Hi Andrew,
> 
> Git::Error::Command should be imported with the rest of Git.pm
> 
> It's a special way of doing exceptions in Perl which I don't see much
> point of...  Looking at git-send-email as an example, it does this:
> 
> 	use Error qw(:try);
> 	use Git;
> 
> 	try {
> 		# something that will throw
> 	} catch Git::Error::Command with {
> 		# error handling
> 	}

> 
> A more standard approach in Perl is just:
> 
> 	eval {
> 		# something that will die
> 	};
> 	if ($@) {
> 		# error handling
> 	}
> 

> On Dec 23, 2009, at 12:09 PM, Eric Wong wrote:
> > Andrew Myrick <amyrick@apple.com> wrote:
> >> One of my projects is failing to clone because merge-base is failing
> >> on one of the revisions; the branch is a partial branch, so merge-base
> >> can't find a common ancestor with trunk.  I'd like to catch the
> >> exception that command_oneline should throw when merge-base fails, but
> >> my perl is very rusty and I'm struggling to get git-svn.perl to grok
> >> the Git::Error::Command class.  What is the appropriate way to import
> >> that class?  Or more generally, is there a better solution to handling
> >> this error case?
> > 
> > Hi Andrew,
> > 
> > Git::Error::Command should be imported with the rest of Git.pm
> > 
> > It's a special way of doing exceptions in Perl which I don't see much
> > point of...  Looking at git-send-email as an example, it does this:
> > 
> > 	use Error qw(:try);
> > 	use Git;
> > 
> > 	try {
> > 		# something that will throw
> > 	} catch Git::Error::Command with {
> > 		# error handling
> > 	}
> 
> I looked at git-send-email's example, but I wanted to do a bit more:
> 	
> 	try {
> 		# command_oneline(...);
> 	} catch Git::Error::Command with {
> 		$E = shift;
> 		if ($E->value() == 1) {
> 			# do something
> 		} else {
> 			# do something else
> 		}
> 	}
>  	
> This is used in perl/Git.pm, but when I tried it in git-svn.perl, it
> fails with the error "Can't locate object method 'value' via package
> 'Git::SVN'".

> Presumably $@ contains the Git::Error::Command object, which leaves me
> in the same spot, unfortunately.
> 
> > But yes, it is Perl after all and TMTOWTDI :)
> 
> Is there ever :)

> On Dec 23, 2009, at 11:54 AM, Andrew Myrick wrote:
> > One of my projects is failing to clone because merge-base is failing
> > on one of the revisions; the branch is a partial branch, so
> > merge-base can't find a common ancestor with trunk.  I'd like to
> > catch the exception that command_oneline should throw when
> > merge-base fails,
> 
> Instead of using the Error.pm module, I was able to handle the
> exception with the more basic eval block.  However, there are some
> details here I would like to discuss with the community.
> 
> When git-svn fetches a partial branch, it appears to refetch all of
> the history of the subdirectory from which the branch was created.
> This creates new commits for the old revisions, and these new commits
> exist as a separate history for the partial branch.   When git-svn
> fetches the revision at which this partial branch is merged back to
> trunk, git-svn attempts to run merge-base to find a common ancestor.
> However, because the partial branch has its own history, the
> merge-base fails, and git-svn dies.
> 
> Naively handling the exception with an eval block and skipping the
> merge ticket works fine in that it brings us back to parity with git
> 1.6.5.7, but it means that the merge parent relationship between trunk
> and the partial branch is lost.  Is there any way to preserve this
> information, or does the separate commit history of the partial branch
> make it fundamentally impossible?

> Andrew Myrick <amyrick@apple.com> wrote:
>> On Dec 23, 2009, at 11:54 AM, Andrew Myrick wrote:
>>> One of my projects is failing to clone because merge-base is failing
>>> on one of the revisions; the branch is a partial branch, so
>>> merge-base can't find a common ancestor with trunk.  I'd like to
>>> catch the exception that command_oneline should throw when
>>> merge-base fails,
>> 
>> Instead of using the Error.pm module, I was able to handle the
>> exception with the more basic eval block.  However, there are some
>> details here I would like to discuss with the community.
>> 
>> When git-svn fetches a partial branch, it appears to refetch all of
>> the history of the subdirectory from which the branch was created.
>> This creates new commits for the old revisions, and these new commits
>> exist as a separate history for the partial branch.   When git-svn
>> fetches the revision at which this partial branch is merged back to
>> trunk, git-svn attempts to run merge-base to find a common ancestor.
>> However, because the partial branch has its own history, the
>> merge-base fails, and git-svn dies.
>> 
>> Naively handling the exception with an eval block and skipping the
>> merge ticket works fine in that it brings us back to parity with git
>> 1.6.5.7, but it means that the merge parent relationship between trunk
>> and the partial branch is lost.  Is there any way to preserve this
>> information, or does the separate commit history of the partial branch
>> make it fundamentally impossible?
> 
> Hi Andrew,
> 
> A method of preserving the $SVN_PATH <=> $PARENT@$REV mapping for
> reusing follow_parent-created branches is definitely desired.
> 
> I've just been lacking time and motivation these days with other
> projects taking priority.  Help (even if it's just to refactor/cleanup
> existing code) would definitely be appreciated here.