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Re: Problems with the documentation of Subversion dump format

From: Johan Corveleyn <jcorvel_at_gmail.com>
Date: Tue, 13 Dec 2011 22:04:33 +0100

On Tue, Dec 13, 2011 at 8:16 PM, Daniel Shahaf <d.s_at_daniel.shahaf.name> wrote:
> C. Michael Pilato wrote on Tue, Dec 13, 2011 at 14:01:45 -0500:
>> On 12/13/2011 01:25 PM, Eric S. Raymond wrote:
>> > C. Michael Pilato <cmpilato_at_collab.net>:
>> >>> Does a file replace differ in any way from a delete plus add of the new text?
>> >>
>> >> In Subversion, yes.  A replacement is, like an add or a delete, an operation
>> >> at the node level, not an operation on the contents of that node.  A replace
>> >> is an addition of a new object[1] -- with its own new line of version
>> >> control history -- that is coincidental with the removal of some previously
>> >> existing object that occupied the same path.
>> >
>> > I still don't understand how this differs from a delete followed by an add.
>> > Explain it to me like I'm reallllyyy stuuupid, please, so I can document it
>> > and you never have to explain it again.
>> >
>> > When I add a file at a given path, it creates new object with a
>> > history that is tracked.  When I delete that path, I destroy the
>> > container as well as the content.  If I subsequently create a new
>> > file at the same path, it's a new object with its own history.
>> >
>> > How is a replace different?
>>
>> Assume your "delete" and subsequent "add" happens in the same commit, it's
>> not different at all.  In fact, the Subversion filesystem API doesn't even
>> recognize a "replace" operation.  There's "delete (file or dir)", there's
>> "make file" and "make dir", and there's "copy (file or dir)".  The "replace"
>> action found in the dumpfile is just a compacting of some delete operation
>> and a subsequent add or copy into a single verb, and that only because it
>> helps sequential processors of the dump stream avoid possibly notifying
>> about multiple actions on the same path.  We favor the likes of:
>>
>>     R   /some/file.txt
>>
>> over:
>>
>>     D   /some/file.txt
>>     A   /some/file.txt
>>
>> in output.
>>
>> (My prior response was the result of my misreading your phrase "delete plus
>> add of the new text" as meaning "removing all the contents of the file, and
>> then adding all new contents of the same file".  I see now that you were
>> talking about "container" operations, not content ones.  Sorry about that.)
>>
>> >> [1] Most of the time.  A replacement can have a copyfrom source, in which
>> >> case its not strictly a new line of history for that object.
>> >
>> > I think I get this part.  When you replace with a copy source, you're
>> > destroying the container that existed at this path, abd replacing it with
>> > a new container that has history extending back through the copy source.
>> > Is that correct?
>>
>> Yup!
>>
>> I was trying to think through the generalities here, too.  I believe they
>> boil down to this:
>>
>>    "delete" stands alone.  It never has text.  Never has properties.
>>    Never has copyfrom.
>>
>>    "add" and "replace" can have text if the added object is a file.  The
>>    text is the contents of the added object as it appears in the committed
>>    revision.  "add" and "replace" of directories can not have text.
>>
>>    "add and replace" can have properties -- the set of properties present
>>    on the added file/directory in the committed revision.
>>
>>    "add and replace" can have copyfrom information, indicating that the
>>    "added" object does not truly represent the creation of a new line of
>>    history, but is instead a continuation of a pre-existing line of
>>    history.  This is still an addition of sorts in that the object is newly
>>    added to the set of its parent directory's list of children.
>>
>> But I haven't double-thunk that for complete accuracy.
>>
>> > So, everything except a delete can include properties and they all
>> > work the same way. Correct?
>>
>> Yes.
>>
>> >>> If a file replace can have a copyfrom source, how does replace with a
>> >>> copyfrom source differ from add with a copyfrom source?
>> >>
>> >> The differ only in the fact that a replace implies the simultaneous deletion
>> >> of some other object which previously lived at that path.
>> >
>> > Got it.  That case I understand, it's how they differ in the non-copyfrom
>> > case that still confuses me.
>>
>> This is replace without copyfrom:
>>
>>    $ svn rm some/file.txt
>>    $ touch some/file.txt
>>    $ svn add some/file.txt
>>    $ svn ci -m "Replace some/file.txt with a new file."
>>
>> This is replace with copyfrom:
>>
>>    $ svn rm some/file.txt
>>    $ touch some/file.txt
>>    $ svn copy someother/differentfile.txt some/file.txt
>>    $ svn ci -m "Replace some/file.txt with a copy of a different file ."
>
> And:
>
>   $ svn rm some/file.txt
>   $ touch some/file.txt
>   $ svn copy some/file.txt_at_HEAD some/file.txt
>   $ svn ci -m "Replace some/file.txt with a copy itself."

And even:

  $ svn mv some/file.txt some/otherfile.txt
  $ svn mv some/otherfile.txt some/file.txt
  $ svn ci -m "Replace some/file.txt with a copy of itself."

(pre-1.7 this would be a replace without copyfrom, breaking the line
of history [1], but that is fixed as of 1.7)

[1] http://subversion.tigris.org/issues/show_bug.cgi?id=3429 - "svn mv
A B; svn mv B A" generates replace without history

-- 
Johan
Received on 2011-12-13 22:05:29 CET

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