Authors mapping

SVN Users and Git Authors

Both Subversion and Git keep authors in commits, but those authors differ.

In SVN, the author is being stored as an unversioned revision property svn:author. Every time a Subversion user makes a commit, SVN creates a new revision and sets this revision svn:author property to that exact user's name, for example, johndoe:

------------------------------------------------------------------------
r163 | johndoe | 2017-06-07 20:22:15 +0500 (Wed, 07 Jun 2017) | 1 line
Changed paths:
   A /project
   A /project/branches
   A /project/tags
   A /project/trunk

initial layout for the project
------------------------------------------------------------------------

????

$ svn proplist -v --revprop --revision 163

Unversioned properties on revision 163:
  svn:author
    johndoe
  svn:date
    2017-06-07T15:22:15.655243Z
  svn:log
    initial layout for the project

Git also stores author name along with commits, but this name differs from that in SVN: whereas SVN stores actual username, Git user identity consists of a name and email:

Git User <gituser@domain.com>

Those name and email don't relate to an actual username that is used to login to Git repository, they are being set in Git configuration, for example then may be set by the commands below:

$ git config --global user.name "John Doe"
$ git config --global user.email johndoe@example.com

The Git user John Doe is then referred as

John Doe <johndoe@example.com>

 This exact line then appears as the author name in every commit John Doe makes.

It worth to mention that Git holds not only author name, but also a committer name:

$ git cat-file commit HEAD
    tree 905df23db37b33320483fc6676bfc684078ed248
    parent 4a0cf06baa9aefaa20a13820265ef401d7b1c2b6
    author John Doe <johndoe@example.com> 1496849115 +0000
    committer Jane Doe <janedoe@example.com> 1496849115 +0000

Pro Git book describes the difference between those names as follows: the author is the person who originally wrote the work, whereas the committer is the person who last applied the work. So, if you send in a patch to a project and one of the core members applies the patch, both of you get credit – you as the author, and the core member as the committer.

SubGit translates Git author name, so committer name doesn't mean much for the SVN-to-Git translation process.

Configuration options

All the configuration options reside in SubGit configuration file, that is situated in subgit subdirectory inside a newly created Git repository:

GIT_REPOS_PATH/subgit/config

There are two configuration options that relate to authors:

• core.authorsFile

  this option represents a path to the authors mapping file or authors mapping helper program. The path is either relative to the Git repository or absolute. There may be more that one authorsFile option set in the file:

  [core]
     authorsFile = subgit/authors.txt
     authorsFile = /etc/authors.txt

  All the mentioned files content is being merged into a full list. If an SVN username appears more than once – only its first occurrence will be applied. For example, if an SVN username johndoe appears in both authors files:

  subgit/authors.txt    
        johndoe = John Doe <johndoe@example.com>
  …
  /etc/authors.txt    
        johndoe = John M. Doe <john_doe@example.com>
  
  

  In this case, the SVN name johndoe is being mapped to Git name John Doe <johndoe@example.com> as it appears first in the list.

• core.defaultDomain

  this option represents a domain name that is being used to construct a Git user email address in case if no explicit authors mapping has been provided. SubGit automatically fills that option with a hostname when subgit configure command is invoked. If the option is not set, Git user email appears empty in commits – a pair of angle brackets with nothing in between:

  Author: john_doe <>

Automatic Authors Mapping

When SubGit starts translation between SVN and Git, it looks for authors mapping files or authors helper programs. If none of them present, it generates the mapping automatically, following these rules for the translation:

• Subversion svnusername is translated to svnusername <svnusername@defaultDomain>> in Git
• Git Author Name <email@domain.com> is translated to Author Name in Subversion

'defaultDomain' here stands for the core.defaultDomain SubGit configuration option. SubGit fills that setting with the hostname during subgit configure process, but it can be changed later. Also, if subgit configure is invoked with --layout auto option, SubGit fills the authors file with automatically generated mapping - i.e. SubGit connects to the SVN, checks through the project history and records all the SVN users found in the history. Then SubGit generates Git names and emails from those SVN usernames according to the rules above and records resulting mapping to the authors file.

Say, a user makes commits using john_doe SVN user; a SVN revision he made may look like:

------------------------------------------------------------------------
    r167 | john_doe | 2017-06-06 10:25:02 +0200 (Tue, 06 Jun 2017) | 1 line
    Changed paths:
       M /project/trunk/foo.c

    minor changes
    ------------------------------------------------------------------------

at some point, the SVN project is being translated to Git. If no explicit authors mapping provided, SubGit will create automatic mapping according to the rules we've mentioned, so the revision 167 we showed above will look like this in Git:

$ git log -v
    commit d5d46afc3aa33240de8b5200e72611d4e0d72a99
    Author: john_doe <john_doe@git.example.com>
    Date:   Thu Jun 6 10:25:02 2017 +0200

        minor changes

supposing Git machine has 'git.example.com' hostname.

And vise versa, if a user John Doe <johndoe@example.com> will make commit to the Git repository:

commit 7faaf52c41a0325d4686f2a6f2851dc3e3739136
    Author: John Doe <johndoe@example.com>
    Date:   Thu Jun 8 20:06:31 2017 +0200

        minor changes to bar.c

being mirrored to SVN it will look like:

------------------------------------------------------------------------
    r173 | John Doe | 2017-06-08 20:06:31 +0200 (Thu, 08 Jun 2017) | 1 line
    Changed paths:
       M /project/trunk/bar.c

    minor changes to bar.c
    ------------------------------------------------------------------------

Note, that since SVN username and Git user.name commonly differ, licensed committers counter might be affected, see the details in chapter 2.

Authors File

The authors mapping file is actually just a text file filled with SVN username - Git author pairs. Each pair maps SVN username to Git author like:

svn_user_name = Git Name <gitname@domain.name>

e.g. for a user named John Doe, the mapping can be set as:

john_doe = John Doe <john_doe@example.com>

Every SVN or Git user that makes commits either to SVN project or to mirrored Git repository is supposed to have the author mapping pair here in the authors file and each author pair must reside on a new line.

During SVN to Git translation, SubGit takes a SVN revision authors name and search the authors file for a match. If there is a matching line in the file - SubGit uses appropriate Git username to create commit in Git; otherwise, if there is not - SubGit will construct a Git commit author name using automatic mapping. And vice versa - during Git commit to SVN revision translate, SubGit searches the file and use appropriate SVN username to create the SVN revision; if there's no matching pair in the file - automatic mapping is used.

Note, that it is possible to map two different SVN usernames to the same Git author - for cases, say, when one team member uses two identities to make commits or some SVN username was renamed some time. In such case there might be such configuration created:

john_doe = John Doe <john_doe@example.com>
    johndoe = John Doe <john_doe@example.com>

Every revision that was made by john_doe or johndoe will be translated to Git commit with author name John Doe <john_doe@example.com>. But note, when this Git user makes commit in Git and this commit is being translated to SVN, the author on the SVN side will be set to first SVN username that matches particular Git name in the authors file. That is if those two authors mapping lines appear in the authors file in that exact order - john_doe first, then johndoe - then SVN revision author will always be set to john_doe when John Doe's Git commits are being translated to SVN.

Similarly, one SVN user can be mapped to different Git authors, e.g.:

jdoe = John Doe <john_doe@example.com>
    jdoe = Jane Doe <jane_doe@example.com>
    jdoe = James Doe <james_doe@example.com>

and again, every Git commit made by those authors will be translated to SVN with revision author set to jdoe; but SVN revisions made by that jdoe SVN user will always be set to first matching Git user in the authors file - John Doe <john_doe@example.com> in this particular case.

Scriptable Authors Mapping

In addition to the authors file, there's another way to provide SVN to Git authors mapping using authors helper program. The authors helper is an executable - script or binary - that is able to read data from standard input and provide its work result to the standard output. The data helper reads from input and the data helper provides to output must fulfill certain format:

• for Git to Subversion mapping: INPUT: Author Name author email OUTPUT: Subversionusername

• for Subversion to Git mapping:

  INPUT: Subversionusername OUTPUT: Author Name author email

Every time SubGit finds an author name during translation, it invokes the authors mapping helper program, passes the name to it and expects the helper to answer with matching author name.

The authors helper program might be extremely useful especially when you have many authors and the authors list is constantly changing - new users are being added, names and emails changes and so on. If you use some catalog to store accounts - LDAP, Active Directory, OpenID and so forth - you can create a script that will gather needed information from the catalog and provide it to SubGit.

During configuration or installation phase SubGit places simple authors.sh script into subgit/samples directory. This script doesn't do much useful, it's just some 'proof of concept' that demonstrates how input data is being read and output data provided. The script as simple as:

while read input
    do
      if [ -z "$name" ]; then
        name="$input"
      elif [ -z "$email" ]; then
       email="$input"
      fi
    done

    if [ -z "$email" ]; then
      echo Full Name
      echo author@email.com
    else
      echo shortSvnUserName
    fi

    exit 0;

Depending on what was sent to its input script returns either Git author name and email or SVN short name. It can be extended to, say, receive the data from catalog or database thereby facilitate the authors mapping.