Authors mapping

SVN Users and Git Authors

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

In SVN, the author is 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:

$ 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, they may be set by the commands:

$ 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 that 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. The default authors file is situated in SubGit directory:

  [core]
     authorsFile = subgit/authors.txt

  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. 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 svn_user_name is being translated to svn_user_name <svn_user_name@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. 

During initial subgit configure call, this setting is being set to the hostname. Also, if subgit configure is invoked with --layout auto option, SubGit connects to specified SVN project, checks its history and generates authors mapping using found SVN usernames and given defaultDomain. Then SubGit fills up the default authors file with the resulting mapping.

For example, suppose, the defaultDomain is set like follows:

[core]
   defaultDomain = example.com

A user made commits in SVN under john_doe SVN account.

When SubGit translates these commits to Git, it sets author name in Git commits in the following way:

Author: john_doe <john_doe@git.example.com>

Similarly, Git commits that made by John Doe <johndoe@example.com> Git user appears in SVN with John Doe author name.

Authors File

The authors mapping file is 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>

For example, a mapping for a user John Doe may look like this:

john_doe = John Doe <john_doe@example.com>

Each mapping pair must appear on a new line.

During SVN to Git translation, SubGit searches all specified authors files for a mapping pair. If the matching pair is found, SubGit uses appropriate author name. If there is no match, then SubGit generates author name according to automatic mapping rules.

It is possible to map more than one SVN username to the same Git author:

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

Revisions that are created either by john_doe or johndoe are being translated to Git commits with author name John Doe <john_doe@example.com>. However, Git commits that are made by John Doe <john_doe@example.com> are being translated to SVN revision using first SVN username in authros files that matches particular Git name – john_doe in this case.

Similarly, one SVN username can be mapped to different Git authors:

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

Again, every Git commit made by those authors will be translated with author set to jdoe. SVN revisions made by jdoe is always set to first matching Git user in the authors files – John Doe <john_doe@example.com> in this particular case.

Changes made to authors files are being applied immediately, there is no need to restart mirroring or reinstall SubGit.

Scriptable Authors Mapping

In addition to the authors files, there is another way to establish 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 send its work result to the standard output. Input and output data must fulfil the following formats:

• for Git to Subversion mapping:

  INPUT: 
     author Name
     author email
  OUTPUT: 
     subversion_user_name

• for Subversion to Git mapping:

  INPUT: 
     subversion_user_name
  OUTPUT: 
     author Name
     author email

Every time SubGit needs to map an author name, it invokes the authors helper, 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.

On configuration phase SubGit creates a simple authors.sh script in samples subdirectory. This script doesn't do much useful, it just demonstrates how input data is being read and output data provided.

#!/bin/sh

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;

For more details on the authors helper see Script-provided authors mapping article.