Dariusz Cieślak's Blog on Software

Let’s say you tracked someone broke HEAD of trunk and you want to reverse that single commit from main branch. Subversion makes very handy syntax for reverse merges:

svn merge -c -19203 https://REPO_URL

In example above you do reverse merge of 19203 revision (note “-” sign before revision number). After that merge:

• Inspect if workspace compiles without errors
• If it’s OK: commit local changeset
• Notify 19203 committer about the change

In order to fix broken commit original author should do the opposite:

svn merge -c 19203 https://REPO_URL

(note there’s no “-” before revision number). Then:

• Correct changeset and test if workspace is not broken
• If it’s OK: commit local changeset (it will be nice to show original revision number in a comment)

You probably already know that Subversion stores some kind of metadata for all files added to repository. It’s called “properties” in Subversion vocabulary. This key-value map is responsible for registering ingored files masks, file attributes, internal file content type etc.

The property I’m going to present today is “mime-type“. It describes file content in similar way to HTTP header “Content-type” telling svn client how to handle the file. Typical values are: “text/plain”, “application/octet-stream”. Especially first part of mime-type is important:

• text/*: line-by-line merges are used, diffs are generated
• any other prefix: no text merges prepared

If you do not set this properly right you may end up with messed binary file (end-of-line conversions) or non-mergable changes in text file (that is marked as binary by mistake).

Of course during adding a file to workspace one can forget to set those properties correctly. Here auto-props comes with help. Auto-props are applied when “svn add” command (from command line or from GUI) is issued. Configuration is placed inside “~/.subversion/config” file. Here’s my config fragment from one of projects.

[auto-props]
*.csv = svn:mime-type=text/plain
*.java = svn:mime-type=text/plain
*.sql = svn:mime-type=text/plain
*.sql = svn:keywords=Author Date Id Revision URL
*.jar = svn:mime-type=application/octet-stream

Besides mime-type svn:keywords is being set in the example. It controls which keywords are expanded in source files.

What’s the purpose of internal project documentation? To help people do their jobs. Developers need the knowledge to be distributed across the team, testers need definition of proper system behaviour, marketing needs information on product features to sell it.

Questions

Important knowledge that may be required by developers doing updates may be summarized in few sentences:

• Who changed recently that line of code?
• When this method have been changed?
• Why algorithm works that way?

There’s simple method of automatically saving and retrieving this kind of information: Subversion (or any other version control system). How?

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