If you’re using Hudson with a CVS-based project and receiving the error “cannot open CVS/Entries for reading” error when Hudson tries to check-out the repository, check to see that the branch tag you’re using is actually a branch tag and not just a revision tag. I was getting this error and it was driving me crazy, because I could run the same “cvs co -r branchname modules” command from the terminal and it worked fine. Until I realized that I’d created the “branchname” tag as a revision tag, not as a branch tag.

How to check whether a given tag is a revision tag or a branch tag:

cvs update -r tagname filename
cvs status filename

Look for the “Sticky Tag” line; if after the tag name it says “revision:”, well, it’s a revision tag. If after the tag name it says “branch:” then it’s a branch tag and, unfortunately, my solution to this maddening and uninformative error doesn’t apply to you. Best of luck.

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Update 2010-08-16: If you’ve gotten past the initial “cannot open CVS/Entries” error but now Hudson is failing when it tries to generate the CVS changelog, that’s due to Hudson issue 6312 which as of today is not fixed. The workaround is to disable CVS changelogs completely, which may be a good idea anyway with projects of any reasonable size. This post on i-proving.ca has the details.