It can append that when we try to access a Mercurial repository we got the following error:

abort: requirement 'fncache' not supported!

It append when a "too old" version of Mercurial is used. Indeed, from version 1.1, Mercurial used a new repository format (fncache), which is no more readable by the former versions. The reverse is not true, and newer version are able to read old repositories.

To solve the problem we can use a version of Mercurial able to read the two type of repository and convert it to the old format using the following command:

hg --config format.usefncache=0 clone --pull A B

With A the initial repository and B the converted repository.

To completely deactivate the new repository format, a modification the configuration file of Mercurial is needed (~/.hgrc):

usefncache = False