RepoError should be importable from mercurial.repo?

[Matt Mackall]

On Mon, 2009-02-02 at 14:10 +0100, Christian Boos wrote:
> Hello,
> 
> In the 08cabecfa8a8 changeset ("errors: move revlog errors"), together 
> with the move of RevLogError and LookupError from mercurial.revlog to 
> mercurial.error, there was also a change which took care of backward 
> compatibility, in mercurial/revlog.py:
> +RevlogError = error.RevlogError
> +LookupError = error.LookupError
> 
> But a few changesets later, in 1d54e2f6c0b7 "error: move repo errors", 
> such a backward compatibility change was not done. Was that on purpose 
> or simply accidental?

Intentional.

Consider the following construct:

import repo
try:
    # some complex operation that may or may not involve repos
except repo.RepoError:
    print "Oops!"

We may be importing repo solely to check for errors that may never be
produced by code that is never called.

Our top-level dispatcher was doing precisely this, repeatedly. It was
effectively importing most of hg simply to be able to catch its errors.

Now I could leave backward-compatibility bits for all of these like
revlog does, but I probably shouldn't. Extensions are the most likely to
do things wrong and negatively impact performance.

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