Troubleshooting SHA1 Failures with Mercurial Repositories

[Paul Boddie]

On Saturday, 13 June 2020 22:31:06 CEST Augie Fackler wrote:
> > On Jun 13, 2020, at 9:24 AM, Paul Boddie <paul at boddie.org.uk> wrote:
> > 
> > I suppose I am left wondering about a few things. Are such simple
> > comparisons of repository histories useful in assessing the prevalence of
> > faulty nodeids? How may faulty nodeids affect the integrity of
> > repositories (considering the quote about censored changesets above)? Are
> > there any compelling practical arguments for converting these faulty
> > repositories if they otherwise function apparently normally? (I realise
> > that combining faulty and converted repositories will result in
> > divergence in the graph at inappropriate places.)
>
> Over the years we’ve gotten a lot pickier about ordering of metadata in
> changeset objects we produce. My guess is that if the original repo passes
> `hg verify` nothing is wrong in the source repo, and that the differences
> you’re seeing are entirely metadata-ordering related (which is to say
> harmless).

So is the page about censored changesets now inaccurate with regard to nodeids 
causing some kind of failure if they do not "encode" the stored content 
according to the fundamental rules of Mercurial? Or did I misunderstand the 
intended message of that text? It sounds like there is nothing corrupt with 
regard to the stored content, merely the metadata (which happened to be used 
to construct the history initially) that is corrupt in some way.

> Was there any specific thing that motivated using `hg convert`?

I think the wiki mentions it as a tool to investigate repository corruption. 
My reasoning was that repository conversion using "hg convert" would rebuild 
the history and recompute the nodeids. In doing so in an environment where 
SHA1 libraries are not generating something arbitrary, I figured that I would 
obtain the "true" nodeids and see where they diverged in the original history 
from what they should have been.

Obviously, if there is a better way of "replaying" the history to see where 
and when the nodeids became bad, I would like to hear about it.

Thanks for the reply!

Paul