Fixing .hg file open ordering

[Bryan O'Sullivan]

Yep, your reasoning here makes sense. While I'm glad that you discovered
after some investigation that we're doing the right thing already, it's a
little troubling that this wasn't easy to understand from reading the code,
and that (from my interpretation of what you wrote) you had to think and
rethink your reading a couple of times to figure out what was going on.
Sounds a bit like a latent future bug, doesn't it?

On Tue, Nov 22, 2016 at 12:19 PM, Ryan McElroy <rm at fb.com> wrote:

> I think that fixing the read open ordering is not sufficient.
>
> I was just chatting with Jun Wu (quark) about this, and after that
> discussion, it seems that we also have to ensure the write order is
> correct. Specifically, bookmarks must be written only after the changelog
> is written, otherwise, regardless of the order that we open files for read,
> we can have a bookmarks file that points to commits that no reader can see
> yet.
>
> Based on a very quick look at the transaction code, it looks like
> filegenerators in transactions are stored in a dict, so we have no
> guarantees about what order they will be called in, so it's very likely
> that we often call the write of the bookmarks file before the write of the
> changelog, making it so that no read-side fix alone can be sufficient to
> solve this problem.
>
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