Re: [PATCH v4 00/13] New remote-hg helper

On Mon, Nov 5, 2012 at 3:13 PM, Michael J Gruber
[off-list ref] wrote:

Felipe Contreras venit, vidit, dixit 02.11.2012 19:01:

quoted

I talked with some people in #mercurial, and apparently there is a
concept of a 'changelog' that is supposed to store these changes, but
since the format has changed, the content of it is unreliable. That's
not a big problem because it's used mostly for reporting purposes
(log, query), not for doing anything reliable.

Is the changelog stored in the repo (i.e. generated by the hg version at
commit time) or generated on the fly (i.e. generated by the hg version
at hand)? See also below.

I don't know. I would expect it to be the former, and then when the
format changes, generated by the tool that did the conversion.

quoted

To reliably see the changes, one has to compare the 'manifest' of the
revisions involved, which contain *all* the files in them.

'manifest' == '(exploded) tree', right? Just making sure my hg fu is not
subzero.

Yeah, the tree. As I said, it contains all the files.

quoted

That's what I was doing already, but I found a more efficient way to
do it. msysGit is using the changelog, which is quite fast, but not
reliable.

Unfortunately while going trough mercurial's code, I found an issue,
and it turns out that 1) is not correct.

In mercurial, a file hash contains also the parent file nodes, which
means that even if two files have the same content, they would not
have the same hash, so there's no point in keeping track of them to
avoid extracting the data unnecessarily, because in order to make sure
they are different, you need to extract the data anyway, defeating the
purpose.

Do I understand correctly that neither the msysgit version nor yours can
detect duplicate blobs (without requesting them) because of that sha1 issue?

That's correct.

I'm really wondering why a file blob hash carries its history along in
the sha1. This appears completely strange to gitters (being brain washed
about "content tracking"), but may be due to hg's extensive use of
delta, or really: delta chains (which do have their merit on the server
side).

It is a surprise to me too. I see absolutely no reason why that would be useful.

It seems like bazaar does store the file hashes without the parent
info, like git.

quoted

Which means mercurial doesn't really behave as one would expect:

# add files with the same content

 $ echo a > a
  $ hg ci -Am adda
  adding a
  $ echo a >> a
  $ hg ci -m changea
  $ echo a > a
  $ hg st --rev 0
  $ hg ci -m reverta
  $ hg log -G --template '{rev} {desc}\n'
  @  2 reverta
  |
  o  1 changea
  |
  o  0 adda

# check the difference between the first and the last revision

  $ hg st --rev 0:2
  M a
  $ hg cat -r 0 a
  a
  $ hg cat -r 2 a
  a

That is really scary. What use is "hg stat --rev" then? Not blaming you
for hg, of course.

On that tangent, I just noticed recently that hg has no python api.
Seriously [1]. They even tell us not to use the internal python api.
msysgit has been lacking support for newer hg, and you've had to add
support for older versions (hg 1.9 will be around on quite some
stable/LTS/EL distro releases) after developing on newer/current ones.
I'm wondering how well that scales in the long term (telling from
git-svn experience: it does not scale well), or whether using some
stable api like 'hgapi' would be a huge bottleneck.

I don't know. I have never really used mercurial until recently. I
don't know how often they change their APIs and/or repository formats.
I would say the burden of updating to newer APIs is probably much less
than the burden of implementing code that accesses their repositories
directly, and eventually possibly rewriting the code when they change
the format.

If we were to access the repository directly, I would choose to use
Ruby for that, but given that 'we' is increasingly looking like 'I'. I
probably wouldn't.

Cheers.

-- 
Felipe Contreras