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The content of this blog is my personal opinion only. Although I am an employee - currently of Imagination Technologies's MIPS group, in the past of other companies such as Intellectual Ventures, Intel, AMD, Motorola, and Gould - I reveal this only so that the reader may account for any possible bias I may have towards my employer's products. The statements I make here in no way represent my employer's position, nor am I authorized to speak on behalf of my employer. In fact, this posting may not even represent my personal opinion, since occasionally I play devil's advocate.

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Tuesday, January 17, 2012

Mercurial whine:
hg merge -r tip
is NOT the same as
hg merge -r default
Because Mercurial's tip may be on a branch. tip is just the most recent changeset, anywhere. I have hit bugs caused by following hg recipes that talk about using -r tip. Sigh.

Tentative checkin for branch only, not to be merged?

Here's another: Working on a branch, I just did a pull, and merged from trunk onto the beranch. I notice that somebody else has a minor bug on the trunk: it looks like they forgot to check in a test reference pattern, although it is also possible that the test has been checked in before the golden behavior is established. I report the bug. But I would like to silence the error, at least on my branch. I can make the change to the reference patterns. However, I do not know if that change is goosd. I just want to silence it, or mark it as a known error. I don't want my change to propagate back to the trunk when eventually I merge and push. I.e. my branch now contains some tentative stuff, as well as some stuff that I am confident will soon be merghed. On CVS I would create a per file branch for the reference, and work off a merged directory. Eventually, I would update to the main branch, ahnd not get5 my patched test. Unclear how to do this in Mercurial, apart from remembering that I have to delete it eventually. Oh, here's a way:
// working on branch B
hg pull
hg merge -r tip
make test
// make change to silence test error
hg branch Bb
// somehow copy old version of patched file to pre brahnch
hg ci // on Bb
hg update -r B
// now work
// when ready to merge, do the usual
hg pull
hg merge
make test
// now do the extra step to be confident that you aren't pushing anything broken
hg update -r Bb
hg merge -r B
// now should have B, except for that one change that was tenative
make test
// see if the bug was fixed by someone else...
// ok, now merge to default branch, and then push
// - since I am paranoid, I might pull and test again
hg update -r default
hg merge -r Bb
make test
hg push
Works, but is complicated. I am quite likely to forget that I am supposed to merge from branch B onto Bb before merging to the trunk. {{Category-VCS}}{{Category-hg}}
AFAICT Mercurial only allows you to merge entire changesets.

Here is an example of why I may want to do a merge at lower granularity:

  • I am working on a branch B
  • I do a pull from the parent, hg pull
  • I merge from the parent on to my branch B, because I am not yet ready to merge back onto the trunk (aka default branch)
  • I run my tests
Now I notice that the .hgignore that I got from the parent is missing a file.  I fix it in my repository, to shut it up.

Then I realize that I should push such a generic change back to the trunk asap.

What I want to do is something like"

  hg clone work-repo quick-fix
  cd quick-fix
  hg update -r default
  hg merge -r B  .hgignore     # to merge just the change to .hgignore into the trunk
  make clean;hg purge
  make test
  hg ci
  ...
  hg push
I.e. I want to merge JUST the change to .hgignore, using "hg merge -r B .hgignore". Instead I do
  hg clone work-repo quick-fix
  cd quick-fix
  hg update -r default
  cp ../work-repo/.hgignore .
  make clean;hg purge
  make test
  hg ci
  ...
  hg push
I.e. "cp ../work-repo/.hgignore ." is used instead of "hg merge -r B .hgignore". Although this works, it makes me unhappy. E.g. I may blithely have overwritten other changes, e.g. if I cloned from the master rather than the work-repo. Not to mention the fact that the "hg push" abnove would push my branch. And my teammates do not want my branch to be pushed, because it has too many fine grain checkins - they want just a single checkin message. But that's another story.