Mercurial: check whether last pull/update introduced changes

Question

I am the process of writing an update script, which pulls the latest version of a number of repositories, and rebuilds the projects. I wanted to make the build conditional, so I tried

hg pull -u && ant clean build

and the variation

hg pull; hg update && ant clean build

However, the ant build is always invoked, even when nothing has changed. I know that I can use hg incoming to check for changes before doing the pull, but this feels wasteful to me.

How can I check for new changes, without having to contact the server twice (once for hg incoming, once for hg pull)?

UPDATE: This is my build script now:

update() {
  TIP=$(hg tip --template "{node"})
  hg pull -u
  if test "$TIP" != $(hg tip --template "{node}"); then
    ant clean build
  fi
}

(cd repo1; update )
(cd repo2; update )

And for people wondering why I do a clean build every time, there are two reasons for that:

1. The repositories depend on each other, and when the API in one of them changes, I need to do a full rebuild to find places where these API changes break code
2. The Java compiler inlines constants, also from other class files. Now, when I change a constant in a class back to a field that can change, all other class files using that constant remain untouched by a build, and this can lead to subtle bugs that I want to avoid.

Answer 1

You should not just run hg incoming twice since it will actually download all the changesets twice then. This because you cannot just take a sneak peek at the remote repository without running a full hg pull.

So save the incoming changesets in a bundle and pull from that instead:

hg incoming --bundle incoming.hg && hg pull --update incoming.hg && echo "Go!"

The hg incoming command acts as a guard for the following commands: the && is short-circuiting so the first command that return a non-zero exit code will make the whole construct fail with that exit code. This means that hg pull and any following commands aren't executed at all when hg incoming signals that there is nothing to pull.

Answer 2

Follow up to @Adam

Quotes from hg help incoming

For remote repository, using --bundle avoids downloading the changesets twice if the incoming is followed by a pull.

...

Returns 0 if there are incoming changes, 1 otherwise.