Re: Reducing redundant build at Travis?

[Lars Schneider <larsxschneider@gmail.com>]

> On 14 Jul 2017, at 17:32, Jeff King <peff@peff.net> wrote:
> 
> On Fri, Jul 14, 2017 at 07:54:16AM -0700, Junio C Hamano wrote:
> 
>>> The "git test" script[1] uses this strategy with git-notes as the
>>> storage, and I've found it quite useful. I don't think we can rely on
>>> git-notes, but I think Travis gives us some storage options. Even just a
>>> best-effort cache directory would probably be sufficient (this is an
>>> optimization, after all).
>> 
>> We do seem to use some persistence to order prove tests already, but
>> I do not think it helps the common case, where my end-of-day push
>> pushes out 'maint' and 'v2.13.3' at the same time, because the push
>> is made with "git push --follow-tags $there maint master next pu"
>> and the new tag happens to be at 'maint'.  It would be nice if
>> Travis runs were sequential, but I often observe that it creates
>> jobs for these multiple branches and tags pushed at the same time,
>> and start running a few of them.
> 
> Ah, right, I didn't think about how these are racing. You'd need storage
> which allows some kind of atomic operation to "claim" the tree as a
> work-in-progress (and anybody who loses the race to get the lock would
> have to spin waiting for the winner to tell them the real status).
> 
> I don't know if Travis's cache storage is up to that challenge. We could
> probably build such a lock on top of third-party storage, but things are
> rapidly getting more complex.

I think we shouldn't go there because of the complexity. I reached out
to TravisCI and asked about the "hash build twice" problem [1]. Unfortunately,
I got no response, yet. The issue could also be considered a feature as you
could perform different actions in your TravisCI configuration based on
the branch name.

I think Junio's original suggestions for the Windows build makes a lot
of sense because it saves Dscho's compute resources:

--- a/ci/run-windows-build.sh
+++ b/ci/run-windows-build.sh
@@ -12,6 +12,12 @@ test -z "$GFW_CI_TOKEN" && echo "GFW_CI_TOKEN not defined" && exit
BRANCH=$1
COMMIT=$2

+if TAG=$(git describe --exact-match "$COMMIT" 2>/dev/null)
+then
+	echo "Tip of $BRANCH exactly at $TAG"
+	exit 0
+fi
+
gfwci () {
	local CURL_ERROR_CODE HTTP_CODE
	CONTENT_FILE=$(mktemp -t "git-windows-ci-XXXXXX")


However, I don't think we need to do the same for the builds that
use TravisCI resources. If they would be concerned about that then 
they wouldn't build the same hash twice in the first place.

- Lars


[1] https://twitter.com/kit3bus/status/885902189692112896