1429502 - Provide an easy way to set MOZ_BUILD_DATE for try pushes

Status: NEW

(Firefox Build System :: General, enhancement)

Description

[(not currently active) Ted Mielczarek]

Firefox builds are almost 100% reproducible, in that you can get the same exact binaries out given the same sources. This is useful for validating changes to the build environment/build options: you can do a build with your changes and diff the resulting binaries. Unfortunately one large source of differences in our builds is the build id (MOZ_BUILD_DATE): it gets baked into the binaries and a bunch of other places. It's not hard to override it, but it would be nice if there was a simple way to force it to a specific value when pushing to try.

It gets set in the decision task nowadays here:
https://dxr.mozilla.org/mozilla-central/rev/c38d22170d6f27e94c3c53093215d20255fab27a/taskcluster/taskgraph/decision.py#206

and propagated down to actual build tasks in transforms. If we had something like `mach try --with-build-id=20180101123456 ...` that would be great. Even nicer would be a parameter that took a link to a push (maybe a treeherder URL?) and fetched info about that push to generate builds with a matching build id.

Comment 1

[Andrew Halberstadt [:ahal]]

It looks like the end result of those transforms is that the buildid ends up in the MOZ_BUILD_DATE environment variable. This means you should already be able to override it with:

./mach try --env MOZ_BUILD_DATE=20180101123456

This should at least work with |mach try fuzzy|, not sure about try syntax (but it probably would there too).

Comment 2

[Gregory Szorc [:gps]]

Should we use the timestamp from the Mercurial changeset to derive MOZ_BUILD_DATE? We may have to ignore the try changeset. But we could look at its first parent.

Comment 3

[Andrew Halberstadt [:ahal]]

Then pushing with/without your changes would produce different buildids anyway.

This might be something we could tie into bug 1425313 though. This bug and that both want to use a previous push as a baseline.

Comment 4

[Mike Hommey [:glandium]]

This is what I did to make a try push mimic another:
https://reviewboard.mozilla.org/r/211664/diff/1-2/

Comment 5

[Mike Hommey [:glandium]]

This worked too:
https://hg.mozilla.org/try/rev/ee432477de4f1baad73050dfadc2016f51de4510
(running mach try fuzzy --env MOZ_BUILD_DATE=20180101020304 --env MOZ_SOURCE_CHANGESET=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa --env MOZ_SOURCE_REPO=https://hg.mozilla.org/try)