TwirlCompiler: Make output reproducible-friendly

Hi,

Currently the TwirlCompiler emits a comment like this:

                  -- GENERATED --
                  DATE: Wed Mar 06 05:39:00 CET 2019
                  SOURCE: /home/bardur/wd/cwconsult/silo/modules/postgresql/src/main/twirl/dk/cwconsult/silo/postgresql/core/impl/LockStreamSQL.scala.txt
                  HASH: aca20403ba81a6a79ef8db751c2a0f13b68222eb
                  MATRIX: 651->1|764->22|791->23|829->35|864->50|892->51
                  LINES: 15->1|20->2|21->3|21->3|21->3|21->3
                  -- GENERATED --

Unfortunately, this contains two pieces of information that are bad for reproducible builds:

• The absolute path of the source file.
• The timestamp of compilation.

(It’s generally also bad for build systems where you ideally want the output of a build step to be stable if its inputs are – to avoid e.g. spurious rebuilds.)

It would be much appreciated if they could either just be outright removed[0] or if we could have an option to disable embedding these ‘fields’. (I’m not sure if the ordering on e.g. MATRIX and LINES output is deterministic, but it should be made so as well.)

If either of these seems acceptable, I think I should be able to code it up fairly quickly.

[0] I’m not really familiar with the Play ecosystem, but one assumes that this may be for template-debugging purposes where a dev server can show the file?

[bjaglin]

For the record, this was released as part of 1.5.1.

[BardurArantsson]

The TL;DR is that the output changes needlessly on recompilation of the same input – which makes interacting with build systems like Pants, Bazel, etc. less than ideal because they’ll assume that because the twirl output file has changed (its hash has changed), they need to recompile the output (and whatever depends on that output).

Does that make sense?