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From Stefan Bodewig <bode...@apache.org>
Subject Re: Reproducible JARs
Date Sun, 23 Jul 2017 10:35:30 GMT
On 2017-07-22, George Bateman wrote:

> I'm currently trying to make Processing <https://processing.org> build
> reproducibly (so building it twice yields the exact same output file).
> Currently this involves, as far as I can tell, unzipping every JAR
> file, touching the files with a constant modification time, and
> re-zipping it. (Even if you were to touch the .class files in advance,
> there's MANIFEST.MF, which is created by Ant and which you have to
> unzip to access.)

The manifest may contain the current time stamp, to make things worse
for you :-)

I'm not sure what you want can be achieved at all. The best you can get
is the exact same jar when the build has been run on the same OS with
the same version of java, javac and zlib (and probably a few other
things I'm missing right now).

Different javacs will create different class files. ZIP (and thus JAR)
creation uses zlib under the covers and different versions may result in
different deflated output.

> Unless there's some efficient way of doing this that I've missed,
> could you advise me on how I'd go about writing a patch for Ant that
> makes reproducible JARs easier? I'd been thinking of adding a
> "modificationtime" attribute to the jar and zip tasks, and giving that
> time to all the files, but I'd be grateful if you could give me a
> rough idea of how Ant works and which files I'd need to be looking at
> and editing.

First of all you'd have to modify the class
org.apache.tools.ant.taskdefs.Zip and add a setModificationtime method
to it - this will create the attribute for both tasks, as the
implementation of <jar> (org.apache.tools.ant.taskdefs.Jar) is a
subclass of the Zip class. You'll need to think about what the value of
the attribute should be - milliseconds since epoch? A formatted string
containing the timestamp to set?

And then you need to look for all places where Zip or Jar invoke setTime
on a ZipEntry instance.

Stefan

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