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From Loren Kratzke <LKrat...@blueorigin.com>
Subject RE: Possible Ivy bug (and suggested fix) in ChainResolver
Date Tue, 24 Mar 2015 21:09:11 GMT
I had to make an important edit (included the wrong snippet of code).

-----Original Message-----
From: Loren Kratzke 
Sent: Tuesday, March 24, 2015 1:09 PM
To: dev@ant.apache.org
Subject: Possible Ivy bug (and suggested fix) in ChainResolver

I have a some observations about how the chain resolver selects a dependency. I think this
may be a bug but I am not sure because the intent of the source code is not entirely clear.
It reads one way, but behaves in a different way. I have pinpointed the exact spots in code
where this happens.

Here is my simple test setup used to debug this issue. I have two resolvers (Filesystem and
URL) configured in a ChainResolver in that order. I publish to one resolver and then the other
repeatedly and consume the result in another project. I use checkModified=true and changingPattern=".*"
on both resolvers.

My artifact is simply a text file with the current date and time so it is easy to see whether
you get fresh or stale artifacts from the repos.

When I consume the published artifact from the other project, I will get the artifact from
the first configured resolver in the chain (Filesystem in this case). But I know from debugging
that the second resolver is also evaluated. So as an experiment, I added force="true" on the
second resolver to see if I could force Ivy to ignore the first result and favor an artifact
returned by the second resolver. Instead, Ivy returned the artifact from the first resolver
even though the second artifact was newer AND the second resolver had force="true".

When I debugged this to see why the first artifact was chosen over the second artifact, I
found something very fishy.

ChainResolver.getDependency() iterates over each resolver in the chain. First it found the
Filesystem resolver and the artifact and next it found the URL resolver and artifact. Next
it calls BasicResolver.getDependency() which will compare the previously resolved artifact
with the current artifact.

This is where it gets very fishy. At the end of the getDependency() method it calls AbstractResolver.checkLatest()
which I assume is intended to return the latest of the two artifacts. But that comparison
never happens. AbstractResolver.isAfter is invoked with two artifacts to be compared and a
null Date. Since the date is null, the two artifacts are never compared and no matter what,
the first artifact will be returned and the second one discarded and a verbose message will
be emitted stating that the second artifact is older than the first artifact, every time.
The message is on line 533 of AbstractResolver. (I am looking at Ivy-2.3.0 so if that line
does not make sense on trunk then let me know.)

[EDITED: Including the correct snippet here]

Line 543 and 544 of AbstractResolver:

            Message.debug("\tmodule revision discarded as older: " + newModuleDesc);
            return previousModuleFound;

The message is not true. The artifact that was kept was the YOUNGER of the two and a comparison
of lastModified never happened (and never can happen in the current code as far as I can tell).

So the actual logic in AbstractResolver.checkLatest() simply returns the first artifact found.
While this is not a bad behavior, it does not seem like it is the intended behavior. I mean,
why go through all the trouble of pretending to compare two artifacts using date methods when
the logic never executes because the passed in Date object is null. And why emit a message
stating that one was determined to be older than the other. That is super fishy.

Furthermore, the next line in ChainResolver.getDependency() after resolver.getDependency()
is called (ChainResolver line105) references isReturnFirst(). That is fishy because none of
that matters any more. The current artifact was rejected on the previous line of code and
the previous (aka first) artifact is now the current artifact and is the one that will be
returned (without a date comparison, and for the arbitrary reason that is was found before
the other one).

I think that the intent of the null Date object is to compare each artifact to a static Date
configured elsewhere (I have no idea where), but if the code were to actually compare the
lastModified dates of the two artifacts, a useful result would happen - Ivy would return the
latest artifact from across multiple repositories.

That is huge because I have never been able to get Ivy to do this. I have never seen anybody
get Ivy to search multiple repositories and return the latest artifact. This is useful for
local development when you publish locally to consume locally modified artifacts. It would
be nice to have the option of picking up newer artifacts from a central repo when those occur
without having to blow away a local repository and its cache.

(By the way, giving my local repo its own cache seems to have solved some other strange issues
I was having. I recommend this to everybody and I think it should be a default in Ivy, but
that is debatable and would need some more research and concensus.)

I think that this is a good feature and should be configurable. I think possibly it was intended
to be configured via ChainResolver.returnFirst="true|false" but that code executed when it
was too late and the decision had already been made. If I were to make this a feature, and
make it configurable, I would configure this using an attribute named returnFirst because
that is the exact facet of functionality that we are talking about here.

Thanks for your attention. Hope I am helping here. I am considering coding this to see if
it works as expected. I would be happy to report my results and provide a patch if anybody
is interested in evaluating this.

L.K.

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