[MNG-5180] merge snapshotVersions in repository metadata correctly

[kwin]

Following this checklist to help us incorporate your
contribution quickly and easily:

• Make sure there is a JIRA issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a JIRA issue. Your pull request should address just this issue, without
  pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [MNG-XXX] SUMMARY, where you replace MNG-XXX
  and SUMMARY with the appropriate JIRA issue. Best practice is to use the JIRA issue
  title in the pull request title and in the first line of the commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the Core IT successfully.

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask on the developers list.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• In any other case, please file an Apache Individual Contributor License Agreement.

This builds upon the work from #371 and supersedes it.

[kwin]

@michael-o This is how I think the merge is reasonable. The same named method in

maven/maven-resolver-provider/src/main/java/org/apache/maven/repository/internal/RemoteSnapshotMetadata.java

Line 72 in 69d6c6d

protected void merge( Metadata recessive )

is IMHO different from Metadata.merge() as the former does a recessive merge but I don't think this is reasonable here, therefore I don't think we have to consider it here any further.

I think it boils down to the question whether multiple or only one snapshotVersion is allowed per extension and classifier in https://maven.apache.org/ref/3.8.3/maven-repository-metadata/repository-metadata.html#class_snapshotVersion.
I currently assume that it is multiple ones but looking at https://repository.apache.org/content/repositories/snapshots/org/apache/maven/maven-core/4.0.0-alpha-1-SNAPSHOT/maven-metadata.xml one could also think, only one SNAPSHOT version is allowed.

Update: I updated the code to be in line with RemoteSnapshotMetadata.merge() now and made sure that only the most recent version per classifier/extension is listed in the metadata.

[michael-o]

@kwin Going through. Do you think that the impl in Resolver is correct?

[michael-o]

Can you show where version and updated are part of an id now?

[kwin]

This is for letting Modello generate a complete equals method which is used e.g. in https://github.com/apache/maven/pull/681/files#diff-a8781d28a4f703eaad1745f9002678d2e273d9189fb9a5f97a79df346f4c1953R129

[kwin]

Do you think that the impl in Resolver is correct?

yes, but you have to keep in mind how its merge is called (

maven/maven-resolver-provider/src/main/java/org/apache/maven/repository/internal/MavenMetadata.java

Line 75 in 69d6c6d

public void merge( File existing, File result )

): The arguments are inverted to the merge from Metadata. For the former the destination is given as argument while for the latter the source is given as argument.

[michael-o]

Please remove the copied classes from Resolver. This makes the test brittle for the future.

[michael-o]

Please provide a backport for 3.8.x

[kwin]

Please provide a backport for 3.8.x

Done in #684