[api-xml-adjuster] Don't needlessly rebuild XML

[jonpryor]

Commit e3abe4b broke caching of the
bin/Build$(Configuration)/api/api-*.xml.* files, reintroducing the
scenario that commit 5c46ee3 fixed. The result is that minutes may
be spent needlessly re-generating API XML.

The cause of the breakage is that @(_ApiParameterDescription)
(introduced in e3abe4b) will contain entries that we don't care
about, e.g. API-4, for which no longer generate bindings. As such, the
_ClassParse target Inputs were looking for files which will never
exist, thus causing the _ClassParse target to always execute.

This could be fixed by making @(_ApiParameterDescription)/etc.
entries conditional on on the file
%(_ApiParameterDescription.ParameterDescription) refers to actually
existing, a'la:

<ItemGroup>
  <_ApiParameterDescription
      Condition="Exists('%(ApiFileDefinition.ParameterDescription)')"
      Include="%(ApiFileDefinition.ParameterDescription)"
  />
</ItemGroup>

However, this is suboptimal, as it means everything must still be
rebuilt if only one file changes:

$ touch src/Mono.Android/Profiles/api-27.params.txt
$ (cd build-tools/api-xml-adjuster ; time msbuild )
real    2m20.205s
# eep!

What we really want is MSBuild Target Batching, but our use of
item groups without item metadata prevents target batching!

Which leads to a realization: the rationalization for commit e3abe4b
was incomplete: The problem wasn't using metadata within Target Inputs
and Outputs; the problem was intermixing strings with metadata.

Meaning the pre-e3abe4b8 api-xml-adjuster.targets was correct
(except for the bit about it mentioning files which didn't exist,
causing needless rebuilds):

<!-- Good! Permits Target Batching -->
<Target Name="_ClassParse"
    Inputs="%(ApiFileDefinition.ParameterDescription)"
    Outputs="%(ApiFileDefinition.ApiAdjustedXml)"
  ...

vs. what e3abe4b was attempting to fix:

<!-- BAD! Intermixes metadata with strings -->
<Target Name="_Make"
    Inputs="$(IntermediateOutputPath)\%(_LibZipTarget.Identity)\Makefile"
  ...

Reviewing e3abe4b, api-xml-adjuster.targets is the only file that
was already properly using item medata within Target Inputs & Outputs.

Fix class-parse.exe and api-xml-adjuster.exe rebuilds by properly
using MSBuild Target Batching, and by filtering
@(ApiFileDefinition) so that it only contains values for which we
have a src/Mono.Android/Profiles/api-*.params.txt file.

With these changes in place, single-file rebuilds are much faster:

$ touch src/Mono.Android/Profiles/api-27.params.txt
$ (cd build-tools/api-xml-adjuster ; time msbuild )
real    0m26.662s

[jonpryor]

@atsushieno: I reworked the patch here from the "simple" fix to something more resilient to rebuilds.

I can't find where you mentioned that touching e.g. src/Mono.Android/Profiles/api-27.params.txt would cause everything to be reconstructed, but that is not ideal. The updated PR addresses that, so that if the params for a single API changes, you only need to re-run class-parse and api-xml-adjuster.exe for that API level, and not everything.

[atsushieno]

Yay, it is much better! (The problem was stated at the commit message in my former attempt to fix: f3f6005 )

[atsushieno]

The build failed with:

03:38:16 	: error : Error building target _DefineApiFiles: Item has already been added. Key in dictionary: 'ParameterDescription'  Key being added: 'ParameterDescription'