[tests] fix for Xamarin.Android.Tools.Bytecode-Tests

[jonathanpeppers]

Context: dotnet/android#978

When bumping java.interop in xamarin-android, a test failure occurred
in the ClassPath.GetDocletType method. This was added in #188, but
since ANDROID_SDK_PATH is not set during java.interop’s test runs the
test was skipped.

The incoming path can be a directory, so the File.OpenText call
will fail in that case. Added a File.Exists check as a fix.

[dnfclas]

@jonathanpeppers,
Thanks for having already signed the Contribution License Agreement. Your agreement was validated by .NET Foundation. We will now review your pull request.
Thanks,
.NET Foundation Pull Request Bot

[jonpryor]

This PR concerns me, because the entire method is starting to concern me. path isn't modified, yet we have Path.Combine(path, "packages.html"), which suggests that path should be a directory.

Then there's line 243:

using (var reader = File.OpenText (Path.Combine (path, "index.html")))

If path is a file, that will throw; there's no File.Exists() check to ensure that if e.g. path were a file and we read filename/index.html -- an invalid filename -- that we won't throw.

Which makes line 255 really weird and suspect: if path must be a directory, then how did this ever work?

Did it ever work? PR #188 was only merged 22 days ago; maybe that code path hasn't actually been used or tested yet, especially since PR #188 contained no unit tests. (Doh!)

[jonpryor]

...Which means this proposed "fix" is Doomed To Fail™: if path must be a directory, then File.Exist(path) will always be false, meaning this code is dead code; it's not executable.

I think we need @Redth to chime in.

[Redth]

So, the ClassParse task just passes in DocumentationPaths strings which usually is a directory, but could be a file, as the ApiXmlDocPaths is intended to be a path to an xml file.

I think this fix should work in this case since File.Exists will indeed skip over directories and this code is only hit for files.

[jonathanpeppers]

Is there a quick test we could add with a file? Looking at the code, I think this part would fail if a file path came in:

using (var reader = File.OpenText (Path.Combine (path, "index.html")))
    reader.ReadBlock (buf, 0, buf.Length);

[jonathanpeppers]

@Redth is going to address in another PR