[Xamarin.Android.Build.Tasks] timestamp fixes for incremental builds (#1930)

Context: https://github.com/xamarin/Xamarin.Forms/blob/42c07d1ae5aa56eb574b7d169499f1a9af7ec080/Xamarin.Forms.ControlGallery.Android/Xamarin.Forms.ControlGallery.Android.csproj

While working on build performance, I noticed two timestamp issues
that are preventing some targets in Xamarin.Android from building
incrementally. I spotted both of these while timing builds of the
`Xamarin.Forms.ControlGallery.Android` project. It is a good test
subject, because it builds PCLs, and seven (or so) Xamarin.Android
library projects.

~~ _LinkAssembliesNoShrink ~~

While timing builds, I noticed the following when
`_LinkAssembliesNoShrink` runs:

    Target Name=_LinkAssembliesNoShrink Project=PagesGallery.Droid.csproj
        Building target "_LinkAssembliesNoShrink" partially, because some output files are out of date with respect to their input files.
        [ResolvedUserAssemblies: Input=C:\Users\myuser\Desktop\Git\Xamarin.Forms\PagesGallery\PagesGallery.Droid\bin\Debug\PagesGallery.Droid.dll, Output=obj\\Debug\android\assets\PagesGallery.Droid.dll] Input file is newer than output file.
        ...
        Skip linking unchanged file: C:\Users\myuser\Desktop\Git\Xamarin.Forms\PagesGallery\PagesGallery.Droid\bin\Debug\PagesGallery.Droid.dll

It seems there is a case where MSBuild is thinking the target needs to
run (due to timestamp values), but the `LinkAssemblies` MSBuild task
does not copy the file. It actually doesn't need to do any work at all
here, since this was a build with no changes.

The fix here is to apply a timestamp for files that get skipped. This
prevents the target from running again when it doesn't need to.

I also removed a spot where a `copysrc` variable looks like it was
being set an extra time--did not seem to be needed.

~~ _GenerateJavaStubs ~~

After fixing `_LinkAssembliesNoShrink`, I noticed another timestamp
issue:

    Building target "_GenerateJavaStubs" completely.
    Input file "obj\\Debug\android\assets\PagesGallery.Droid.dll" is newer than output file "obj\\Debug\android\typemap.jm".

Looking at the `GenerateJavaStubs` MSBuild task, it was never setting
the timestamp on either `typemap.mj` or `typemap.jm`.

Fixing these two issues, I was able to get a build with no-changes in
this Xamarin.Forms project down from ~23 seconds to ~13 seconds. I
also updated the `CheckTimestamps` test to validate these changes.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tasks/GenerateJavaStubs.cs

@@ -306,7 +306,8 @@ void UpdateWhenChanged (string path, Action<Stream> generator)
 			var np  = path + ".new";
 			using (var o = File.OpenWrite (np))
 				generator (o);
-			Files.CopyIfChanged (np, path);
+			MonoAndroidHelper.CopyIfChanged (np, path);
+			MonoAndroidHelper.SetLastAccessAndWriteTimeUtc (path, DateTime.UtcNow, Log);
 			File.Delete (np);
 		}
 	}

src/Xamarin.Android.Build.Tasks/Tasks/LinkAssemblies.cs

@@ -152,16 +152,18 @@ bool Execute (DirectoryAssemblyResolver res)
 				foreach (var assembly in ResolvedAssemblies) {
 					var copysrc = assembly.ItemSpec;
 					var filename = Path.GetFileName (assembly.ItemSpec);
+					var assemblyDestination = Path.Combine (copydst, filename);
 
 					if (options.LinkNone) {
 						if (skiplist.Any (s => Path.GetFileNameWithoutExtension (filename) == s)) {
 							// For skipped assemblies, skip if there is existing file in the destination.
 							// We cannot just copy the linker output from *current* run output, because
 							// it always renew the assemblies, in *different* binary values, whereas
 							// the dll in the OptionalDestinationDirectory must retain old and unchanged.
-							if (File.Exists (Path.Combine (copydst, filename)))
+							if (File.Exists (assemblyDestination)) {
+								MonoAndroidHelper.SetLastAccessAndWriteTimeUtc (assemblyDestination, DateTime.UtcNow, Log);
 								continue;
-							copysrc = assembly.ItemSpec;
+							}
 						} else {
 							// Prefer fixup assemblies if exists, otherwise just copy the original.
 							copysrc = Path.Combine (OutputDirectory, filename);
@@ -171,7 +173,6 @@ bool Execute (DirectoryAssemblyResolver res)
 					else if (!MonoAndroidHelper.IsForceRetainedAssembly (filename))
 						continue;
 
-					var assemblyDestination = Path.Combine (copydst, filename);
 					if (MonoAndroidHelper.CopyIfChanged (copysrc, assemblyDestination)) {
 						MonoAndroidHelper.SetLastAccessAndWriteTimeUtc (assemblyDestination, DateTime.UtcNow, Log);
 					}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -158,7 +158,7 @@ public void CheckTimestamps ()
 				var output = Path.Combine (Root, b.ProjectDirectory, proj.OutputPath);
 				if (Directory.Exists (output))
 					Directory.Delete (output, true);
-				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				Assert.IsTrue (b.Build (proj), "first build should have succeeded.");
 
 				//Absolutely non of these files should be *older* than the starting time of this test!
 				var files = Directory.EnumerateFiles (intermediate, "*", SearchOption.AllDirectories).ToList ();
@@ -167,6 +167,21 @@ public void CheckTimestamps ()
 					var info = new FileInfo (file);
 					Assert.IsTrue (info.LastWriteTimeUtc > start, $"`{file}` is older than `{start}`, with a timestamp of `{info.LastWriteTimeUtc}`!");
 				}
+
+				//Build again after a code change, checking a few files
+				proj.MainActivity = proj.DefaultMainActivity.Replace ("clicks", "CLICKS");
+				proj.Touch ("MainActivity.cs");
+				start = DateTime.UtcNow;
+				Assert.IsTrue (b.Build (proj), "second build should have succeeded.");
+
+				foreach (var file in new [] { "typemap.mj", "typemap.jm" }) {
+					var info = new FileInfo (Path.Combine (intermediate, "android", file));
+					Assert.IsTrue (info.LastWriteTimeUtc > start, $"`{file}` is older than `{start}`, with a timestamp of `{info.LastWriteTimeUtc}`!");
+				}
+
+				//One last build with no changes
+				Assert.IsTrue (b.Build (proj), "third build should have succeeded.");
+				Assert.IsTrue (b.Output.IsTargetSkipped ("_LinkAssembliesNoShrink"), "`_LinkAssembliesNoShrink` should be skipped!");
 			}
 		}