[msbuild] Fix detecting already signed executables.

[rolfbjarne]

The MSBuild tasks will codesign an executable if the executable's timestamp is
later than _CodeSignature/CodeResources's timestamp (or if
_CodeSignature/CodeResources doesn't exist).

Unfortunately, the codesign executable modifies both of those files, and the
executable last. This means that even just after running codesign, the
executable's timestamp might be later than _CodeSignature/CodeResources's
timestamp (due to HFS+'s one-second timestamp resolution, this might happen
all within the same second, which means that this is a random issue: the
problem only occurs if the executable was modified at least a second later
than _CodeSignature/CodeResources.)

So make sure to touch _CodeSignature/CodeResources after running codesign,
so that the next time a build occurs (with no modifications), we don't resign
needlessly.

Fixes this (random) test failure when running the MSBuild tests:

1) Test Failure : Xamarin.iOS.Tasks.TargetTests.RebuildExecutable_NoModifications
     #1: ../MySingleView/bin/iPhoneSimulator/Debug/MySingleView.app/MySingleView
  Expected: 2017-11-30 10:04:20.000
  But was:  2017-11-30 10:04:22.000

Fixes https://github.com/xamarin/maccore/issues/592.

[spouliot]

make sense but are not all our bots using APFS ?

[monojenkins]

Build success

[dalexsoto]

woow, what a subtle bug...

[rolfbjarne]

@spouliot not all our bots, but the High Sierra bots should. This is a good question, because this should be 100% reproducible on APFS... yet it isn't.

Test failure from a High Sierra bot (log, wrench revision) :

Errors and Failures:
1) Test Failure : Xamarin.iOS.Tasks.TargetTests.RebuildExecutable_NoModifications
     #1: ../MySingleView/bin/iPhoneSimulator/Debug/MySingleView.app/MySingleView
  Expected: 2018-03-09 19:48:09.000
  But was:  2018-03-09 19:48:11.000

the timestamps clearly have a one-second resolution.

I logged onto the bot, to confirm its file system:

$ diskutil info /
   [...]
   File System Personality:  APFS
   Type (Bundle):            apfs
   Name (User Visible):      APFS

I tested that mono returns sub-second times:

$ csharp -e 'System.IO.File.GetLastWriteTimeUtc ("/Users/builder/MonkeyWrench.log").ToString ("HH:mm:ss.fffffff")'
"11:22:45.2788357"

so I have no idea why this is random on High Sierra as well.

[rolfbjarne]

So I figured out why our High Sierra bots don't fail: mono didn't support sub-second timestamp resolution on non-Windows platforms until mono 5.12 (mono/mono@e0e185d).

I tested for exactly this on the bot, but it turns out that the bot had mono 5.12 installed from another test run when I tested it 😠

[chamons]

There are Codesign invocations in Xamarin.Mac.Common.targets, does it need a fix as well for this issue?