Provide MOAR info when a JDK path is invalid

Context: dotnet/android#2004

Sometimes the xamarin-android-tools bump on xamarin-android works.

Sometimes it doesn't.

(Insert gnashing of teeth and tearing of hair here.)

Even "better", the path it's complaining about is entirely
non-sensical, and not reproducible except on certain machines:

	Xamarin.Android.Common.targets(691,2): warning : Not a valid JDK directory: `/var/folders/19/hjm21bn17913_0ck7nh3thdr0000gn/T/tmp731daee6.tmp/microsoft_dist_openjdk_1.8.999`

That path *looks* like a path that `AndroidSdkInfoTests.cs` would
generate, except that test *isn't executed* -- or built! -- so
*where is that path coming from?!*

Surely it comes from `AppDomain.GetData()`, says I.  So I add some
logging to print out the value of the `AppDomain.GetData()` value,
and...it has no value.

So ***REALLY***, where is `/var/folders/...` coming from?

To answer *that* question, update `JdkInfo.TryGetJdkInfo()` to print
out a "category" for what directory we're loading.  With luck, this
bit of contextual information will help us narrow down where it's
coming from.  Maybe.

Author: jonpryor

src/Xamarin.Android.Tools.AndroidSdk/JdkInfo.cs

@@ -229,7 +229,7 @@ public static IEnumerable<JdkInfo> GetKnownSystemJdkInfos (Action<TraceLevel, st
 		static IEnumerable<JdkInfo> GetConfiguredJdks (Action<TraceLevel, string> logger)
 		{
 			return GetConfiguredJdkPaths (logger)
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "monodroid-config.xml"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -246,7 +246,7 @@ static IEnumerable<string> GetConfiguredJdkPaths (Action<TraceLevel, string> log
 		internal static IEnumerable<JdkInfo> GetMacOSMicrosoftJdks (Action<TraceLevel, string> logger)
 		{
 			return GetMacOSMicrosoftJdkPaths ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "$HOME/Library/Developer/Xamarin/jdk"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -265,14 +265,14 @@ static IEnumerable<string> GetMacOSMicrosoftJdkPaths ()
 			return Directory.EnumerateDirectories (jdks);
 		}
 
-		static JdkInfo TryGetJdkInfo (string path, Action<TraceLevel, string> logger)
+		static JdkInfo TryGetJdkInfo (string path, Action<TraceLevel, string> logger, string sourceCategory)
 		{
 			JdkInfo jdk = null;
 			try {
 				jdk = new JdkInfo (path);
 			}
 			catch (Exception e) {
-				logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`");
+				logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`; via category: {sourceCategory}");
 				logger (TraceLevel.Verbose, e.ToString ());
 			}
 			return jdk;
@@ -290,7 +290,7 @@ static IEnumerable<JdkInfo> GetJavaHomeEnvironmentJdks (Action<TraceLevel, strin
 			var java_home = Environment.GetEnvironmentVariable ("JAVA_HOME");
 			if (string.IsNullOrEmpty (java_home))
 				yield break;
-			var jdk = TryGetJdkInfo (java_home, logger);
+			var jdk = TryGetJdkInfo (java_home, logger, "JAVA_HOME");
 			if (jdk != null)
 				yield return jdk;
 		}
@@ -300,7 +300,7 @@ static IEnumerable<JdkInfo> GetLibexecJdks (Action<TraceLevel, string> logger)
 		{
 			return GetLibexecJdkPaths (logger)
 				.Distinct ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "/usr/libexec/java_home"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -339,7 +339,7 @@ static IEnumerable<JdkInfo> GetJavaAlternativesJdks (Action<TraceLevel, string>
 		{
 			return GetJavaAlternativesJdkPaths ()
 				.Distinct ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "/usr/sbin/update-java-alternatives"))
 				.Where (jdk => jdk != null);
 		}
 
@@ -372,7 +372,7 @@ static IEnumerable<JdkInfo> GetLibJvmJdks (Action<TraceLevel, string> logger)
 		{
 			return GetLibJvmJdkPaths ()
 				.Distinct ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "/usr/lib/jvm directory listing"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -394,7 +394,7 @@ static IEnumerable<string> GetLibJvmJdkPaths ()
 		static IEnumerable<JdkInfo> GetPathEnvironmentJdks (Action<TraceLevel, string> logger)
 		{
 			return GetPathEnvironmentJdkPaths ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "$PATH"))
 				.Where (jdk => jdk != null);
 		}
 

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -119,7 +119,8 @@ JdkInfo TryGetJdkInfo (string path)
 					jdk = new JdkInfo (path);
 				}
 				catch (Exception e) {
-					logger (TraceLevel.Warning, e.ToString ());
+					logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`; via category: Windows");
+					logger (TraceLevel.Verbose, e.ToString ());
 				}
 				return jdk;
 			}