Allow an optional locator to be provided to JdkInfo (#43)

Context: dotnet/android#2004 (comment)

There are a two (related) "usability" issues with
`JdkInfo.GetKnownSystemJdkInfos()`:

 1. It returns duplicate paths, and
 2. There's no way of knowing where a path "came from"

For example:

	$ csharp -r:bin/Debug/Xamarin.Android.Tools.AndroidSdk.dll
	csharp> using Xamarin.Android.Tools;
	csharp> JdkInfo.GetKnownSystemJdkInfos();
	{
	  JdkInfo(Version=1.8.0.77, Vendor="Oracle Corporation", HomePath="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home"),
	  Not a valid JDK directory: `/Library/Java/JavaVirtualMachines/1.6.0_65-b14-462.jdk/Contents/Home`
	  System.ArgumentException: Could not find required file `jvm` within `/Library/Java/JavaVirtualMachines/1.6.0_65-b14-462.jdk/Contents/Home`; is this a valid JDK?
	  Parameter name: homePath
	    at Xamarin.Android.Tools.JdkInfo.ValidateFile (System.String name, System.String path) [0x00025] in <087d9a26808b4d2d9c5d2844064ae952>:0
	    at Xamarin.Android.Tools.JdkInfo..ctor (System.String homePath, System.String locator) [0x00120] in <087d9a26808b4d2d9c5d2844064ae952>:0
	    at Xamarin.Android.Tools.JdkInfo.TryGetJdkInfo (System.String path, System.Action`2[T1,T2] logger, System.String locator) [0x00004] in <087d9a26808b4d2d9c5d2844064ae952>:0
	  JdkInfo(Version=1.8.0.77, Vendor="Oracle Corporation", HomePath="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home"),
	}

Notice that on my machine `jdk1.8.0_77.jdk` is listed *twice*.

Why is it listed twice?  It's listed twice because it comes from two
different "locators": `JdkInfo.GetPathEnvironmentJdks()`, which reads
`$PATH`, and `JdkInfo.GetLibexecJdks()`, which parses the output of
`/usr/libexec/java_home -X`.  If `$JAVA_HOME` were set to the same
location, it would be listed three times.

As-is, there's no way of knowing where a path is "coming from"; the
"locator" -- what found the path -- isn't known.

We could `.Distinct()` the output so that we don't have duplicates,
but I *like* the idea of duplicates, as it shows that we're using
multiple locators.

Then we hit [xamarin-android PR #2004][0], in which the
`xamarin-android-tools` bump within `xamarin-android` would fail,
*depending on which machine ran the unit tests*:

	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`

Where did that path come from?!

Update `JdkInfo.TryGetJdkInfo()` so that it takes an additional
"locator" parameter, which is publicly visible as the new
`JdkInfo.Locator` property, so that when we encounter an invalid JDK
directory, we know where that path came from:

	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`; via locator: monodroid-config.xml

*Now* we can see that the invalid (bizarro!) JDK path is coming from
`monodroid-config.xml`, allowing us to deduce that
`monodroid-config.xml` is "bad" (because the unit tests in a4aad18
incorrectly restored the original `monodroid-config.xml` file, meaning
the state of the machine after running that unit test is forever
suspect!).

The additional inclusion of Locator information makes it much easier
to reason about things, and the inclusion of duplicates makes sense:

	$ csharp -r:bin/Debug/Xamarin.Android.Tools.AndroidSdk.dll
	csharp> using Xamarin.Android.Tools;
	csharp> JdkInfo.GetKnownSystemJdkInfos();
	{
	  JdkInfo(Version=1.8.0.77, Vendor="Oracle Corporation", HomePath="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home", Locator="$PATH"),
	  Not a valid JDK directory: `/Library/Java/JavaVirtualMachines/1.6.0_65-b14-462.jdk/Contents/Home`
	  System.ArgumentException: Could not find required file `jvm` within `/Library/Java/JavaVirtualMachines/1.6.0_65-b14-462.jdk/Contents/Home`; is this a valid JDK?
	  Parameter name: homePath
	    at Xamarin.Android.Tools.JdkInfo.ValidateFile (System.String name, System.String path) [0x00025] in <087d9a26808b4d2d9c5d2844064ae952>:0
	    at Xamarin.Android.Tools.JdkInfo..ctor (System.String homePath, System.String locator) [0x00120] in <087d9a26808b4d2d9c5d2844064ae952>:0
	    at Xamarin.Android.Tools.JdkInfo.TryGetJdkInfo (System.String path, System.Action`2[T1,T2] logger, System.String locator) [0x00004] in <087d9a26808b4d2d9c5d2844064ae952>:0
	  JdkInfo(Version=1.8.0.77, Vendor="Oracle Corporation", HomePath="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home", Locator="`/usr/libexec/java_home -X`")
	}

[0]: dotnet/android#2004

Author: jonpryor

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

@@ -16,6 +16,8 @@ public class JdkInfo {
 
 		public      string                              HomePath                    {get;}
 
+		public      string                              Locator                     {get;}
+
 		public      string                              JarPath                     {get;}
 		public      string                              JavaPath                    {get;}
 		public      string                              JavacPath                   {get;}
@@ -76,9 +78,15 @@ public JdkInfo (string homePath)
 			javaVersion         = new Lazy<Version> (GetJavaVersion, LazyThreadSafetyMode.ExecutionAndPublication);
 		}
 
+		public JdkInfo (string homePath, string locator)
+			: this (homePath)
+		{
+			Locator             = locator;
+		}
+
 		public override string ToString()
 		{
-			return $"JdkInfo(Version={Version}, Vendor=\"{Vendor}\", HomePath=\"{HomePath}\")";
+			return $"JdkInfo(Version={Version}, Vendor=\"{Vendor}\", HomePath=\"{HomePath}\", Locator=\"{Locator}\")";
 		}
 
 		public bool GetJavaSettingsPropertyValues (string key, out IEnumerable<string> value)
@@ -229,7 +237,7 @@ 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 +254,7 @@ 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 +273,14 @@ 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 locator)
 		{
 			JdkInfo jdk = null;
 			try {
-				jdk = new JdkInfo (path);
+				jdk = new JdkInfo (path, locator);
 			}
 			catch (Exception e) {
-				logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`");
+				logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`; via locator: {locator}");
 				logger (TraceLevel.Verbose, e.ToString ());
 			}
 			return jdk;
@@ -290,7 +298,7 @@ static IEnumerable<JdkInfo> GetJavaHomeEnvironmentJdks (Action<TraceLevel, strin
 			var java_home = Environment.GetEnvironmentVariable ("JAVA_HOME");
 			if (string.IsNullOrEmpty (java_home))
 				yield break;
-			var jdk = TryGetJdkInfo (java_home, logger);
+			var jdk = TryGetJdkInfo (java_home, logger, "$JAVA_HOME");
 			if (jdk != null)
 				yield return jdk;
 		}
@@ -300,7 +308,7 @@ 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 -X`"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -339,7 +347,7 @@ 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 -l`"))
 				.Where (jdk => jdk != null);
 		}
 
@@ -372,7 +380,7 @@ static IEnumerable<JdkInfo> GetLibJvmJdks (Action<TraceLevel, string> logger)
 		{
 			return GetLibJvmJdkPaths ()
 				.Distinct ()
-				.Select (p => TryGetJdkInfo (p, logger))
+				.Select (p => TryGetJdkInfo (p, logger, "`ls /usr/lib/jvm/*`"))
 				.Where (jdk => jdk != null)
 				.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 		}
@@ -394,7 +402,7 @@ 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

@@ -112,28 +112,29 @@ protected override string GetJavaSdkPath ()
 
 		internal static IEnumerable<JdkInfo> GetJdkInfos (Action<TraceLevel, string> logger)
 		{
-			JdkInfo TryGetJdkInfo (string path)
+			JdkInfo TryGetJdkInfo (string path, string locator)
 			{
 				JdkInfo jdk = null;
 				try {
-					jdk = new JdkInfo (path);
+					jdk = new JdkInfo (path, locator);
 				}
 				catch (Exception e) {
-					logger (TraceLevel.Warning, e.ToString ());
+					logger (TraceLevel.Warning, $"Not a valid JDK directory: `{path}`; via category: {locator}");
+					logger (TraceLevel.Verbose, e.ToString ());
 				}
 				return jdk;
 			}
 
-			IEnumerable<JdkInfo> ToJdkInfos (IEnumerable<string> paths)
+			IEnumerable<JdkInfo> ToJdkInfos (IEnumerable<string> paths, string locator)
 			{
-				return paths.Select (TryGetJdkInfo)
+				return paths.Select (p => TryGetJdkInfo (p, locator))
 					.Where (jdk => jdk != null)
 					.OrderByDescending (jdk => jdk, JdkInfoVersionComparer.Default);
 			}
 
-			return ToJdkInfos (GetPreferredJdkPaths ())
-				.Concat (ToJdkInfos (GetOpenJdkPaths ()))
-				.Concat (ToJdkInfos (GetOracleJdkPaths ()));
+			return ToJdkInfos (GetPreferredJdkPaths (), "Preferred Registry")
+				.Concat (ToJdkInfos (GetOpenJdkPaths (), "OpenJDK"))
+				.Concat (ToJdkInfos (GetOracleJdkPaths (), "Oracle JDK"));
 		}
 
 		private static IEnumerable<string> GetPreferredJdkPaths ()