[macOS] fix DirectoryNotFoundException on clean systems (#110)

@PureWeen was hitting a problem building a .NET 6 app on macOS:
Task "ResolveSdks"
...
at System.IO.Enumeration.FileSystemEnumerator`1.CreateDirectoryHandle(String path, Boolean ignoreNotFound)
at System.IO.Enumeration.FileSystemEnumerator`1.Init()
at System.IO.Enumeration.FileSystemEnumerator`1..ctor(String directory, Boolean isNormalized, EnumerationOptions options)
at System.IO.Enumeration.FileSystemEnumerable`1..ctor(String directory, FindTransform transform, EnumerationOptions options, Boolean isNormalized)
at System.IO.Enumeration.FileSystemEnumerableFactory.UserDirectories(String directory, String expression, EnumerationOptions options)
at System.IO.Directory.InternalEnumeratePaths(String path, String searchPattern, SearchTarget searchTarget, EnumerationOptions options)
at Xamarin.Android.Tools.AndroidSdkBase.FindBestNDK(String androidSdkPath)
at Xamarin.Android.Tools.AndroidSdkBase.GetAllAvailableAndroidNdks()+MoveNext()
at Xamarin.Android.Tools.AndroidSdkBase.GetValidNdkPath(String ctorParam)
at Xamarin.Android.Tools.AndroidSdkBase.Initialize(String androidSdkPath, String androidNdkPath, String javaSdkPath)
at Xamarin.Android.Tools.AndroidSdkInfo..ctor(Action`2 logger, String androidSdkPath, String androidNdkPath, String javaSdkPath)
at Xamarin.Android.Tasks.MonoAndroidHelper.RefreshAndroidSdk(String sdkPath, String ndkPath, String javaPath, TaskLoggingHelper logHelper)
at Xamarin.Android.Tasks.ResolveSdks.RunTask()
at Microsoft.Android.Build.Tasks.AndroidTask.Execute()
This was a relatively new Mac which didn't have an Android NDK
installed.
A way to workaround the problem was to create a directory that was
being checked for:
mkdir -p ~/Library/Android/sdk
It appears this problem was introduced in b2d9fdf, as:
1. `~/Library/Android/sdk` is returned from
[`GetAllAvailableAndroidNdks()`][0]
2. `GetAllAvailableAndroidNdks()` calls `GetAllAvailableAndroidSdks()`
looking for any `ndk-bundle` folders it can find.
3. `FindBestNDK()` is passed a directory that doesn't exist, and the
`Directory.EnumerateDirectories()` call [throw][1].
The fix here is to update `FindBestNDK()` to check `Directory.Exists()`
before trying to traverse the directory `androidSdkPath`, as the values
from `GetSdkFromEnvironmentVariables()` may not exist either.
I was able to add a test for this scenario by adding an invalid
directory to `$ANDROID_HOME`.
[0]: https://github.com/xamarin/xamarin-android-tools/blob/b2d9fdf8782875ffb63da48cf8e0b385b04f7c25/src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkUnix.cs#L113-L115
[1]: https://github.com/xamarin/xamarin-android-tools/blob/b2d9fdf8782875ffb63da48cf8e0b385b04f7c25/src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs#L172

Author: jonathanpeppers

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

@@ -152,7 +152,7 @@ protected virtual IEnumerable<string> GetAllAvailableAndroidNdks ()
 // Look in PATH
 foreach(varndkStackinProcessUtils.FindExecutablesInPath(NdkStack)){
 varndkDir=Path.GetDirectoryName(ndkStack);
-if(ndkDir==null)
+if(string.IsNullOrEmpty(ndkDir))
 continue;
 yieldreturnndkDir;
 }
@@ -161,12 +161,19 @@ protected virtual IEnumerable<string> GetAllAvailableAndroidNdks ()
 foreach(varsdkinGetAllAvailableAndroidSdks()){
 if(sdk==AndroidSdkPath)
 continue;
-yieldreturnFindBestNDK(sdk);
+varndkDir=FindBestNDK(sdk);
+if(string.IsNullOrEmpty(ndkDir))
+continue;
+yieldreturnndkDir;
 }
 }
 
 stringFindBestNDK(stringandroidSdkPath)
 {
+if(!Directory.Exists(androidSdkPath)){
+returnString.Empty;
+}
+
 varndkInstances=newSortedDictionary<Version,string>(Comparer<Version>.Create((Versionl,Versionr)=>r.CompareTo(l)));
 
 foreach(stringndkPathinDirectory.EnumerateDirectories(androidSdkPath,"ndk*",SearchOption.TopDirectoryOnly)){

tests/Xamarin.Android.Tools.AndroidSdk-Tests/AndroidSdkInfoTests.cs

@@ -146,6 +146,28 @@ public void Ndk_PathInSdk()
 }
 }
 
+[Test]
+publicvoidNdk_AndroidSdkDoesNotExist()
+{
+CreateSdks(outstringroot,outstringjdk,outstringndk,outstringsdk);
+
+Action<TraceLevel,string>logger=(level,message)=>{
+Console.WriteLine($"[{level}] {message}");
+if(level==TraceLevel.Error)
+Assert.Fail(message);
+};
+
+varoldAndroidHome=Environment.GetEnvironmentVariable("ANDROID_HOME");
+try{
+Environment.SetEnvironmentVariable("ANDROID_HOME","/i/dont/exist");
+// Check that this doesn't throw
+newAndroidSdkInfo(logger,androidSdkPath:sdk,androidNdkPath:null,javaSdkPath:jdk);
+}finally{
+Environment.SetEnvironmentVariable("ANDROID_HOME",oldAndroidHome);
+Directory.Delete(root,recursive:true);
+}
+}
+
 [Test]
 publicvoidConstructor_SetValuesFromPath()
 {