[Xamarin.Build.Download] Guard against race condition when multiple processes are downloading same artifact.

Util/Xamarin.Build.Download/source/Xamarin.Build.Download/Xamarin.Build.Download.csproj

@@ -8,7 +8,7 @@
 
     <PackageId>Xamarin.Build.Download</PackageId>
     <Title>Xamarin Build-time Download Support</Title>
-    <PackageVersion>0.11.3</PackageVersion>
+    <PackageVersion>0.11.4</PackageVersion>
     <Authors>Microsoft</Authors>
     <Owners>Microsoft</Owners>
     <PackageProjectUrl>https://go.microsoft.com/fwlink/?linkid=865061</PackageProjectUrl>

Util/Xamarin.Build.Download/source/Xamarin.Build.Download/XamarinDownloadArchives.cs

@@ -109,6 +109,7 @@ bool IsValidDownload(string cachedHashFile, string fileToHash, string expectedHa
 				} catch { }
 			}
 
+			LogMessage ("Hash mismatch for file: " + fileToHash, MessageImportance.High);
 			return false;
 		}
 
@@ -152,6 +153,10 @@ async Task<bool> MakeSureLibraryIsInPlace (XamarinBuildDownload xbd, Cancellatio
 					return false;
 				}
 
+				// Check flag again in case someone else downloaded this while we were waiting for the lock
+				if (File.Exists (flagFile))
+					return true;
+
 				if (!File.Exists (xbd.CacheFile) || !IsValidDownload (xbd.DestinationDir + ".sha256", xbd.CacheFile, xbd.Sha256)) {
 					try {
 						int progress = -1;