[XABT] Add ArtifactFilename metadata for AndroidMavenLibrary item.

src/Xamarin.Android.Build.Tasks/Properties/Resources.resx

@@ -985,15 +985,11 @@ To use a custom JDK path for a command line build, set the 'JavaSdkDirectory' MS
   </data>
   <data name="XA4236" xml:space="preserve">
     <value>Cannot download Maven artifact '{0}:{1}'.
-- {2}: {3}
-- {4}: {5}</value>
+{2}</value>
     <comment>The following are literal names and should not be translated: Maven
 {0} - Maven artifact group id
 {1} - Maven artifact id
-{2} - The .jar filename we tried to download
-{3} - The HttpClient reported download exception message
-{4} - The .aar filename we tried to download
-{5} - The HttpClient provided download exception message</comment>
+{2} - The filenames we tried to download and the HttpClient reported download exception messages</comment>
   </data>
   <data name="XA4237" xml:space="preserve">
     <value>Cannot download POM file for Maven artifact '{0}'.

src/Xamarin.Android.Build.Tasks/Tasks/MavenDownload.cs

@@ -87,8 +87,11 @@ public async override System.Threading.Tasks.Task RunTaskAsync ()
 		if (repository is null)
 			return null;
 
+		// Allow user to override the Maven filename of the artifact
+		var maven_override_filename = item.GetMetadataOrDefault ("ArtifactFilename", null);
+
 		// Download artifact
-		var artifact_file = await MavenExtensions.DownloadPayload (repository, artifact, MavenCacheDirectory, Log, CancellationToken);
+		var artifact_file = await MavenExtensions.DownloadPayload (repository, artifact, MavenCacheDirectory, maven_override_filename, Log, CancellationToken);
 
 		if (artifact_file is null)
 			return null;

src/Xamarin.Android.Build.Tasks/Utilities/ITaskItemExtensions.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Xml.Linq;
 using Microsoft.Android.Build.Tasks;
@@ -23,7 +24,8 @@ public static IEnumerable<XElement> ToXElements (this ICollection<ITaskItem> ite
 			}
 		}
 
-		public static string GetMetadataOrDefault (this ITaskItem item, string name, string defaultValue)
+		[return: NotNullIfNotNull (nameof (defaultValue))]
+		public static string? GetMetadataOrDefault (this ITaskItem item, string name, string? defaultValue)
 		{
 			var value = item.GetMetadata (name);
 

src/Xamarin.Android.Build.Tasks/Utilities/MavenExtensions.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Java.Interop.Tools.Maven.Models;
@@ -77,7 +78,7 @@ public static bool TryParseArtifacts (string id, TaskLoggingHelper log, out List
 		return result;
 	}
 
-	public static bool TryParseJavaArtifact (this ITaskItem task, string type, TaskLoggingHelper log, [NotNullWhen (true)]out Artifact? artifact, out bool attributesSpecified)
+	public static bool TryParseJavaArtifact (this ITaskItem task, string type, TaskLoggingHelper log, [NotNullWhen (true)] out Artifact? artifact, out bool attributesSpecified)
 	{
 		var result = TryParseJavaArtifacts (task, type, log, out var artifacts, out attributesSpecified);
 
@@ -130,45 +131,57 @@ public static bool TryParseJavaArtifacts (this ITaskItem task, string type, Task
 	}
 
 	// Returns artifact output path
-	public static async Task<string?> DownloadPayload (CachedMavenRepository repository, Artifact artifact, string cacheDir, TaskLoggingHelper log, CancellationToken cancellationToken)
+	public static async Task<string?> DownloadPayload (CachedMavenRepository repository, Artifact artifact, string cacheDir, string? mavenOverrideFilename, TaskLoggingHelper log, CancellationToken cancellationToken)
 	{
 		var output_directory = Path.Combine (cacheDir, repository.Name, artifact.GroupId, artifact.Id, artifact.Version);
 
 		Directory.CreateDirectory (output_directory);
 
-		var filename = $"{artifact.Id}-{artifact.Version}";
-		var jar_filename = Path.Combine (output_directory, Path.Combine ($"{filename}.jar"));
-		var aar_filename = Path.Combine (output_directory, Path.Combine ($"{filename}.aar"));
+		var files_to_check = new List<string> ();
+
+		if (mavenOverrideFilename.HasValue ()) {
+			files_to_check.Add (Path.Combine (output_directory, mavenOverrideFilename));
+		} else {
+			files_to_check.Add (Path.Combine (output_directory, $"{artifact.Id}-{artifact.Version}.jar"));
+			files_to_check.Add (Path.Combine (output_directory, $"{artifact.Id}-{artifact.Version}.aar"));
+		}
 
 		// We don't need to redownload if we already have a cached copy
-		if (File.Exists (jar_filename))
-			return jar_filename;
+		foreach (var file in files_to_check) {
+			if (File.Exists (file))
+				return file;
+		}
 
-		if (File.Exists (aar_filename))
-			return aar_filename;
+		// Try to download the file from Maven
+		var results = new List<(string file, string error)> ();
 
-		if (await TryDownloadPayload (repository, artifact, jar_filename, cancellationToken) is not string jar_error)
-			return jar_filename;
+		foreach (var file in files_to_check) {
+			if (await TryDownloadPayload (repository, artifact, file, cancellationToken) is not string error)
+				return file;
 
-		if (await TryDownloadPayload (repository, artifact, aar_filename, cancellationToken) is not string aar_error)
-			return aar_filename;
+			results.Add ((file, error));
+		}
 
-		log.LogCodedError ("XA4236", Properties.Resources.XA4236, artifact.GroupId, artifact.Id, Path.GetFileName (jar_filename), jar_error, Path.GetFileName (aar_filename), aar_error);
+		// Couldn't download the artifact, construct an error message for the user
+		var error_builder = new StringBuilder ();
+
+		foreach (var error in results)
+			error_builder.AppendLine ($"- {Path.GetFileName (error.file)}: {error.error}");
+
+		log.LogCodedError ("XA4236", Properties.Resources.XA4236, artifact.GroupId, artifact.Id, error_builder.ToString ().TrimEnd ());
 
 		return null;
 	}
 
 	// Return value is download error message, null represents success (async methods cannot have out parameters)
 	static async Task<string?> TryDownloadPayload (CachedMavenRepository repository, Artifact artifact, string filename, CancellationToken cancellationToken)
 	{
-		var maven_filename = $"{artifact.Id}-{artifact.Version}{Path.GetExtension (filename)}";
-
 		try {
-			if ((await repository.GetFilePathAsync (artifact, maven_filename, cancellationToken)) is string path) {
+			if ((await repository.GetFilePathAsync (artifact, filename, cancellationToken)) is string path) {
 				return null;
 			} else {
 				// This probably(?) cannot be hit, everything should come back as an exception
-				return $"Could not download {maven_filename}";
+				return $"Could not download {filename}";
 			}
 
 		} catch (Exception ex) {