[Xamarin.Android.Build.Tasks] honour minSdkVersion from library uses-sdk Manifest Files.

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

@@ -90,6 +90,8 @@ public class GenerateJavaStubs : AndroidTask
 
 		public string SupportedOSPlatformVersion { get; set; }
 
+		public string LibraryMinSdk { get; set; } = string.Empty;
+
 		public ITaskItem[] Environments { get; set; }
 
 		[Output]
@@ -345,6 +347,7 @@ void Run (DirectoryAssemblyResolver res, bool useMarshalMethods)
 				SdkDir = AndroidSdkDir,
 				TargetSdkVersion = AndroidSdkPlatform,
 				MinSdkVersion = minSdkVersion,
+				LibraryMinSdkVersion = LibraryMinSdk,
 				Debug = Debug,
 				MultiDex = MultiDex,
 				NeedsInternet = NeedsInternet,

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

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Xml;
 using System.Xml.Linq;
 using Microsoft.Build.Utilities;
 using Microsoft.Build.Framework;
@@ -22,7 +23,7 @@ public class GetImportedLibraries : AndroidTask
 		[Required]
 		public string TargetDirectory { get; set; }
 
-		public string CacheFile { get; set;} 
+		public string CacheFile { get; set;}
 
 		[Output]
 		public ITaskItem [] Jars { get; set; }
@@ -33,6 +34,9 @@ public class GetImportedLibraries : AndroidTask
 		[Output]
 		public ITaskItem [] ManifestDocuments { get; set; }
 
+		[Output]
+		public string LibraryMinSdk { get; set; } = string.Empty;
+
 		public override bool RunTask ()
 		{
 			if (!Directory.Exists (TargetDirectory)) {
@@ -43,6 +47,7 @@ public override bool RunTask ()
 			var manifestDocuments = new List<ITaskItem> ();
 			var nativeLibraries   = new List<ITaskItem> ();
 			var jarFiles          = new List<ITaskItem> ();
+			int libraryMinSdk  = -1;
 			foreach (var file in Directory.EnumerateFiles (TargetDirectory, "*", SearchOption.AllDirectories)) {
 				if (file.EndsWith (".so", StringComparison.OrdinalIgnoreCase)) {
 					if (AndroidRidAbiHelper.GetNativeLibraryAbi (file) != null)
@@ -55,11 +60,23 @@ public override bool RunTask ()
 						var directory = Path.GetFileName (Path.GetDirectoryName (file));
 						if (IgnoredManifestDirectories.Contains (directory))
 							continue;
-						manifestDocuments.Add (new TaskItem (file));
+						var item = new TaskItem (file);
+						XDocument doc = XDocument.Load (file);
+						var minAttr = doc.Root.Element ("uses-sdk")?.Attribute (ManifestDocument.AndroidXmlNamespace + "minSdkVersion");
+						if (minAttr != null) {
+							if (int.TryParse (minAttr.Value, out int minSDK))
+								libraryMinSdk = Math.Max (libraryMinSdk, minSDK);
+						}
+						manifestDocuments.Add (item);
 					}
 				}
 			}
 
+			XAttribute minSdk = null;
+			if (libraryMinSdk != -1) {
+				LibraryMinSdk = libraryMinSdk.ToString ();
+				minSdk = new XAttribute ("libraryMinSdk", LibraryMinSdk);
+			}
 			ManifestDocuments = manifestDocuments.ToArray ();
 			NativeLibraries = nativeLibraries.ToArray ();
 			Jars = jarFiles.ToArray ();
@@ -68,17 +85,13 @@ public override bool RunTask ()
 				var document = new XDocument (
 							new XDeclaration ("1.0", "UTF-8", null),
 							new XElement ("Paths",
-									new XElement ("ManifestDocuments", ManifestDocuments.Select(e => new XElement ("ManifestDocument", e.ItemSpec))),
+									new XElement ("ManifestDocuments", minSdk, ManifestDocuments.Select(e => new XElement ("ManifestDocument", e.ItemSpec))),
 									new XElement ("NativeLibraries", NativeLibraries.Select(e => new XElement ("NativeLibrary", e.ItemSpec))),
 									new XElement ("Jars", Jars.Select(e => new XElement ("Jar", e.ItemSpec)))
 						));
 				document.SaveIfChanged (CacheFile);
 			}
 
-			Log.LogDebugTaskItems ("  NativeLibraries: ", NativeLibraries);
-			Log.LogDebugTaskItems ("  Jars: ", Jars);
-			Log.LogDebugTaskItems ("  ManifestDocuments: ", ManifestDocuments);
-
 			return true;
 		}
 	}

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

@@ -1,21 +1,21 @@
-// 
+//
 // ReadImportedLibrariesCache.cs
-//  
+//
 // Author:
 //       Dean Ellis <dean.ellis@xamarin.com>
-// 
+//
 // Copyright (c) 2013 Xamarin Inc.
-// 
+//
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
 // in the Software without restriction, including without limitation the rights
 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 // copies of the Software, and to permit persons to whom the Software is
 // furnished to do so, subject to the following conditions:
-// 
+//
 // The above copyright notice and this permission notice shall be included in
 // all copies or substantial portions of the Software.
-// 
+//
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
@@ -39,7 +39,7 @@ public class ReadImportedLibrariesCache : AndroidTask
 		public override string TaskPrefix => "RIL";
 
 		[Required]
-		public string CacheFile { get; set;} 
+		public string CacheFile { get; set;}
 
 		[Output]
 		public ITaskItem [] Jars { get; set; }
@@ -50,6 +50,9 @@ public class ReadImportedLibrariesCache : AndroidTask
 		[Output]
 		public ITaskItem [] ManifestDocuments { get; set; }
 
+		[Output]
+		public string LibraryMinSdk { get; set; }
+
 		public override bool RunTask ()
 		{
 			if (!File.Exists (CacheFile)) {
@@ -60,10 +63,11 @@ public override bool RunTask ()
 			Jars = doc.GetPathsAsTaskItems ("Jars", "Jar");
 			NativeLibraries = doc.GetPathsAsTaskItems ("NativeLibraries", "NativeLibrary");
 			ManifestDocuments = doc.GetPathsAsTaskItems ("ManifestDocuments", "ManifestDocument");
-
-			Log.LogDebugTaskItems ("  NativeLibraries: ", NativeLibraries);
-			Log.LogDebugTaskItems ("  Jars: ", Jars);
-			Log.LogDebugTaskItems ("  ManifestDocuments: ", ManifestDocuments);
+			var minSdk = doc.Element ("Paths")?.Element ("ManifestDocuments")?.Attribute ("libraryMinSdk");
+			LibraryMinSdk = string.Empty;
+			if (minSdk != null)
+				if (int.TryParse (minSdk.Value, out int minSDK))
+					LibraryMinSdk = minSdk.Value;
 
 			return !Log.HasLoggedErrors;
 		}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs

@@ -685,6 +685,33 @@ public void ModifyManifest ([Values (true, false)] bool isRelease)
 			}
 		}
 
+		[Test]
+		public void EmptyUsesSdkElementBuildsOK ([Values (false, true)] bool addReference)
+		{
+			var proj = new XamarinAndroidApplicationProject () {
+				IsRelease = true,
+			};
+			if (addReference)
+				proj.PackageReferences.Add (KnownPackages.AndroidXSecurityCrypto);
+			proj.AndroidManifest = @"<?xml version=""1.0"" encoding=""utf-8""?>
+<manifest xmlns:android=""http://schemas.android.com/apk/res/android"" xmlns:tools=""http://schemas.android.com/tools"" android:versionCode=""1"" android:versionName=""1.0"" package=""foo.foo"">
+	<uses-sdk />
+	<application android:label=""foo"">
+	</application>
+</manifest>";
+			using (var b = CreateApkBuilder ($"temp/{TestName}", cleanupAfterSuccessfulBuild: true, cleanupOnDispose: false)) {
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				var manifestFile = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "android", "AndroidManifest.xml");
+				var doc = XDocument.Load (manifestFile);
+				var ns = XNamespace.Get ("http://schemas.android.com/apk/res/android");
+				var usesSdk = doc.Element ("manifest")?.Element ("uses-sdk");
+				Assert.IsNotNull (usesSdk, "Should have found a uses-sdk element.");
+				var expected = addReference ? "23" : "19";
+				Assert.AreEqual (expected, usesSdk.Attribute (ns + "minSdkVersion")?.Value, $"minSdkVersion should have been '{expected}' but was '{usesSdk.Attribute ("minSdkVersion")?.Value}'");
+
+			}
+		}
+
 		[Test]
 		public void MergeLibraryManifest ()
 		{

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/KnownPackages.cs

@@ -328,6 +328,16 @@ public static class KnownPackages
 				}
 			}
 		};
+		public static Package AndroidXSecurityCrypto = new Package {
+			Id = "Xamarin.AndroidX.Security.SecurityCrypto",
+			Version = "1.0.0.9",
+			TargetFramework = "MonoAndroid90",
+			References = {
+				new BuildItem.Reference("Xamarin.AndroidX.Security.SecurityCrypto") {
+					MetadataValues = "HintPath=..\\packages\\Xamarin.AndroidX.Security.SecurityCrypto.1.0.0.9\\lib\\MonoAndroid90\\Xamarin.AndroidX.Security.SecurityCrypto.dll"
+				}
+			}
+		};
 		public static Package XamarinGoogleAndroidMaterial = new Package {
 			Id = "Xamarin.Google.Android.Material",
 			Version = "1.0.0.1",

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

@@ -87,6 +87,7 @@ internal class ManifestDocument
 		public string SdkDir { get; set; }
 		public string TargetSdkVersion { get; set; }
 		public string MinSdkVersion { get; set; }
+		public string LibraryMinSdkVersion { get; set; } = string.Empty;
 		public bool Debug { get; set; }
 		public bool MultiDex { get; set; }
 		public bool NeedsInternet { get; set; }
@@ -119,13 +120,17 @@ public string VersionCode {
 
 		public string GetMinimumSdk () {
 			int defaultMinSdkVersion = MonoAndroidHelper.SupportedVersions.MinStableVersion.ApiLevel;
+			Console.WriteLine ($"DEBUG GetMinimumSdk: {defaultMinSdkVersion}");
 			var minAttr = doc.Root.Element ("uses-sdk")?.Attribute (androidNs + "minSdkVersion");
 			if (minAttr == null) {
 				int minSdkVersion;
+				Console.WriteLine ($"DEBUG GetMinimumSdk: Parsing {MinSdkVersionName}");
 				if (!int.TryParse (MinSdkVersionName, out minSdkVersion))
 					minSdkVersion = defaultMinSdkVersion;
+				Console.WriteLine ($"DEBUG GetMinimumSdk: Using {Math.Min (minSdkVersion, defaultMinSdkVersion)}");
 				return Math.Min (minSdkVersion, defaultMinSdkVersion).ToString ();
 			}
+			Console.WriteLine ($"DEBUG GetMinimumSdk: Found {minAttr.Value}");
 			return minAttr.Value;
 		}
 
@@ -309,10 +314,15 @@ public IList<string> Merge (TaskLoggingHelper log, TypeDefinitionCache cache, Li
 
 			if (uses.Attribute (androidNs + "minSdkVersion") == null) {
 				int minSdkVersion;
-				if (!int.TryParse (MinSdkVersionName, out minSdkVersion))
+				if (!int.TryParse (MinSdkVersionName, out minSdkVersion)) {
 					minSdkVersion = XABuildConfig.NDKMinimumApiAvailable;
-				minSdkVersion = Math.Min (minSdkVersion, XABuildConfig.NDKMinimumApiAvailable);
-				uses.SetAttributeValue (androidNs + "minSdkVersion", minSdkVersion.ToString ());
+				}
+				if (!string.IsNullOrEmpty (LibraryMinSdkVersion) && int.TryParse (LibraryMinSdkVersion, out minSdkVersion)) {
+					uses.SetAttributeValue (androidNs + "minSdkVersion", minSdkVersion.ToString ());
+				} else {
+					minSdkVersion = Math.Min (minSdkVersion, XABuildConfig.NDKMinimumApiAvailable);
+					uses.SetAttributeValue (androidNs + "minSdkVersion", minSdkVersion.ToString ());
+				}
 			}
 
 			string targetSdkVersion;

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1545,6 +1545,7 @@ because xbuild doesn't support framework reference assemblies.
       LinkingEnabled="$(_LinkingEnabled)"
       HaveMultipleRIDs="$(_HaveMultipleRIDs)"
       IntermediateOutputDirectory="$(IntermediateOutputPath)"
+      LibraryMinSdk="$(_LibraryMinSdk)"
       Environments="@(AndroidEnvironment);@(LibraryEnvironments)">
     <Output TaskParameter="GeneratedBinaryTypeMaps" ItemName="_AndroidTypeMapping" Condition=" '$(_InstantRunEnabled)' == 'True' " />
   </GenerateJavaStubs>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.EmbeddedResource.targets

@@ -102,6 +102,7 @@ This file is used by all project types, including binding projects.
       <Output TaskParameter="NativeLibraries"   ItemName="ExtractedNativeLibraryImports" />
       <Output TaskParameter="NativeLibraries"   ItemName="AndroidNativeLibrary" />
       <Output TaskParameter="ManifestDocuments" ItemName="ExtractedManifestDocuments" />
+      <Output TaskParameter="LibraryMinSdk"     PropertyName="_LibraryMinSdk" />
     </ReadImportedLibrariesCache>
   </Target>