Allow using LinkWith attribute without a native library. (#997)

This makes it possible to set linker flags per assembly:

    [assembly: LinkWith (LinkerFlags = "-lsqlite3")]

Which is required when incremental builds is enabled and a particular assembly
needs special linker flags (because we don't propagate the global -gcc_flags
to each dylib we build when doing incremental builds).

Also add an option to set the dlsym mode for an assembly (using the LinkWith
attribute).

docs/website/binding_types_reference_guide.md

@@ -2122,6 +2122,7 @@ public enum LinkTarget {
 
 [AttributeUsage(AttributeTargets.Assembly, AllowMultiple=true)]
 public class LinkWithAttribute : Attribute {
+    public LinkWithAttribute ();
     public LinkWithAttribute (string libraryName);
     public LinkWithAttribute (string libraryName, LinkTarget target);
     public LinkWithAttribute (string libraryName, LinkTarget target, string linkerFlags);
@@ -2149,6 +2150,13 @@ into the resulting assembly, allowing users to ship a single DLL that contains
 both the unmanaged dependencies as well as the command line flags necessary to
 properly consume the library from Xamarin.iOS.
 
+It' s also possible to not provide a `libraryName`, in which case the
+`LinkWith` attribute can be used to only specify additional linker flags:
+
+ ``` csharp
+[assembly: LinkWith (LinkerFlags = "-lsqlite3")]
+ ```
+
  <a name="LinkWithAttribute_Constructors" class="injected"></a>
 
 
@@ -2163,6 +2171,9 @@ Note that the LinkTarget argument is inferred by Xamarin.iOS and does not need t
 Examples:
 
 ```
+// Specify additional linker:
+[assembly: LinkWith (LinkerFlags = "-sqlite3")]
+
 // Specify library name for the constructor:
 [assembly: LinkWith ("libDemo.a");
 

src/ObjCRuntime/LinkWithAttribute.cs

@@ -40,6 +40,13 @@ namespace XamCore.ObjCRuntime {
 		x86_64 = Simulator64
 	}
 	
+	public enum DlsymOption
+	{
+		Default,
+		Required,
+		Disabled,
+	}
+
 	[AttributeUsage(AttributeTargets.Assembly, AllowMultiple=true)]
 #if XAMCORE_2_0
 	sealed
@@ -62,6 +69,10 @@ namespace XamCore.ObjCRuntime {
 		{
 			LibraryName = libraryName;
 		}
+
+		public LinkWithAttribute ()
+		{
+		}
 		
 		public bool ForceLoad {
 			get; set;
@@ -98,5 +109,9 @@ namespace XamCore.ObjCRuntime {
 		public bool SmartLink {
 			get; set;
 		}
+
+		public DlsymOption Dlsym {
+			get; set;
+		}
 	}
 }

tests/introspection/ApiTypoTest.cs

@@ -134,6 +134,7 @@ namespace Introspection
 			"dlerror",
 			"Dlfcn",
 			"dlopen",
+			"Dlsym",
 			"dlsym",
 			"Dng",
 			"Dns",

tests/mtouch/MTouch.cs

@@ -1961,6 +1961,31 @@ class Test {
 			}
 		}
 
+		[Test]
+		public void LinkWithNoLibrary ()
+		{
+			using (var tool = new MTouchTool ()) {
+				tool.Profile = Profile.Unified;
+				tool.CreateTemporaryApp (@"
+using System;
+using System.Runtime.InteropServices;
+using ObjCRuntime;
+[assembly: LinkWith (Dlsym = DlsymOption.Required)]
+class C {
+	[DllImport (""libsqlite3"")]
+	static extern void sqlite3_column_database_name16 ();
+	static void Main ()
+	{
+	}
+}
+");
+				tool.NoFastSim = true;
+				tool.Dlsym = false;
+				tool.Linker = MTouchLinker.LinkSdk;
+				Assert.AreEqual (0, tool.Execute (MTouchAction.BuildDev), "build");
+			}
+		}
+
 #region Helper functions
 		static string CompileUnifiedTestAppExecutable (string targetDirectory, string code = null, string extraArg = "")
 		{

tools/common/Assembly.cs

@@ -164,7 +164,8 @@ namespace Xamarin.Bundler {
 				string libraryName = linkWith.LibraryName;
 				
 				// Remove the resource from the assembly at a later stage.
-				AddResourceToBeRemoved (libraryName);
+				if (!string.IsNullOrEmpty (libraryName))
+					AddResourceToBeRemoved (libraryName);
 
 				// We can't add -dead_strip if there are any LinkWith attributes where smart linking is disabled.
 				if (!linkWith.SmartLink)
@@ -202,59 +203,65 @@ namespace Xamarin.Bundler {
 				if (linkWith.IsCxx)
 					EnableCxx = true;
 
-				path = Path.Combine (Cache.Location, libraryName);
-				if (path.EndsWith (".framework", StringComparison.Ordinal)) {
 #if MONOTOUCH
-					if (App.DeploymentTarget.Major < 8) {
-						throw ErrorHelper.CreateError (1305, "The binding library '{0}' contains a user framework ({0}), but embedded user frameworks require iOS 8.0 (the deployment target is {1}). Please set the deployment target in the Info.plist file to at least 8.0.",
-							FileName, Path.GetFileName (path), App.DeploymentTarget);
-					}
+				if (linkWith.Dlsym != DlsymOption.Default)
+					App.SetDlsymOption (FullPath, linkWith.Dlsym == DlsymOption.Required);
 #endif
-					var zipPath = path + ".zip";
-					if (!Application.IsUptodate (FullPath, zipPath)) {
-						Application.ExtractResource (assembly.MainModule, libraryName, zipPath, false);
-						Driver.Log (3, "Extracted third-party framework '{0}' from '{1}' to '{2}'", libraryName, FullPath, zipPath);
-						LogLinkWithAttribute (linkWith);
+
+				if (!string.IsNullOrEmpty (libraryName)) {
+					path = Path.Combine (Cache.Location, libraryName);
+					if (path.EndsWith (".framework", StringComparison.Ordinal)) {
+#if MONOTOUCH
+						if (App.DeploymentTarget.Major < 8) {
+							throw ErrorHelper.CreateError (1305, "The binding library '{0}' contains a user framework ({0}), but embedded user frameworks require iOS 8.0 (the deployment target is {1}). Please set the deployment target in the Info.plist file to at least 8.0.",
+								FileName, Path.GetFileName (path), App.DeploymentTarget);
+						}
+#endif
+						var zipPath = path + ".zip";
+						if (!Application.IsUptodate (FullPath, zipPath)) {
+							Application.ExtractResource (assembly.MainModule, libraryName, zipPath, false);
+							Driver.Log (3, "Extracted third-party framework '{0}' from '{1}' to '{2}'", libraryName, FullPath, zipPath);
+							LogLinkWithAttribute (linkWith);
+						} else {
+							Driver.Log (3, "Target '{0}' is up-to-date.", path);
+						}
+
+						if (!File.Exists (zipPath)) {
+							ErrorHelper.Warning (1302, "Could not extract the native framework '{0}' from '{1}'. " +
+								"Please ensure the native framework was properly embedded in the managed assembly " +
+								"(if the assembly was built using a binding project, the native framework must be included in the project, and its Build Action must be 'ObjcBindingNativeFramework').",
+								libraryName, zipPath);
+						} else {
+							if (!Directory.Exists (path))
+								Directory.CreateDirectory (path);
+
+							if (Driver.RunCommand ("/usr/bin/unzip", string.Format ("-u -o -d {0} {1}", Driver.Quote (path), Driver.Quote (zipPath))) != 0)
+								throw ErrorHelper.CreateError (1303, "Could not decompress the native framework '{0}' from '{1}'. Please review the build log for more information from the native 'unzip' command.", libraryName, zipPath);
+						}
+
+						if (Frameworks == null)
+							Frameworks = new HashSet<string> ();
+						Frameworks.Add (path);
 					} else {
-						Driver.Log (3, "Target '{0}' is up-to-date.", path);
+						if (!Application.IsUptodate (FullPath, path)) {
+							Application.ExtractResource (assembly.MainModule, libraryName, path, false);
+							Driver.Log (3, "Extracted third-party binding '{0}' from '{1}' to '{2}'", libraryName, FullPath, path);
+							LogLinkWithAttribute (linkWith);
+						} else {
+							Driver.Log (3, "Target '{0}' is up-to-date.", path);
+						}
+
+						if (!File.Exists (path))
+							ErrorHelper.Warning (1302, "Could not extract the native library '{0}' from '{1}'. " +
+							"Please ensure the native library was properly embedded in the managed assembly " +
+							"(if the assembly was built using a binding project, the native library must be included in the project, and its Build Action must be 'ObjcBindingNativeLibrary').",
+								libraryName, path);
+
+						if (LinkWith == null)
+							LinkWith = new List<string> ();
+						LinkWith.Add (path);
 					}
-
-					if (!File.Exists (zipPath)) {
-						ErrorHelper.Warning (1302, "Could not extract the native framework '{0}' from '{1}'. " +
-							"Please ensure the native framework was properly embedded in the managed assembly " +
-							"(if the assembly was built using a binding project, the native framework must be included in the project, and its Build Action must be 'ObjcBindingNativeFramework').",
-							libraryName, zipPath);
-					} else {
-						if (!Directory.Exists (path))
-							Directory.CreateDirectory (path);
-
-						if (Driver.RunCommand ("/usr/bin/unzip", string.Format ("-u -o -d {0} {1}", Driver.Quote (path), Driver.Quote (zipPath))) != 0)
-							throw ErrorHelper.CreateError (1303, "Could not decompress the native framework '{0}' from '{1}'. Please review the build log for more information from the native 'unzip' command.", libraryName, zipPath);
-					}
-
-					if (Frameworks == null)
-						Frameworks = new HashSet<string> ();
-					Frameworks.Add (path);
-				} else {
-					if (!Application.IsUptodate (FullPath, path)) {
-						Application.ExtractResource (assembly.MainModule, libraryName, path, false);
-						Driver.Log (3, "Extracted third-party binding '{0}' from '{1}' to '{2}'", libraryName, FullPath, path);
-						LogLinkWithAttribute (linkWith);
-					} else {
-						Driver.Log (3, "Target '{0}' is up-to-date.", path);
-					}
-
-					if (!File.Exists (path))
-						ErrorHelper.Warning (1302, "Could not extract the native library '{0}' from '{1}'. " +
-						"Please ensure the native library was properly embedded in the managed assembly " +
-						"(if the assembly was built using a binding project, the native library must be included in the project, and its Build Action must be 'ObjcBindingNativeLibrary').",
-							libraryName, path);
-
-					if (LinkWith == null)
-						LinkWith = new List<string> ();
-					LinkWith.Add (path);
 				}
-
 			}
 
 			if (exceptions != null && exceptions.Count > 0)
@@ -297,6 +304,9 @@ namespace Xamarin.Bundler {
 			case 2:
 				linkWith = new LinkWithAttribute ((string) cargs [0].Value, (LinkTarget) cargs [1].Value); 
 				break;
+			case 0:
+				linkWith = new LinkWithAttribute ();
+				break;
 			default: 
 			case 1: 
 				linkWith = new LinkWithAttribute ((string) cargs [0].Value); 
@@ -329,6 +339,9 @@ namespace Xamarin.Bundler {
 				case "SmartLink":
 					linkWith.SmartLink = (bool) property.Argument.Value;
 					break;
+				case "Dlsym":
+					linkWith.Dlsym = (DlsymOption) property.Argument.Value;
+					break;
 				default: 
 					break;
 				}

tools/mtouch/Application.cs

@@ -155,6 +155,16 @@ namespace Xamarin.Bundler {
 		List<Abi> abis;
 		HashSet<Abi> all_architectures; // all Abis used in the app, including extensions.
 
+		public void SetDlsymOption (string asm, bool dlsym)
+		{
+			if (DlsymAssemblies == null)
+				DlsymAssemblies = new List<Tuple<string, bool>> ();
+
+			DlsymAssemblies.Add (new Tuple<string, bool> (Path.GetFileNameWithoutExtension (asm), dlsym));
+
+			DlsymOptions = DlsymOptions.Custom;
+		}
+
 		public void ParseDlsymOptions (string options)
 		{
 			bool dlsym;
@@ -186,13 +196,6 @@ namespace Xamarin.Bundler {
 		{
 			string asm;
 
-			switch (DlsymOptions) {
-			case DlsymOptions.All:
-				return true;
-			case DlsymOptions.None:
-				return false;
-			}
-
 			if (DlsymAssemblies != null) {
 				asm = Path.GetFileNameWithoutExtension (assembly);
 				foreach (var tuple in DlsymAssemblies) {
@@ -201,6 +204,13 @@ namespace Xamarin.Bundler {
 				}
 			}
 
+			switch (DlsymOptions) {
+			case DlsymOptions.All:
+				return true;
+			case DlsymOptions.None:
+				return false;
+			}
+
 			if (EnableLLVMOnlyBitCode)
 				return false;