[dotnet-linker] Improve error reporting by consolidating it in the LinkerConfiguration class. (#9992)

* Continue using our own error handling logic, and print our problems to stderr.
* Also use the linker's messaging facilities to report a more generic error,
  in case stderr doesn't show up for some reason.

src/ObjCRuntime/ErrorHelper.cs

@@ -37,7 +37,15 @@ namespace ObjCRuntime {
 			return new ProductException (code, false, innerException, message, args);
 		}
 
-		static void CollectExceptions (Exception ex, List<Exception> exceptions)
+		internal static IList<Exception> CollectExceptions (IEnumerable<Exception> exceptions)
+		{
+			var rv = new List<Exception> ();
+			foreach (var ex in exceptions)
+				CollectExceptions (ex, rv);
+			return rv;
+		}
+
+		internal static void CollectExceptions (Exception ex, List<Exception> exceptions)
 		{
 			AggregateException ae = ex as AggregateException;
 

tools/common/ErrorHelper.tools.cs

@@ -272,12 +272,9 @@ namespace Xamarin.Bundler {
 
 		public static void Show (IEnumerable<Exception> list)
 		{
-			List<Exception> exceptions = new List<Exception> ();
+			var exceptions = CollectExceptions (list);
 			bool error = false;
 
-			foreach (var e in list)
-				CollectExceptions (e, exceptions);
-
 			foreach (var ex in exceptions)
 				error |= ShowInternal (ex);
 

tools/dotnet-linker/LinkerConfiguration.cs

@@ -283,6 +283,29 @@ namespace Xamarin.Linker {
 
 			document.Save (Path.Combine (ItemsDirectory, itemName + ".items"));
 		}
+
+		public void Report (params Exception [] exceptions)
+		{
+			Report ((IList<Exception>) exceptions);
+		}
+
+		public void Report (IList<Exception> exceptions)
+		{
+			// We can't really use the linker's reporting facilities and keep our own error codes, because we'll
+			// end up re-using the same error codes the linker already uses for its own purposes. So instead show
+			// a generic error using the linker's Context.LogMessage API, and then print our own errors to stderr.
+			// Since we print using a standard message format, msbuild will parse those error messages and show
+			// them as msbuild errors.
+			var list = ErrorHelper.CollectExceptions (exceptions);
+			var allWarnings = list.All (v => v is ProductException pe && !pe.Error);
+			if (!allWarnings) {
+				// Revisit the error code after https://github.com/mono/linker/issues/1596 has been fixed.
+				var msg = MessageContainer.CreateErrorMessage ("Failed to execute the custom steps.", 1999, Platform.ToString ());
+				Context.LogMessage (msg);
+			}
+			// ErrorHelper.Show will print our errors and warnings to stderr.
+			ErrorHelper.Show (list);
+		}
 	}
 }
 

tools/dotnet-linker/Steps/ConfigurationAwareStep.cs

@@ -14,7 +14,7 @@ namespace Xamarin.Linker {
 
 		protected void Report (Exception exception)
 		{
-			ErrorHelper.Show (exception);
+			Configuration.Report (exception);
 		}
 
 		protected void Report (List<Exception> exceptions)

tools/dotnet-linker/Steps/ConfigurationAwareSubStep.cs

@@ -7,14 +7,12 @@ namespace Xamarin.Linker {
 	public abstract class ConfigurationAwareSubStep : ExceptionalSubStep {
 		protected override void Report (Exception exception)
 		{
-			ErrorHelper.Show (exception);
+			Configuration.Report (exception);
 		}
 
 		protected void Report (List<Exception> exceptions)
 		{
-			// Maybe there's a better way to show errors that integrates with the linker?
-			// We can't just throw an exception or exit here, since there might be only warnings in the list of exceptions.
-			ErrorHelper.Show (exceptions);
+			Configuration.Report (exceptions);
 		}
 	}
 }