[#1710] Don't touch the project again if it was already built

The window of opportunity for the autobuild is the time between
forgetting the last built state and the touch of the project in its own
job. Check if there is a known built state before touching the project
again.

Signed-off-by: Sebastian Zarnekow <[email protected]>

org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/impl/BuilderStateDiscarder.java

@@ -8,6 +8,9 @@
  *******************************************************************************/
 package org.eclipse.xtext.builder.impl;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.util.Map;
 
 import org.apache.log4j.Logger;
@@ -34,12 +37,34 @@
 public class BuilderStateDiscarder {
 
 	private static final Logger logger = Logger.getLogger(BuilderStateDiscarder.class);
+	private static final MethodHandle InternalBuilder_getLastBuiltTree;
 
+	static {
+		try {
+			@SuppressWarnings("restriction")
+			MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(
+					org.eclipse.core.internal.events.InternalBuilder.class, MethodHandles.lookup());
+			@SuppressWarnings("restriction")
+			MethodHandle handle = lookup.findVirtual(
+					org.eclipse.core.internal.events.InternalBuilder.class, 
+					"getLastBuiltTree", 
+					MethodType.methodType(org.eclipse.core.internal.watson.ElementTree.class));
+			InternalBuilder_getLastBuiltTree = handle;
+		} catch (ReflectiveOperationException e) {
+			throw new ExceptionInInitializerError(e);
+		}
+	}
+
 	/**
 	 * Returns true, if the given builderArguments indicate that we should discard the built state
 	 * for the given projects.
 	 */
 	public boolean forgetLastBuildState(Iterable<IProject> toUpdate, Map<String, String> builderArguments) {
+		/*
+		 * Implementation note:
+		 * This is executed with the Workspace.lock being acquired, but the tree might be locked for modifications,
+		 * so we may not directly touch a project.
+		 */
 		if (canHandleBuildFlag(builderArguments)) {
 			for (IProject project : toUpdate) {
 				XtextBuilder builder = BuildManagerAccess.findBuilder(project);
@@ -61,7 +86,8 @@ public boolean forgetLastBuildState(Iterable<IProject> toUpdate, Map<String, Str
 	protected void touchProject(IProject project) {
 		Job touchJob = Job.create("Touch project " + project.getName(), progressMonitor -> {
 			try {
-				if (project.isAccessible()) {
+				// Check if there is already a known built tree before touching the project
+				if (project.isAccessible() && !hasBeenBuilt(project)) {
 					project.touch(progressMonitor);
 				}
 				return Status.OK_STATUS;
@@ -77,6 +103,17 @@ protected void touchProject(IProject project) {
 
 	}
 
+	protected boolean hasBeenBuilt(IProject project) {
+		@SuppressWarnings("restriction")
+		org.eclipse.core.internal.events.InternalBuilder builder = BuildManagerAccess.findBuilder(project);
+		try {
+			return InternalBuilder_getLastBuiltTree.invoke(builder) != null;
+		} catch (Throwable e) {
+			logger.error("Failed to dermine if the project was already built. Assuming it wasn't built yet", e);
+			return  false;
+		}
+	}
+
 	protected boolean canHandleBuildFlag(Map<String, String> builderArguments) {
 		return IBuildFlag.FORGET_BUILD_STATE_ONLY.isSet(builderArguments)
 				|| IBuildFlag.RECOVERY_BUILD.isSet(builderArguments);