Offer ObjectReaders advice about a RevWalk

By giving the reader information about the roots of a revision
traversal, some readers may be able to prefetch information from
their backing store using background threads in order to reduce
data access latency.  However this isn't typically necessary so
the default reader implementation doesn't react to the advice.

Change-Id: I72c6cbd05cff7d8506826015f50d9f57d5cda77e
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java

@@ -44,9 +44,13 @@
 package org.eclipse.jgit.lib;
 
 import java.io.IOException;
+import java.util.Collection;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.revwalk.ObjectWalk;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.storage.pack.ObjectReuseAsIs;
 
 /**
@@ -173,6 +177,44 @@ public abstract class ObjectReader {
 		return open(objectId, typeHint).getSize();
 	}
 
+	/**
+	 * Advice from a {@link RevWalk} that a walk is starting from these roots.
+	 *
+	 * @param walk
+	 *            the revision pool that is using this reader.
+	 * @param roots
+	 *            starting points of the revision walk. The starting points have
+	 *            their headers parsed, but might be missing bodies.
+	 * @throws IOException
+	 *             the reader cannot initialize itself to support the walk.
+	 */
+	public void walkAdviceBeginCommits(RevWalk walk, Collection<RevCommit> roots)
+			throws IOException {
+		// Do nothing by default, most readers don't want or need advice.
+	}
+
+	/**
+	 * Advice from an {@link ObjectWalk} that trees will be traversed.
+	 *
+	 * @param ow
+	 *            the object pool that is using this reader.
+	 * @param min
+	 *            the first commit whose root tree will be read.
+	 * @param max
+	 *            the last commit whose root tree will be read.
+	 * @throws IOException
+	 *             the reader cannot initialize itself to support the walk.
+	 */
+	public void walkAdviceBeginTrees(ObjectWalk ow, RevCommit min, RevCommit max)
+			throws IOException {
+		// Do nothing by default, most readers don't want or need advice.
+	}
+
+	/** Advice from that a walk is over. */
+	public void walkAdviceEnd() {
+		// Do nothing by default, most readers don't want or need advice.
+	}
+
 	/**
 	 * Release any resources used by this reader.
 	 * <p>

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/MergeBaseGenerator.java

@@ -137,6 +137,7 @@ class MergeBaseGenerator extends Generator {
 		for (;;) {
 			final RevCommit c = pending.next();
 			if (c == null) {
+				walker.reader.walkAdviceEnd();
 				walker.reader.release();
 				return null;
 			}

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/ObjectWalk.java

@@ -91,6 +91,10 @@ public class ObjectWalk extends RevWalk {
 
 	private RevObject last;
 
+	private RevCommit firstCommit;
+
+	private RevCommit lastCommit;
+
 	/**
 	 * Create a new revision and object walker for a given repository.
 	 *
@@ -235,6 +239,9 @@ public class ObjectWalk extends RevWalk {
 				}
 				continue;
 			}
+			if (firstCommit == null)
+				firstCommit = r;
+			lastCommit = r;
 			pendingObjects.add(r.getTree());
 			return r;
 		}
@@ -295,11 +302,19 @@ public class ObjectWalk extends RevWalk {
 			treeWalk = treeWalk.next();
 		}
 
+		if (firstCommit != null) {
+			reader.walkAdviceBeginTrees(this, firstCommit, lastCommit);
+			firstCommit = null;
+			lastCommit = null;
+		}
+
 		last = null;
 		for (;;) {
 			final RevObject o = pendingObjects.next();
-			if (o == null)
+			if (o == null) {
+				reader.walkAdviceEnd();
 				return null;
+			}
 			if ((o.flags & SEEN) != 0)
 				continue;
 			o.flags |= SEEN;
@@ -403,6 +418,8 @@ public class ObjectWalk extends RevWalk {
 		treeWalk = new CanonicalTreeParser();
 		currentTree = null;
 		last = null;
+		firstCommit = null;
+		lastCommit = null;
 	}
 
 	@Override
@@ -412,6 +429,8 @@ public class ObjectWalk extends RevWalk {
 		treeWalk = new CanonicalTreeParser();
 		currentTree = null;
 		last = null;
+		firstCommit = null;
+		lastCommit = null;
 	}
 
 	private void addObject(final RevObject o) {

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/PendingGenerator.java

@@ -128,7 +128,9 @@ class PendingGenerator extends Generator {
 			for (;;) {
 				final RevCommit c = pending.next();
 				if (c == null) {
-					walker.reader.release();
+					walker.reader.walkAdviceEnd();
+					if (!(walker instanceof ObjectWalk))
+						walker.reader.release();
 					return null;
 				}
 
@@ -174,6 +176,7 @@ class PendingGenerator extends Generator {
 					c.disposeBody();
 			}
 		} catch (StopWalkException swe) {
+			walker.reader.walkAdviceEnd();
 			walker.reader.release();
 			pending.clear();
 			return null;

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevWalk.java

@@ -172,7 +172,7 @@ public class RevWalk implements Iterable<RevCommit> {
 
 	int carryFlags = UNINTERESTING;
 
-	private final ArrayList<RevCommit> roots;
+	final ArrayList<RevCommit> roots;
 
 	AbstractRevQueue queue;
 

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/StartGenerator.java

@@ -85,6 +85,8 @@ class StartGenerator extends Generator {
 		final TreeFilter tf = w.getTreeFilter();
 		AbstractRevQueue q = walker.queue;
 
+		w.reader.walkAdviceBeginCommits(w, w.roots);
+
 		if (rf == RevFilter.MERGE_BASE) {
 			// Computing for merge bases is a special case and does not
 			// use the bulk of the generator pipeline.