RevWalk: integrate commit-graph with commit parsing

RevWalk#createCommit() will inspect the commit-graph file to find the
specified object's graph position and then return a new RevCommitCG
instance.

RevCommitGC is a RevCommit with an additional "pointer" (the position)
to the commit-graph, so it can load the headers and metadata from there
instead of the pack. This saves IO access in walks where the body is not
needed (i.e. #isRetainBody is false and #parseBody is not invoked).

RevWalk uses automatically the commit-graph if available, no action
needed from callers. The commit-graph is fetched on first access from
the reader (that internally can keep it loaded and reuse it between
walks).

The startup cost of reading the entire commit graph is small. After
testing, reading a commit-graph with 1 million commits takes less than
50ms. If we use RepositoryCache, it will not be initialized util the
commit-graph is rewritten.

Bug: 574368
Change-Id: I90d0f64af24f3acc3eae6da984eae302d338f5ee
Signed-off-by: kylezhao <kylezhao@tencent.com>

1 files changed, 24 insertions, 1 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
index 6b644cef90..e155a25638 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevCommit.java
@@ -105,7 +105,12 @@ public class RevCommit extends RevObject {
 
 	static final RevCommit[] NO_PARENTS = {};
 
-	private RevTree tree;
+	/**
+	 * Tree reference of the commit.
+	 *
+	 * @since 6.5
+	 */
+	protected RevTree tree;
 
 	/**
 	 * Avoid accessing this field directly. Use method
@@ -657,6 +662,24 @@ public class RevCommit extends RevObject {
 	}
 
 	/**
+	 * Get the distance of the commit from the root, as defined in
+	 * {@link org.eclipse.jgit.internal.storage.commitgraph.CommitGraph}
+	 * <p>
+	 * Generation number is
+	 * {@link org.eclipse.jgit.lib.Constants#COMMIT_GENERATION_UNKNOWN} when the
+	 * commit is not in the commit-graph. If a commit-graph file was written by
+	 * a version of Git that did not compute generation numbers, then those
+	 * commits in commit-graph will have generation number represented by
+	 * {@link org.eclipse.jgit.lib.Constants#COMMIT_GENERATION_NOT_COMPUTED}.
+	 *
+	 * @return the generation number
+	 * @since 6.5
+	 */
+	int getGeneration() {
+		return Constants.COMMIT_GENERATION_UNKNOWN;
+	}
+
+	/**
 	 * Reset this commit to allow another RevWalk with the same instances.
 	 * <p>
 	 * Subclasses <b>must</b> call <code>super.reset()</code> to ensure the