1 files changed, 144 insertions, 0 deletions
diff --git a/java/com/google/gerrit/server/git/receive/HackPushNegotiateHook.java b/java/com/google/gerrit/server/git/receive/HackPushNegotiateHook.java
new file mode 100644
index 0000000000..bf3d270e43
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+++ b/java/com/google/gerrit/server/git/receive/HackPushNegotiateHook.java
@@ -0,0 +1,144 @@
+// Copyright (C) 2010 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.git.receive;
+
+import static org.eclipse.jgit.lib.RefDatabase.ALL;
+
+import com.google.common.collect.Sets;
+import com.google.common.flogger.FluentLogger;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Map;
+import java.util.Set;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.transport.AdvertiseRefsHook;
+import org.eclipse.jgit.transport.BaseReceivePack;
+import org.eclipse.jgit.transport.ServiceMayNotContinueException;
+import org.eclipse.jgit.transport.UploadPack;
+
+/**
+ * Advertises part of history to git push clients.
+ *
+ * <p>This is a hack to work around the lack of negotiation in the send-pack/receive-pack wire
+ * protocol.
+ *
+ * <p>When the server is frequently advancing master by creating merge commits, the client may not
+ * be able to discover a common ancestor during push. Attempting to push will re-upload a very large
+ * amount of history. This hook hacks in a fake negotiation replacement by walking history and
+ * sending recent commits as {@code ".have"} lines in the wire protocol, allowing the client to find
+ * a common ancestor.
+ */
+public class HackPushNegotiateHook implements AdvertiseRefsHook {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  /** Size of an additional ".have" line. */
+  private static final int HAVE_LINE_LEN = 4 + Constants.OBJECT_ID_STRING_LENGTH + 1 + 5 + 1;
+
+  /**
+   * Maximum number of bytes to "waste" in the advertisement with a peek at this repository's
+   * current reachable history.
+   */
+  private static final int MAX_EXTRA_BYTES = 8192;
+
+  /**
+   * Number of recent commits to advertise immediately, hoping to show a client a nearby merge base.
+   */
+  private static final int BASE_COMMITS = 64;
+
+  /** Number of commits to skip once base has already been shown. */
+  private static final int STEP_COMMITS = 16;
+
+  /** Total number of commits to extract from the history. */
+  private static final int MAX_HISTORY = MAX_EXTRA_BYTES / HAVE_LINE_LEN;
+
+  @Override
+  public void advertiseRefs(UploadPack us) {
+    throw new UnsupportedOperationException("HackPushNegotiateHook cannot be used for UploadPack");
+  }
+
+  @Override
+  public void advertiseRefs(BaseReceivePack rp) throws ServiceMayNotContinueException {
+    Map<String, Ref> r = rp.getAdvertisedRefs();
+    if (r == null) {
+      try {
+        r = rp.getRepository().getRefDatabase().getRefs(ALL);
+      } catch (ServiceMayNotContinueException e) {
+        throw e;
+      } catch (IOException e) {
+        throw new ServiceMayNotContinueException(e);
+      }
+    }
+    rp.setAdvertisedRefs(r, history(r.values(), rp));
+  }
+
+  private Set<ObjectId> history(Collection<Ref> refs, BaseReceivePack rp) {
+    Set<ObjectId> alreadySending = rp.getAdvertisedObjects();
+    if (alreadySending.isEmpty()) {
+      alreadySending = idsOf(refs);
+    }
+
+    int max = MAX_HISTORY - Math.max(0, alreadySending.size() - refs.size());
+    if (max <= 0) {
+      return Collections.emptySet();
+    }
+
+    // Scan history until the advertisement is full.
+    RevWalk rw = rp.getRevWalk();
+    rw.reset();
+    try {
+      for (Ref ref : refs) {
+        try {
+          if (ref.getObjectId() != null) {
+            rw.markStart(rw.parseCommit(ref.getObjectId()));
+          }
+        } catch (IOException badCommit) {
+          continue;
+        }
+      }
+
+      Set<ObjectId> history = Sets.newHashSetWithExpectedSize(max);
+      try {
+        int stepCnt = 0;
+        for (RevCommit c; history.size() < max && (c = rw.next()) != null; ) {
+          if (c.getParentCount() <= 1
+              && !alreadySending.contains(c)
+              && (history.size() < BASE_COMMITS || (++stepCnt % STEP_COMMITS) == 0)) {
+            history.add(c);
+          }
+        }
+      } catch (IOException err) {
+        logger.atSevere().withCause(err).log("error trying to advertise history");
+      }
+      return history;
+    } finally {
+      rw.reset();
+    }
+  }
+
+  private static Set<ObjectId> idsOf(Collection<Ref> refs) {
+    Set<ObjectId> r = Sets.newHashSetWithExpectedSize(refs.size());
+    for (Ref ref : refs) {
+      if (ref.getObjectId() != null) {
+        r.add(ref.getObjectId());
+      }
+    }
+    return r;
+  }
+}