Index: oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/ReadOnlyFileStore.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/ReadOnlyFileStore.java	(revision 9f1e8bdc6c91ba750ac1eea540fe59ac89ef9114)
+++ oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/ReadOnlyFileStore.java	(date 1535624179000)
@@ -19,6 +19,7 @@
 package org.apache.jackrabbit.oak.segment.file;
 
 import static org.apache.jackrabbit.oak.segment.DefaultSegmentWriterBuilder.defaultSegmentWriterBuilder;
+import static org.apache.jackrabbit.oak.segment.file.Reclaimers.newOldReclaimer;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -28,6 +29,7 @@
 import java.util.UUID;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
+import java.util.function.Consumer;
 
 import com.google.common.io.Closer;
 import com.google.common.util.concurrent.UncheckedExecutionException;
@@ -35,6 +37,7 @@
 import org.apache.jackrabbit.oak.segment.Segment;
 import org.apache.jackrabbit.oak.segment.SegmentId;
 import org.apache.jackrabbit.oak.segment.SegmentWriter;
+import org.apache.jackrabbit.oak.segment.compaction.SegmentGCOptions;
 import org.apache.jackrabbit.oak.segment.file.tar.TarFiles;
 import org.jetbrains.annotations.NotNull;
 import org.slf4j.Logger;
@@ -55,6 +58,7 @@
 
     @NotNull
     private final SegmentWriter writer;
+    private final int gcRetainedGenerations;
 
     private ReadOnlyRevisions revisions;
 
@@ -75,6 +79,8 @@
                 .build();
 
         writer = defaultSegmentWriterBuilder("read-only").withoutCache().build(this);
+        gcRetainedGenerations = builder.getGcOptions().getRetainedGenerations();
+
         log.info("TarMK ReadOnly opened: {} (mmap={})", directory,
                 memoryMapping);
     }
@@ -165,4 +171,10 @@
     public Set<SegmentId> getReferencedSegmentIds() {
         return tracker.getReferencedSegmentIds();
     }
+
+    @Override
+    public void collectBlobReferences(Consumer<String> collector) throws IOException {
+        tarFiles.collectBlobReferences(collector,
+            newOldReclaimer(SegmentGCOptions.GCType.FULL, revisions.getHead().getSegmentId().getGcGeneration(), gcRetainedGenerations));
+    }
 }
Index: oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentBlobReferenceRetriever.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentBlobReferenceRetriever.java	(revision 9f1e8bdc6c91ba750ac1eea540fe59ac89ef9114)
+++ oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentBlobReferenceRetriever.java	(date 1535624179000)
@@ -23,7 +23,7 @@
 
 import org.apache.jackrabbit.oak.plugins.blob.BlobReferenceRetriever;
 import org.apache.jackrabbit.oak.plugins.blob.ReferenceCollector;
-import org.apache.jackrabbit.oak.segment.file.FileStore;
+import org.apache.jackrabbit.oak.segment.file.AbstractFileStore;
 
 /**
  * Implementation of {@link BlobReferenceRetriever} to retrieve blob references from the
@@ -31,9 +31,9 @@
  */
 public class SegmentBlobReferenceRetriever implements BlobReferenceRetriever {
 
-    private final FileStore store;
+    private final AbstractFileStore store;
 
-    public SegmentBlobReferenceRetriever(FileStore store) {
+    public SegmentBlobReferenceRetriever(AbstractFileStore store) {
         this.store = store;
     }
 
Index: oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/AbstractFileStore.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/AbstractFileStore.java	(revision 9f1e8bdc6c91ba750ac1eea540fe59ac89ef9114)
+++ oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/file/AbstractFileStore.java	(date 1535624247000)
@@ -28,6 +28,7 @@
 import java.util.HashSet;
 import java.util.Set;
 import java.util.UUID;
+import java.util.function.Consumer;
 
 import org.apache.jackrabbit.oak.api.jmx.CacheStatsMBean;
 import org.apache.jackrabbit.oak.segment.CachingSegmentReader;
@@ -270,4 +271,17 @@
         return new Segment(tracker, segmentReader, id, buffer);
     }
 
+    /**
+     * Finds all external blob references that are currently accessible
+     * in this repository and adds them to the given collector. Useful
+     * for collecting garbage in an external data store.
+     * <p>
+     * Note that this method only collects blob references that are already
+     * stored in the repository (at the time when this method is called), so
+     * the garbage collector will need some other mechanism for tracking
+     * in-memory references and references stored while this method is
+     * running.
+     * @param collector  reference collector called back for each blob reference found
+     */
+    public abstract void collectBlobReferences(Consumer<String> collector) throws IOException;
 }